Development of ZnO components for Flash sintering at low temperatures

Nuno Gonçalo

Ferreira

Desenvolvimento de componentes de ZnO a baixa

temperatura por sinterização Flash

Development of ZnO components for Flash sintering

at low temperatures

Nuno Gonçalo Ferreira 3 2018

Nuno Gonçalo

Ferreira

Desenvolvimento de componentes de ZnO a

baixa temperatura por sinterização Flash

Development of ZnO components for Flash sintering

at low temperatures

Internship report presented to the University of Aveiro to fulfil the requirement for awarding the degree of Master in Materials Science and Engineering carried out under the supervision of Prof. Doctor Ana Maria Oliveira e Rocha Senos, Associate Professor in the Department of Materials and Ceramics Engineering, co-supervision of Doctor Oleksandr Tkach, Associate Researcher in the Department of Materials and Ceramics Engineering and internship supervision of Rauschert Portuguesa Eng.º Diamantino Dias.

Nuno Gonçalo Ferreira 5 The Board Examiners

President Prof. Doutor Pedro Manuel Lima de Quintanilha Mantas

Auxiliar professor from University of Aveiro, Portugal

Prof. Doutor Luís Miguel Nunes Pereira

Associated prodessor from University Nova de Lisboa

Prof. Doutora Ana Maria de Oliveira e Rocha Senos

Nuno Gonçalo Ferreira 7 Agradecimentos Tudo parece impossível até que seja feito”, disse Nelson Mandela,

laureado com o Prémio Nobel da paz em 1993. Esta frase traduz bem o sentimento de alguém que tem em mãos uma tarefa que, no início, parece difícil de concretizar, por todos os obstáculos que se colocam aquando da realização da mesma.

Ora, o sucesso de qualquer trabalho é sempre o resultado do esforço e empenho de um conjunto de pessoas que se unem em prol de um objetivo comum.

Por isso, quero expressar aqui a minha gratidão a todos os meus professores ao longo dos cinco anos do curso pelas suas qualidades científicas e pedagógicas que tornaram este meu percurso académico tão estimulante.

Em especial, tenho de deixar o meu grande apreço à minha orientadora, Professora Doutora Ana Senos, e ao meu co-orientador, Doutor Oleksandr Tkach, por toda a disponibilidade dispensada ao longo da realização deste trabalho.

Ao colega Ricardo Serrazina, aluno de doutoramento, pelas aprendizagens partilhadas e pela ajuda sempre pronta.

À Marina, companheira de todas as horas, que tem partilhado comigo as angústias e conquistas nesta fase da vida.

Por fim, mas não menos importante, agradeço o amor, o estímulo e apoio incondicional dos meus pais, Luísa e Pedro.

keywords ZnO, Al-doped ZnO, Flash sintering, Microstructure, Electrical conductivity.

summary Flash sintering is a non-conventional sintering method proposed in 2010. Flash combines the use of electric field and temperature to promote densification of materials in seconds (<60 s) at lowered temperatures.

The Flash temperature (Tf) is defined as the temperature at

which the power supply switches from voltage to current control mode and sintering starts. The combination of the electric field and furnace atmosphere during Flash experiments gives a possibility to significantly decrease Tf of

ZnO among other materials. Conventional sintering temperature of pure ZnO is close to 1200 ºC, while the use of a water assisted Flash sintered apparatus allows the densification of ZnO to occur at room temperature. However, a comprehensive study of the water role and the effect of other atmospheres on the Flash sintering of ZnO is yet missing. In the present work, the influence of such atmospheres as air, argon (Ar), and nitrogen/hydrogen mixture (N2/H2) on Flash

sintering of pure ZnO is studied with and without water.. So far, it is possible to Flash sinter ZnO between 25 and 30 ºC using wet Ar and N2/H2, respectively. However, without the

water vapour, the Flash temperature increases to 100 ºC in Ar and 144 ºC in N2/H2, highlighting the important role of the

water in the ZnO conductivity and defect chemistry, which directly influences the value of Tf and the microstructural

evolution in the Flash sintering. The influence of doping ZnO with Al both on the onset temperature of Flash sintering and on the microstructure of the sintered pellets is also studied under the conditions previously used for undoped ZnO. In addition, an energetic sustainability study of Flash sintered commercial alumina, Rapox Brown from Rauschert, is presented as a result of a partnership between the University of Aveiro and Rauschert Portuguesa.

palavras-chave ZnO, ZnO dopado com Al, Sinterização Flash, Condutividade elétrica.

resumo Sinterização Flash é uma técnica não convencional de sinterização proposta em 2010. Flash combina o uso de um campo elétrico e de temperatura de forma a sintetizar um material em menos de 60 s e a temperaturas mais baixas aquelas a que tem de ser sujeito quando sinterizado convencionalmente. A temperatura de Flash (Tf) é definida como aquela aquando se dá a troca de modo voltagem para modo amperagem e a sinterização é iniciada. A combinação de um campo elétrico e uma atmosfera controlada dentro de um forno durante o Flash abre portas para o decréscimo da Tf de ZnO, entre outros. ZnO, normalmente, é sinterizado a temperaturas próximas de 1200 ºC, no entanto com o uso de vapor de água este é possível ser feito à temperatura ambiente. No entanto, um estudo aprofundado ainda não foi feito do papel deste elemento e de outras atmosferas na temperatura e morfologia das amostras aquando sinterizadas por Flash.

No presente trabalho, a influência de diferentes atmosferas, tal como air, árgon (Ar) e azoto/hidrogénio (N2/H2) na sinterização

Flash do ZnO com e sem vapor de água.. Até ao momento, foi possível sinterizar ZnO, usando Flash, a temperaturas entre 25 (Ar) e 30ºC (N2/H2) usando vapor de água, no entanto, sem vapor

de água estas temperaturas aumentam para 100 ºC em Ar e 144 ºC em N2/H2. Isto realça a importância da água na condutividade

e na química de defeitos que influenciará diretamente a Tf e a

evolução microestrutural durante o processo de sinterização. ZnO foi dopando com Al (AZO) e foi estudada a sua influência a nível da temperatura de Flash e, ainda, as alterações a nível microestrutural. As condições usadas para sinterizar os AZO’s foram as mesmas que para o ZnO puro.

Por fim, foi feito um estudo de sustentabilidade energética da sinterização Flash numa parceria entre a Rauschert Portuguesa e a Universidade de Aveiro. Para isso, foi sinterizada uma alumina comercial, Rapox Brown.

Nuno Gonçalo Ferreira i

Index

List of Tables ... vi

List of Abbreviations ... vii

List of Symbols ... viii

1. Introduction ... 3

1.1. Motivation and objectives ... 4

1.2. Structure of the master thesis ... 5

2. State of the art ... 9

2.1. Characteristics of undoped and Al-doped ZnO ... 9

2.2. Sintering - Basic concepts ... 13

2.3. Conventional sintering ... 14

2.3.1 Thermal energy and capillary forces... 14

2.3.2 Mechanisms of sintering ... 15

2.3.3 The effects of pores and grain boundaries ... 16

2.3.4 Sintering stress ... 17

2.3.5 Stages of sintering ... 18

2.3.6 Liquid phase sintering ... 19

2.3.7 Conventional sintering of ZnO based ceramics ... 20

2.4. Flash sintering ... 22

2.4.1. Flash sintering variables ... 23

2.4.2. Stages and mechanisms of Flash sintering... 27

2.4.3. Flash sintering of ZnO ... 30

3. Experimental details... 37

3.1. Powders preparation ... 37

Master Thesis

Nuno Gonçalo Ferreira ii

3.3. Thermal consolidation techniques ... 39

3.4. Characterization techniques ... 41

3.4.1. Particle size distribution – Malvern ... 41

3.4.2. Dilatometry ... 42

3.4.3. Differential thermal analysis and thermogravimetric analysis ... 42

3.4.4. X-ray diffraction ... 43

3.4.5. Specific Surface Area ... 44

3.4.6. Scanning electron microscopy ... 45

3.4.7. Energy-dispersive X-ray spectroscopy ... 46

3.4.8. Density measurements ... 47

4. Results and Discussion ... 51

Part I ... 51

4.1. Powders Characterization ... 51

4.1.1. Undoped ZnO... 52

4.1.2. Al-doped ZnO powders... 54

4.2.1. Establishing FS conditions ... 58

4.2.2. Undoped ZnO... 63

4.2.1. Al-doped ZnO ... 71

Part II ... 76

4.3. Internship at Rauschert Portuguesa ... 76

4.3.1. Experimental details and ceramic microstructure ... 76

4.3.2. Cost-efficiency study ... 78

5. Conclusions ... 85

6. Future work ... 89

Nuno Gonçalo Ferreira iii

List of Figures

Fig. 1- Typical time and temperature for sintering ceramic materials using different technologies (adapted from [4]). ... 3

Fig. 2- Crystal structure of ZnO [6]. ... 9 Fig. 3- Typical point defects observed in undoped ZnO – a) Oxygen vacancies ( 𝑉𝑂 ∙∙) and b) Zn2+_{in a interstitial position (𝑍𝑛𝑖 ∙). ... 10}

Fig. 4- Schematic Brouwer diagram showing the transition from intrinsic defect control in range I, to extrinsic defect control, from donor impurities, in range II, as the oxygen partial pressure is increased [13]. ... 11

Fig. 5- Schematics of the lattice of AZO – a) Al3+ occupying a Zn2+ site in the ZnO lattice and b) Al3+ occupying adjacent Zn2+ sites with formation of oxygen vacancy. .. 12

Fig. 6- Schematic Brouwer diagram for ZnO: Al (AZO). Vertical dotted line separates different electroneutrality regimes. Fractions indicate line slopes, n represents the concentration of electrons, [𝐴𝑙𝑍𝑛.] the Al donors and [𝑉𝑍𝑛′′] the charged Zn vacancies (adapted from [19]). ... 13

Fig. 7- Schematic representation of material transport paths during sintering and corresponding sintering mechanisms (adapted from[24][25]) ... 16

Fig. 8- Isolated pores at triple junctions: a) pore will shrink, b) stable pore and c) pore will grow [12]. ... 17

Fig. 9- Relative density as a function of sintering time [31]. ... 19 Fig. 10- Schematic illustration of the densification mechanisms inherent to classic liquid phase sintering [32]. ... 20

Fig. 11- Process parameters that influence the FS behaviour. ... 23 Fig. 12- FS apparatus configurations: (a) sample is suspended in a furnace using two Pt wire electrodes adapted from Cologna et al. [45]); (b) sample is pressed between two electrodes (adapted from Caliman et al. [46]); (c) a commercially available SPS device adapted for FS [47], d) contactless mode where a plasma is used for carrying current across the samples, Saunders et al.[48]. ... 24

Fig. 13- Possible specimen geometries used in FS studies. (a) dogbone ; (b) rectangular bar; (c) cylindrical pellets, of various height to diameter ratios. ... 26

Fig. 14- Three stages of FS [44]. ... 28 Fig. 15- Schematics of FS setup with atmosphere control. ... 39

Master Thesis

Nuno Gonçalo Ferreira iv Fig. 16- Plotted thermic cycle and applied electric field. ... 40 Fig. 17- a) Schematic representation and b) image of the Flash sintering setup developed at UA. ... 41

Fig. 18- Schematic Representation of Bragg’s Law. ... 44 Fig. 19- SEM Hitachi S-4100 and SEM Hitachi SU-70 available at UA. ... 46 Fig. 20- XRD pattern of as received ZnO powder with indicated reflections for wurtzite phase. ... 52

Fig. 21- Particle size distribution of ZnO powder as received and milled determined by Malvern. ... 53

Fig. 22- SEM micrographs of as received a) and milled b) undoped ZnO precursor powder. ... 53

Fig. 23- a) Densification integral curve; b) derivative curves of undoped ZnO (~60% GD) for heating rates of 5, 10 and 20 ºC/min. ... 54

Fig. 24- XRD patterns of Al-doped ZnO powders with Al content of 0, 0.5, 1 and 1.5 at.% calcined at 450 °C in whole 2Θ range of 20-80° a) and in highlighted area around (101) wurzite peak b). ... 55

Fig. 25- a) XRD patterns of undoped and 1 at.% Al-doped ZnO powders calcined at 450 and 1000 °C in whole 2Θ range of 20-80° a) and in highlighted area around (101) wurzite peak b). ... 56

Fig. 26- SEM micrographs of 1Al powders. ... 57 Fig. 27- a) densification integral curve; b) derivative curves of undoped AZO (~60% ± 3 green density) for heating rate of 20 ºC/min ... 57

Fig. 28- Optical magnifier and SEM micrographs of (a) the cathode side (-), (b) the anode side (+) and (c) and (d) overall look of fracture surface of undoped ZnO flash sintered with 200 V/cm and 75 mA/mm2. ... 59

Fig. 29- SEM micrographs of ZnO samples Flash sintered with 200 V/cm and 75 mA/mm2 in Ar. a) overall look of the ZnO samples in the cathode side and anode side, (b) 30 s, (c) 60 s and (d) 120 s sintering time, images of the center of the specimen. ... 60

Fig. 30- The onset Flash temperature of ZnO as a function of 200 and 300 V/cm electric field and air, Ar and N2/H2 atmospheres... 61

Fig. 31- Images of ZnO samples sintered with an applied electric field of 300 V/cm2

... 62 Fig. 32- The onset Flash temperature of ZnO sintered at 200 V.cm-1 and 75 mA.mm-2 as a function air, Ar and N2/H2 atmospheres with and without water vapour. ... 64

Nuno Gonçalo Ferreira v Fig. 33- Optical magnifier and SEM micrographs of ZnO Flash sintered in Ar at 200 V.cm-1 and 75 mA.mm-2 taken at locations a), b), c) and d) marked at optical image. .. 65

Fig. 34- SEM micrographs of (a) and (b) Cathode (+), (c) transition from cathode to anode and (d) anode (-) areas of ZnO_w sintered in air. ... 66

Fig. 35- Optical magnifier and SEM micrographs of ZnO_w Flash sintered in wet Ar at 200 V/cm and 75 mA/mm2 taken at the optical magnifier. ... 68

Fig. 36- Average relative density vs flash temperature in a wet and dry atmosphere of Ar, N2/H2 and air, respectively. ... 69

Fig. 37- SEM micrographs of undoped ZnO sintered without water (left micrographs) and with water (right micrographs). ... 70

Fig. 38- Influence of atmosphere with and without water vapour on FS of ZnO_w, 0.5Al_w, 1Al_w and 1.5Al_w. ... 72

Fig. 39- SEM micrographs of 1Al Flash sintered at 200 V/cm and 75 mA/mm2 _{in Ar:}

a) distinction between anode and cathode side and b), c), and d) highlighted areas from a). ... 73

Fig. 40- Optical magnifier and SEM micrographs of 1Al_w Flash sintered at 200 V/cm and 75 mA/mm2 in wet Ar. ... 74

Fig. 41- CIP available at Rauschert Portuguesa and a pressed pellet. ... 76 Fig. 42- Conventional sintering cycle of Rapox Brown ceramics at Rauschert Portuguesa. ... 77

Fig. 43- Images of the Flash sintered Rapox Brown ceramics ... 77 Fig. 44- Comparison between FS (left) vs CS (right) sintered Rapox Brown ceramics. ... 78 Fig. 45- Electrical furnace available at Rauschert Portuguesa and a Schematic representation of Rauschert conventional sintering furnace. ... 79

Fig. 46- Comparison between CS and FS sintering cycles of Rapox Brown. ... 80 Fig. 47- Cumulative power consumption for FS of 1 kg of Rapox Brown. ... 81 Fig. 48- Temperature and cumulative energy consumption for 1 kg of Flash sintered Rapox, with 5 ºC/min heating and cooling rate cycle. ... 81

Master Thesis

vi

List of Tables

Table 1- Properties of ZnO [7],[8]. ... 9

Table 2- Mechanism of Sintering in polycrystalline materials [24]. ... 16

Table 3- State of the art of FS of ZnO. ... 32

Table 4- Main cationic impurities of the aluminium nitrate reagent... 37

Table 5- Designation of produced powders. ... 38

Table 6- Powders characterization powders: mean particle size (MPS) by coulter, specific surface area (SSA) and mean grain size (GBET) calculated by BET and the mean particle size and crystalline size determined by XRD. ... 51

Table 7- Summarized conditions and results of FS in undoped ZnO with varying applied electric field, dwell time and atmosphere. ... 58

Table 8- FS results of undoped: effect of different dry and wet atmospheres for pure ZnO. ... 63

Table. 9- FS results of doped ZnO: effect of different dry and wet atmospheres. .... 71 Table. 10- Conditions used of ZnO and AZO flash sintered in this work. ... Erro! Marcador não definido.

Table. 11 - Compilation of the results concerning the cost-effective study ... Erro! Marcador não definido.

Nuno Gonçalo Ferreira vii

List of Abbreviations

AC- Alternative current; AZO- Al doped ZnO;

BET- Brunauner. Emmet and Teller CC- Current control;

CS- Conventional sintering;

DTA- Differential thermal analysis;

EDS- Energy Dispersive X-ray spectroscopy; FAST- Field Assisted Sintering Techniques; FS- Flash sintering;

GB- Grain boundaries;

GSD- Granulate size distribution; MW- Microwave;

rpm- Rotation per minute;

SEM- Scanning electron microscopy; SSA- Specific surface area;

TG- Thermogravimetric analysis; UA- University of Aveiro; UV- Ultraviolet;

Master Thesis

Nuno Gonçalo Ferreira viii

List of Symbols

l- Length of the sample;

L- Crystalline size;

l0- Initial length of the sample;

m- Mass;

n- Electron concentration;

𝑁- Number of atoms; 𝑁0- Number of atom sites;

ng- Number of grains;

p(O2)- Oxygen partial pressure;

Q- Activation energy; r- Radius of the pore; R- Gas constant; t- Time;

T- Temperature; Tf- Flash temperature;

V- Voltage;

Vg- Adsorbed gas volume;

VO- Oxygen vacancy;

Nuno Gonçalo Ferreira ix - Stress on the surface;

- Surface free energy;  -Bragg’s angle - Wavelength;

ρ0- Green density;

ρg- Geometric density;

ρr- Relative density;

ρtheo- Theoretical density;

e’- Free electron (negatively charged);

𝑉_𝑂′′- Oxygen vacancy positively charged (+2); 𝑍𝑛_𝑖 - Zn interstitial;

𝑍𝑛_𝑖•- interstitial Zn positively charged (+1); 𝑍𝑛_𝑖𝑥- Zn atom in interstitial position

CHAPTER 1

3

1. Introduction

Nowadays, with the increasing technology and energy consumption it is fundamental to develop or utilize advanced technologies in order to prevent the overload on the natural or fossil resources. Among the manufacturing techniques, ceramics processing tends to be very energy consuming, since their sintering involves a very high processing temperature. Therefore, new technologies known as non-conventional sintering techniques are being developed in order to reduce the energy consumption, while maintaining or even improving the characteristics of the resulting ceramic materials [1].

Moreover, very long and high-temperature heat treatments, required to consolidate the ceramic materials and to form a polycrystalline structure, limit the range of materials, which can be co-processed with the ceramics, to those with melting points above the sintering temperature [2]. As shown in Fig. 1, alternatives to conventional ceramic sintering, including Hot Pressing, Microwave (MW) mediated heating as well as Field Assisted Sintering Techniques (FAST), can reduce the temperatures required for sintering to full density. However, with exception of FAST / Flash, these techniques require specialised equipment, and, particularly in the case of pressure-assisted methods, significantly limit the geometry of the produced samples [3].

Fig. 1- Typical time and temperature for sintering ceramic materials using different technologies

Introduction

4

1.1. Motivation and objectives

Flash sintering (FS) is a FAST sintering method that can be an alternative for conventional and even non-conventional sintering technologies mentioned above and shown in Fig. 1. This technique was developed by Rishi Raj from University of Colorado at Boulder in 2010 [5]and it has captured attention of researchers due to drastic reduction of sintering temperature and time. Moreover, when applied industrially, such advantageous possibilities as low energy consumption and thereby low cost and CO2

(carbon dioxide) emissions are anticipated. However, FS technique, giving a possibility to sinter in < 60 s, is known to undergo non-equilibrium processes. Therefore, all the FS variables have to be mastered, for each particular material with high accuracy, thus, needing further research and development in a multidisciplinary approach (involving sintering, light emission, electrical properties, new phase formation, non-conventional microstructures).

This master thesis is aimed to give a contribution to this topic based on the knowledge attained so far. Another purpose of this study is to enlarge the knowledge on FS of a particular system of pure and doped ZnO (zinc oxide), because from 88 papers on FS found so far only 3 works are published on ZnO. It is also important to mention that ZnO is a material that is used in a wide range of applications and can even substitute materials like GaN (gallium nitride) and Ln2O3 (lanthanum oxide) used in ultraviolet (UV) light

emission and in optoelectronic devices.

The main objectives of this thesis can be summarized as:

(i) to understand the influence of sintering atmosphere both on the densification process and on the onset Flash temperature, performing FS of undoped ZnO in different gas flows with and without water;

(ii) To perceive the influence of aluminium (Al) trivalent cation in the ZnO lattice (Al doped ZnO, AZO) on the onset Flash temperature and developed microstructure by comparison between undoped ZnO and AZO Flash sintered in the atmospheres of same type;

(iii) To analyse the energetic sustainability of FS in the processing of industrial productsI on the example of commercial alumina, Rapox Brown from Rauschert, as result of a partnership between the University of Aveiro (UA) and Rauschert Portuguesa.

5

1.2. Structure of the master thesis

The thesis is structured according to the following scheme. The current chapter provides a brief introduction into the research topic, the motivation and the main objectives of this research work.

Chapter 2 presents the fundamental background on ZnO and AZO as the materials of this study; an overview related to FS and its variables studied so far as well as the reported mechanisms and phenomenology behind FS.

The description of experimental procedures of the powders and ceramics preparation and characterization by a variety of techniques is performed in chapter 3.

Chapter 4 constitutes one of the key points of this dissertation, presenting the results on the use of FS for preparation of undoped ZnO and AZO ceramics. The influence of powder composition and sintering atmosphere on the structure, morphology and composition of FS samples is analysed in this chapter. Chapter 4 is divided in two sections: the first part is regarding experimental work done in UA and the second part concerns results obtained at Rauschert Portuguesa.

At the end of the thesis the main conclusions as well as further work prospects are presented.

CHAPTER 2

9

2. State of the art

2.1. Characteristics of undoped and Al-doped ZnO

ZnO has the Wurtzite-type crystal structure. Each hexagonal cell of its structure contains two molecules of ZnO, where zinc (Zn) atoms are tetrahedrally surrounded by oxygen (O) atoms. Perpendicularly to c-axis, the unit cell consists of alternating Zn and O planes, resulting in a classical polar structure, as shown in Fig. 2.

Fig. 2- Crystal structure of ZnO [6].

Intrinsically ZnO is a wide bandgap n-type semiconductor with the principal properties presented in Table 1.

Table 1- Properties of ZnO [7],_[8].

Properties Value

Stable phase Hexagonal compact

ao 3.2495 co 5.19 Density 5.61 g/cm3 Melting point 1975 ºC Thermal conductivity 0.6 / 1-1.2 W/(m K) Band gap 3.2 – 3.7 eV Electrical conductivity 10-8 _{– 10}4 _Ω/cm

Hall mobility in monocrystals

type-n (T = 300K) 200 cm

2_{/(V s)}

Dielectric constant 8.656

The remarkable properties of ZnO are used in many areas from rubber and concrete industries to medicine and cosmetics, but specially in optoelectronics (gas sensors and

ZnO based ceramics

10 devices that emit UV radiation). The worldwide use of ZnO exceeds 1.2 million tonnes annually [9]. ZnO offers numerous advantages like low cost (resource availability), high electron mobility and strong resistance to high energy radiation, which makes it suitable for space applications. In addition, ZnO is thermally and chemically stable and compatible with lithography processes in microelectronics [10],[11].

The electrical conductivity of ZnO is affected by additional electrons resulting from oxygen vacancies Eq. (1) or by the ionization of interstitial zinc atoms, as shown in Eqs. (2) and (3): 𝑂_𝑂𝑥 ↔ 1 2𝑂2+ 𝑉𝑂 ∙∙_{+ 2𝑒}′ ₍₁₎ 𝑍𝑛𝑂 (𝑠) + 𝑥𝑍𝑛 (𝑔) ↔ 𝑍𝑛𝑂 + 𝑍𝑛_𝑖𝑥 (2) 𝑍𝑛_𝑖𝑥→ 𝑍𝑛_𝑖∙ + 𝑒′ (3)

where 𝑉_𝑂∙∙ represents an oxygen vacancy positively charged (+2), and 𝑒′ represents the free electron (negatively charged) generated to keep charge neutrality. The 𝑍𝑛_𝑖𝑥 represents 𝑍𝑛 atom in an interstitial position without change in the oxidation state, and 𝑍𝑛_𝑖∙ correnponds to an interstitial 𝑍𝑛 positively charged (+1). Thus, free charge carriers in pure ZnO have been attributed to the presence of native defects, such as Zn interstitial (Zni) and oxygen vacancies (VO) [11]. A schematic representation of the typical defects

responsible for the electrical properties in undoped ZnO is presented in Fig. 3.

Fig. 3- Typical point defects observed in undoped ZnO – a) Oxygen vacancies ( 𝑉𝑂∙∙) and b) Zn2+in a

11 These point defects are always present in the material and can act as shallow n-type donors, with energy levels of about 30 meV below conduction band [12].

The properties of ZnO can be affected by the sintering atmosphere. In Fig. 4, the schematic Brouwer diagram, presenting the defect structure of ZnO, shows a plateau in electron concentration n at intermediate oxygen pressure and increase in n at reduced

p(O2). The plateau is attributed to electroneutrality between electrons and a donor ion,

while the upturn in n at low p(O2) is a result of an increase in doubly charged oxygen

vacancies, which yields the -1/6 slope relationship [13].

Fig. 4- Schematic Brouwer diagram showing the transition from intrinsic defect control in range I, to

extrinsic defect control, from donor impurities, in range II, as the oxygen partial pressure is increased [13].

The variation of ZnO electrical properties, made through the addition of charge carriers, can be done by doping. The donor dopants can be either from group III atoms (Ga, Al, etc.), or group VII atoms (Br or F). These atoms can be incorporated substitutionally at the Zn or O sites, respectively, acting as a shallow donor.

In this thesis, the use of Al as a dopant has the objective of enhancing the electrical conductivity to promote lower flash temperature. Al is a trivalent element with an ionic radius of 67.5 pm that is significantly smaller than that of Zn, which is 88.0 pm [14]. This fact causes lattice distortion when Al substitutes for Zn in ZnO lattice. The effects of doping ZnO with trivalent Al cation are:

ZnO based ceramics

12 ii. Variation of the crystallographic parameters, confirming that the Al cations enter into the ZnO lattice, i.e. they are incorporated as substitutional ions, as shown in Fig. 5 [16]. This behaviour follows the Eq. (4), Fig. 5 a), and Eq. (5), Fig. 5 b).

𝐴𝑙𝑍𝑛𝑥  𝐴𝑙𝑍𝑛. + 𝑒′ (4)

𝑂𝑜𝑥  𝑉𝑂..+ 1

2 𝑂2 (𝑔) + 2𝑒

′ ₍₅₎

iii. Above the solubility limit of Al3+ a formation of second phases (ZnAl2O4 - spinel),

Al-O clusters or the amorphous gahnite spinel phase (Zn2Al2O4), Fig. 5 b)

[17][18].

The AZO properties can be affected by the sintering atmosphere according to the Brouwer diagram in Fig. 6, which shows the AZO log-log plot of defects concentration versus oxygen partial pressure p(O2) [19].

a) b)

Fig. 5- Schematics of the lattice of AZO – a) Al3+ _{occupying a Zn}2+ _{site in the ZnO lattice and b) Al}3+

13 As seen from the figure, the formation of Zn vacancies results in ionic compensation of the donor species (𝐴𝑙_𝑍𝑛∙ ) rather than in the carrier generation in the high-p(O2) regime.

The electroneutrality condition is 2 [𝑉_𝑍𝑛′′] = [𝐴𝑙_𝑍𝑛∙ ]. As p(O2) is reduced, the electron

population steadily rises until electrons become the prevalent species compensating the donors. In low-p(O2) regime, the electroneutrality condition becomes n = [𝐴𝑙_𝑍𝑛∙ ]. This

means that the electron population of AZO can increase exponentially through the control of p(O2) during sintering.

2.2. Sintering - Basic concepts

The use of sintering process dates to the beginnings of civilization. It is believed that the first material to be sintered were bricks. However, detailed studies of the sintering process just began in the 20’s of last century. Currently, sintering theories remain matters of extreme relevance and subject of intense investigation in ceramicist’s research because of three factors:

1. The difficulty to obtain a quantitative theoretical description that expresses exactly the complex interactions between the geometric and thermodynamic factors occurring during the sintering process;

2. The emergence of new consolidate nanostructured ceramic materials, which are obtained from nanometric powders [20];

Fig. 6- Schematic Brouwer diagram for ZnO: Al (AZO). Vertical dotted line separates different

electroneutrality regimes. Fractions indicate line slopes, n represents the concentration of electrons, [𝐴𝑙_𝑍𝑛. _{] the Al donors and [𝑉}

ZnO based ceramics

14 3. The reduction of energy consumption and time, in order to transform sintering

into a more environmental friendly process.

The sintering process plays a predominant role in the production of ceramics. Ceramic sintering means that ceramic bodies, i.e., systems of consolidated powders, are burned at elevated temperatures to generate a microstructure with the desired properties [21]. Generally, the properties of ceramics are strongly dependent on the microstructure. Therefore, it is important to control with precision microstructural features, such as: size and shape of the grains; the size, amount, and distribution of pores in the structure; as well as the nature and distribution of any secondary phases, in order to achieve the desired properties. For the most of applications, microstructural control usually means the attainment of high density, a desired grain size, and a homogeneous microstructure. Thus, the detailed understanding of the sintering process is of considerable importance where the main subjects of sintering studies are the densification and the grain growth during sintering.

2.3. Conventional sintering

2.3.1 Thermal energy and capillary forces

Densification in polycrystalline solids is a process, by which pores are eliminated from a compact of particles. The pore elimination happens by means of mass transport with movement of atoms from the particles to the inter-particle pore spaces. This movement is driven by a reduction in the surface area; the energy for mass transport is supplied by increasing the temperature of the material. Material is transferred from regions of higher chemical potential (source) to regions of lower chemical potential (sink). Sintering or sinter bonding becomes evident in most materials around one half of the melting temperature. This is highly dependent on the size of the particles, which are bonding. Smaller particle has higher surface area and thus more energy per unit volume, this energy can then facilitate sintering at lower temperatures. Sintering is an irreversible thermodynamic process, and in accord with other irreversible processes, sintering is accompanied by a lowering of the free energy of the system. The sources that give rise to this lowering of free energy are commonly referred to as the driving forces for sintering. Three possible driving forces are: the curvature of the particle surfaces, externally applied

15 pressures and chemical reactions. The sintering process is thermally activated (in most cases) and relies on the ability of the atoms in the sintering body to achieve enough energy to escape from the current position and move into a new adjacent position. The Arrhenius temperature relation that describes an array of vacant atom sites, 𝑁0, and the number of

atoms with enough energy to move, 𝑁, is [22]:

𝑁

𝑁₀

= 𝑒

−𝑄/𝑅𝑇

_.

(6)

The Arrhenius terms are 𝑄 as the activation energy, 𝑅 as the gas constant and T as the temperature. According to Eq. (6), higher temperatures will produce more active atoms and thus faster sintering rates. This is one of many important parameters in the overall sintering characteristics of a polycrystalline solid. Others include heating rate, particle size, and secondary phases.

2.3.2 Mechanisms of sintering

A polycrystalline solid can be simplified as a system of particles as shown in Fig. 7. According to this figure, there are six distinct mechanisms, which allow for the transport of matter during solid state sintering [23], as listed also in Table 2. Each mechanism is distinguished by the diffusive path involved, the source, from which matter is drawn, and the sink, to which matter is transported. All six of the mechanisms lead to bonding between the particles and neck growth. However, only mechanism 4, 5 and 6 contribute to a change in bulk density. Mechanisms 1, 2 and 3 may impede the densification by contributing to neck growth without causing the particle centres to move closer together. In other words, by reducing the curvature of the neck surface they also reduce the driving force for sintering. Table 2 describes the source and sink for each mechanism. The point here is that, in sintering it is often desirable to enhance the densification mechanisms (4, 5 and 6) at the expense of the non-densifying mechanisms, in order to produce a fine-grained ceramic.

ZnO based ceramics

16 Fig. 7- Schematic representation of material transport paths during sintering and corresponding

sintering mechanisms (adapted from[24][25])

Table 2- Mechanism of Sintering in polycrystalline materials [24].

Mechanism Source Sink Densifying

Surface diffusion Surface Neck No

Lattice diffusion Surface Neck No

Gas phase transport Surface Neck No

Grain boundary diffusion Grain boundary Neck Yes

Lattice diffusion Grain boundary Neck Yes

Viscous flow Dislocation Neck Yes

2.3.3 The effects of pores and grain boundaries

In polycrystalline materials the grain boundaries also play an important role in determining bulk densities by determining the equilibrium shape of the pore. Considering again the hypothetical example of a pore surrounded by three spherical grains, shown in Fig. 8, the balance of forces where the junction meets the grain boundary can be represented by Eq. (7) [26].

𝜃 = 2 𝑐𝑜𝑠

(

2 𝛾𝑠𝑣

)

The surface tension of the solid vapour interface is represented by 𝛾𝑠𝑣, this term is

tangential to the interface, as shown in Fig. 8. The surface energy of the grain boundary is 𝛾_𝑠𝑠, this term always remains in the plane of the grain-boundary; and oriented vertically in Fig. 8. The dihedral angle 𝜃 can predict the stability of the situation, as shown in Fig.

17 8 for a pore formed by 3 grains. In this case, dihedral angles less than 𝜋/3 bring pore instability and it will grow. Conversely, dihedral angles greater than 𝜋/3 will lead to pore closure. This relationship can be made more universal as proposed by Raj [27] and Lange [28]. Each author demonstrated that the pore will shrink provided 𝜃 > 𝛹. Here 𝛹 is defined as 𝜋/ng and ng is number of grains touching the pore (coordination number), i.e.

in Fig. 8 𝛹 = 𝜋/3.

Fig. 8- Isolated pores at triple junctions: a) pore will shrink, b) stable pore and c) pore will grow [12].

2.3.4 Sintering stress

The sintering stress or pressure, from a thermodynamic perspective, was rigorously developed by Raj [29] by analysing the coupling between the change in the interface energy and the reduction in the total volume of the sintered polycrystal. Reduction in the surface energies can occur by either removal of pores or a coalescence of pores. In a polycrystal this is accompanied by the concomitant coarsening of the grains during the sintering process, thus, reducing the overall grain boundary area and energy available. The term sintering stress is used to describe the accumulated stress from interfacial energies acting over the curved surfaces in a sintering system [26]. This is the thermodynamic energy employed to reduce the available surface energy and turn it into mechanical work.

The sintering stress is associated directly with the curvature of the inter-particle neck as demonstrated in Eq. (7). Raj worked out the sintering pressure given three assumptions: 1) that the ratio of the number and type of pores per grain remain constant 2) that the

ZnO based ceramics

18 pores have a quasi-equilibrium shape and 3) that the grains are nearly spherical. The sintering stress 𝜎𝑠𝑖𝑛𝑡. is then captured by two terms:

𝜎

=

𝐺

+

2𝛾𝑠𝑣

±𝑟 (8)

The first one represents the grain with average grain size 𝐺. The surface energies are represented by 𝛾_𝑠𝑠 for the grain boundary and 𝛾_𝑠𝑣 for the surface to vapour interface. The second term reflects the energy contribution from the pores with the radius of ±𝑟. Note that it can be either positive or negative depending on the curvature of the pore surface. If the surface is convex with the radius pointing towards the centre of the pore then r, and this whole term, is positive adding to the sintering pressure. A concave pore, pointing towards the centre of the grain works against the densification. With small grain sizes the first term trends to remain dominate. At very small pore sizes the second term can become dominating. This is the same relationship as described by Fig. 8.

The predictions of sintering stress were confirmed by Ducamp and Raj [30]. Ducamp demonstrated that sintering pressure of glass-powder increases from 0.1 MPa at relative densities of 0.6 to 0.3 MPa at near 0.9. It should be mentioned here that glass sinters by vitreous processes eliminating the complexity added by grain boundaries in polycrystalline materials. Overall, in polycrystalline materials the sintering pressure may become smaller or larger at higher densities depending on which term in Eq. 8 remains dominate.

2.3.5 Stages of sintering

The general process of sintering can be demonstrated succinctly by a plot of the relative density against the time as shown in Fig. 9.

The process of sintering progresses through three stages:

I. Initial stage – Rapid inter-particle neck growth by diffusion, vapour transport, plastic and viscous flow. The large initial differences in surface curvature are removed in this stage and densification accompanies neck growth.

II. Intermediate Stage – Once the neck formation is over, the pores reach equilibrium shape based on the particle shape-size, packing and surface energy. The pores remain interconnected and shrink in volume. This stage

19 covers densification from 60% to 85% of theoretical density at which point pores become isolated from their neighbour pores.

III. Final Stage – The microstructure of the final stage can develop in many ways. The final stage of sintering begins when the pores become isolated from each other. In some cases, the pores continue to shrink, eventually disappearing completely. It should be noted here that pores are removed at a much slower rate than in stage II. In other cases, the pores may reach an equilibrium with the surrounding densifying solid and shrink only to a point, preventing the sintering compact from ever fully achieving the asymptotic theoretical density in the polycrystalline state.

Fig. 9- Relative density as a function of sintering time [31].

2.3.6 Liquid phase sintering

In classic liquid phase sintering (LPS), two powders are mixed, with one powder being the additive/dopant and the second powder being the main component. On liquid formation, a rearrangement densification starts, followed by solution reprecipitation with concomitant grain growth and grain shape accommodation, and finished by the final solid-skeletal densification, as presented in Fig. 10. If there is a high liquid content, full density can be achieved via rearrangement upon liquid formation. At intermediate liquid content, the solid grain packing inhibits densification, requiring the participation of solution-reprecipitation events, where mass transport through the liquids controls grain

ZnO based ceramics

20 reshaping. At low-liquid content, densification is dominated by solid-state sintering of rigid solid skeleton.

Fig. 10- Schematic illustration of the densification mechanisms inherent to classic liquid phase

sintering [32].

In practice, liquid-phase sintering to high density must occur in a short time, even though mass transport is very rapid in the presence of a liquid phase, it is not sufficient to ensure rapid densification. Grain rearrangement upon liquid formation is the most important stage if rapid sintering densification in a process such as FS [33].

A liquid phase enhances sintering densification due to the interparticle capillary forces. Once a wetting liquid forms it penetrates existing solid-grain boundaries and forms a pendular bind that provides a capillary force on the grain contacts. This capillary force results in a sintering force that causes grain rearrangement, densification and contact flattening.

2.3.7 Conventional sintering of ZnO based ceramics

Over the last years many studies related to sintering and grain growth of microcrystalline ZnO powder compacts have been published [34]–[37]. High final densities (95 – 98 %) with grain sizes around 9 μm were reported for the sintering of high

21 purity ZnO powders (~99.9%) at temperatures between 1100 and 1200 °C. Gupta and Coble [36],[37] studied the densification and grain growth at the intermediate and final stages of ZnO powder compacts sintered in air and oxygen atmosphere by isothermal sintering techniques. They found an increase in density and a reduction in grain size when ZnO is sintered in oxygen rich atmosphere instead of atmospheric air atmosphere. The authors have also observed a good agreement between diffusion coefficients from the sintering data and the ones related to the lattice diffusion coefficients directly measured for zinc. Their results were also consistent with the expected effect of oxygen pressure on Zn diffusivity when the transport of Zn in the lattice occurs by an interstitial mechanism. Hence, they conclude that lattice diffusion of interstitial Zn is the main transport mechanism for the densification and grain growth of ZnO. The sintering kinetics of ZnO in the initial and intermediate stages was also investigated using a constant heating rate sintering technique. The author also stated that after a short initial period of particle rearrangement, the densification of ZnO was controlled by lattice diffusion of interstitial Zn. In another study, Han et al. [38] reported an activation energy of ~320 kJ/mol in the earlier sintering stage for the sintering of ZnO and conclude that the densification in the earlier stage of sintering is controlled by a combination of both grain boundary sliding and volume diffusion mechanisms.

The influence of atmosphere during the sintering of ZnO has been extensively studied. According to Coble [39], the sintering atmosphere influences the densification rate, grain growth and/or brings limitations in final densities of sintered ceramic oxides. In addition to the previous studies on sintering in oxygen, the effect of nitrogen and helium atmospheres on the densification of ZnO powder compacts was also investigated. The densification rate was found to be inversely proportional to the oxygen partial pressure, i.e., the higher is the oxygen partial pressure, the lower the densification rate became. This behaviour is a consequence of the change in the number of vacancies when oxygen partial pressure varies. For a sintering process controlled by diffusion, an increase in the vacancies concentration in the lattice causes an increase of the diffusion coefficient of the lattice, and thus, an increase in the densification rate [39]. Quadir and Readdey [40], observed an enhancement of vapour transport during sintering in hydrogen atmosphere, which led to a decrease in the densification rate and in the final density, mainly through a rapid particle coarsening, which is accentuated with hydrogen pressure and temperature.

ZnO based ceramics

22 Many experiments have been performed to investigate the role of different dopants during sintering of ZnO. Trivalent dopants, such as Al, Ga, and In, are found to decrease the electrical resistivity of ZnO. This decrease together with the modification of the crystallographic parameters indicates that when those cations enter into the ZnO lattice, they occupy substitutional positions. In general, the sintering is retarded when doping with trivalent cations. Komatsu et al. [41] studied the shrinkage of ZnO doped with 1.2 mol % Al under isothermal conditions at temperatures in the range 800–1000 °C in air. The authors reported a slight decrease in the activation energy for densification, from 193 kJ/mol for undoped ZnO to 176 kJ/mol for Al-doped ZnO. Quadir and Readey[40] investigated the effect of Al doping (1–10 mol % Al) on grain growth of ZnO sintered in hydrogen. They detected the presence of ZnAl2O4 spinel phase for higher doping

concentrations and a retardation of ZnO grain growth with the Al addition. Nunes and Bradt [42], studied the effect of aluminium oxide (Al2O3) doping concentration during

liquid phase sintering in air of ZnO also doped with bismuth oxide (Bi2O3). They have

detected the formation of the spinel phase, ZnAl2O4, and the inhibition of ZnO grain

growth with Al addition, even in the presence of the Bi2O3-rich liquid phase. The authors

proposed that the retarded grain growth is a result of the spinel particle pinning the ZnO grain boundaries and consequently reducing their mobility. Han et al. [43] also observed a strong pinning effect of the formed spinel when the solubility limit of Al in ZnO lattice is overcame. This author determined that the solubility of Al in ZnO lattice is ~0.06 mol% at 1200 ºC

2.4.

Flash sintering

FS is a recent innovation which was first described in 2010 by Cologna et al. [44] in a paper entitled “Flash sintering of nanograin zirconia in <5 s at 850 °C”. The paper describes a process, which involves the combination of temperature and an applied electrical field through packed powders to cause its rapid sintering.

In the following subchapters, various FS methods will be described and compared in terms of FS performance, advantages and limitations. The sample geometry, the methods and materials for making electrical contact, and industrial techniques developed to date will also be described. Finally, FS mechanisms will be presented.

23

2.4.1. Flash sintering variables

Despite the technology behind FS is quite simple, its understanding and process control is much more complex. Many process parameters can influence the FS behaviour, onset temperature, the densification behaviour and the microstructure development. The main variables are summarized in Fig. 11

FS apparatus consists of three main components: high-temperature furnace, power supply and a specimen. For monitoring the changes in the samples, equipment that determines the voltage, current, and sample displacement/shrinkage during the heat-treatment is added.

Despite its simplicity many configurations/setups, presented in Fig. 12, can be used depending on the type of technique and final properties.

ZnO based ceramics

24 Fig. 12- FS apparatus configurations: (a) sample is suspended in a furnace using two Pt wire

electrodes adapted from Cologna et al. [45]); (b) sample is pressed between two electrodes (adapted from Caliman et al. [46]); (c) a commercially available SPS device adapted for FS [47], d) contactless mode

where a plasma is used for carrying current across the samples, Saunders et al.[48].

From four configurations presented in Fig. 12, those shown in Fig. 12 (a) and (d) are pressureless, while those illustrated in Fig. 12 (b) and (c) are pressure-assisted. Using these apparatus, it is also possible to have atmosphere control.

The first configuration, shown in Fig. 12 (a), uses a vertical tube furnace with a dogbone-shaped sample suspended horizontally with platinum (Pt) wires as the electrodes [44], [49]–[51]. These are inserted through the top of the furnace and connected to the power supply. Devices monitoring current and voltage are included in the power circuit as well. At the base of the tube furnace, a camera with suitable filters that are focused on

25 measuring the dimensional variations (shrinkage or expansion) in the sample during FS is mounted [44].

The second FS configuration, presented in Fig. 12 (b), uses a dilatometer [52]. This type of apparatus requires small pressure to keep the sample in place, providing the contacts between sample and the electrodes for the Flash event to occur. In this technique, a disc shaped specimen that is placed between two electrodes made of platina and supported by alumina push-rods is used. This system is inside a furnace coupled with a dilatometer to test mechanical properties and to monitor the densification.

Fig. 12 c) is similar to SPS or hot-pressing apparatus. This design has been used in Flash spark plasma sintering (FSPS) [47], [53]–[56], where SPS apparatus is used without graphite mould, while high heating rates are provided by induction furnace [57]. This apparatus uses uniaxial pressure that can be applied with a wide range of pressure values, from that only promoting electrical contact to up to twenty tons. In this setup the ceramic powder is loaded into an insulator-lined die and the electrodes are the top and bottom graphite plungers, an approach originally described by Zapata-Solvas et al. [58].

Pressure-assisted sintering techniques have the advantage of improving the electrical contact at the interfaces between the sample and the electrodes. Cylindrical graphite punches can be used as electrodes [58],[59]. For materials susceptible to carbon contamination, refractory metal foils (Ta, Mo and W) can be inserted between the sample and the graphite punches. Continuous throughput FS techniques are currently under development. As shown in Figure 5 (d), arc plasma has been used to pass electrical current through the sample in a contactless mode [48].

Ceramic materials moulded into dogbones [44], bars [60] with rectangular cross-sections and cylindrical pellets of various diameter to height ratios [61]–[63] have been used for FS, as shown in Fig. 13.

ZnO based ceramics

26 Fig. 13- Possible specimen geometries used in FS studies. (a) dogbone ; (b) rectangular bar; (c)

cylindrical pellets, of various height to diameter ratios.

FS samples generally have small dimensions. As the voltage and current from lab-scale power supplies are relatively low, the control of maximum electric field and current density is a balance between the sample size and these two properties of the power supply. The initial (maximum) electric field E0 can be calculated by Eq. (9)

𝐸₀ = 𝑉

𝑙 , (9)

where V is the voltage and l is the length of the sample. Therefore, the initial electric field can be increased by changing the dimensions of the sample, even if the voltage is limited. The current density is calculated with Eq. (10).

𝐽 = 𝐼

𝐴 , (10)

where I is the current and A is the surface area of the pellet in contact with the electrode. From Eq. (10) it is clear that the current density can be controlled by changing the surface area. To ensure a higher 𝐽 it is necessary to have smaller samples.

Use of dogbone-shaped specimens does not have practical application [47], being however very relevant for academic researchers not only for the monitoring of shrinkage in section with different current densities during FS. [39]. It is also relevant for future applications of FS of ceramics with more complex geometries and larger specimens. There is evidence that the handle-sections of Flash-sintered dogbone specimens experience higher electric fields and hence higher temperatures, leading to larger grain growth under DC electric fields [62]. It means that for practical applications of the samples, these sections need to be removed. It has also been suggested that drilling holes into the green state dogbone samples for electrode wires can introduce damage [62].

Most FS research has employed DC power supplies (V and I in the range of 10–5000 V and 0.5–15 A, respectively)[1],[6],[19]. This is due to their lower cost compared to AC supplies. However, there are studies performed using AC power supplies (V and I in the

27 range of 10–62 V and 0.1–3 A, respectively), operating in the low- to mid-frequency range of 50–1000 Hz as well as at 1 MHz [64]–[66]. In the DC mode the sample behaves as a resistive load, whereas in the AC mode the sample can show capacitive/inductive effects. The implication of tuning AC frequency during FS was studied by Gittings et al. [67] for hydroxyapatite-based bio-ceramics. The temperature and frequency dependence of the complex conductivity was investigated between 200 and 1000 °C with current frequency varying from 0 (DC) to 1 MHz (AC). A strong increase (by five orders of magnitude) in the real part of the room temperature impedance was demonstrated when the AC frequency was increased up to 1 MHz.

The most common electrodes used for FS have been made of Pt metal in the form of ink, paste, and/or wires and plates. Pt is largely used due to its high melting temperature and good electrical conductivity.

The method, by which electrodes are attached to the ceramic sample, is largely dependent on the specimen geometry. In the case of dogbone samples, electrode wires are placed through holes made in the dogbone handle regions [14]. Wires twisted around each end are used for rectangular bars [68]. The electrodes used for pellets or rectangular bars are usually Pt sheets placed against the top and bottom surfaces [69] with wires attached to the power supply.

2.4.2. Stages and mechanisms of Flash sintering

Since the beginning of study of FS, the sample was subjected to a set electric field value during the heating [44]. FS occurred spontaneously at an onset temperature of the furnace. However, this approach has some problems: the sample temperature is not known and at high heating rates the temperature of the sample could differ from the one in the furnace.

With the technological and theoretical development, the parameters required for attaining dense samples became better established and, consequently, it was easier for later experiments to introduce an isothermal hold in temperature at a suitable level. So, experiments carried out in this way established that FS can also occur after an incubation time at a particular temperature [70]. In addition, it has been established using measurements of the optical spectrum of the emission that the glowing of the samples during FS is due to electroluminescence, rather than black body radiation.

ZnO based ceramics

28 Jha et al.[71] compiled the studies made of earlier isothermal experiments with FS of 3Y-TZP, dividing FS process into three distinct stages shown in Fig. 14:

• Stage I: Pre-heating of the furnace temperature (“incubation time”) previous to the occurrence of the Flash event. In this stage, the power supply is under voltage control (VC) and the sample heats by only Joule heating, provided by the furnace, and/or Joule heating plus additional conduction heating. This stage has a duration cycle that can last between 1 s to several hours until the sample reaches the temperature, at which it will start to conduct. The temperature recorded for Flash is the one of the furnace when Flash occurs.

• Stage II: The Flash process, occurring at isothermal furnace temperature. The power supply is switched from VC to current control (CC) and sintering occurs within 1 s to 5 s. Electroluminescence and some grain growth is observed.

• Stage III: Maintenance of the constant Flash state applied by power supply that is still under CC to the sample for less than 60 s. Finally, the sample is sintered followed by rapid grain growth. At this point the furnace can be turned off and the sample cooled during this stage.

Fig. 14- Three stages of FS [44].

Concluding, the length of each stage depends on the material and the process conditions (electric field and furnace temperature). It should also be noted that the

29 switching of the power supply from voltage control to current control is an essential step in FS stages, otherwise it will enter in an uncontrollable state of electrical runaway, which creates the possibility of melting or exploding the sample. Thus, by switching from VC to CC, the power becomes constant and the sample enters at a steady state condition as the voltage and current are both stable, allowing high densification of the specimens.

Up to now three main mechanisms of FS are suggested.

i) Local heating of grain boundaries

In the original report on FS, Cologna et al. [44] reviewed possible explanations for the rapid sintering and suggested local Joule heating at the particle-particle contacts; the high electrical resistance of grain boundaries relatively to the grains may accelerate grain boundary diffusion sufficiently to cause the rapid densification observed. Chaim[72] has extended this idea to include local melting of the grain boundaries, giving further possibilities for the acceleration of sintering by including liquid phase sintering mechanisms.

These ideas have not been completely accepted by the scientific community, on the basis that they do not consider the heat flow [73]. In submicron powders such as those used in FS, therefore, local heating at particle-particle contacts cannot be sustained over the timescale of several seconds in which FS takes place. This appears to rule out the simple idea of local heating and/or melting.

ii) Nucleation of avalanches of lattice defects

Being a recent area of study, another possible mechanism that has been proposed is the production of lattice defects, such as Frenkel defects. During FS experiments it was postulated that the Frenkel pairs occur in an avalanche and they are ionised into charge neutral defects and electron-hole pairs [73].

This is credible since it offers a single mechanism that can explain, almost completely, the observed aspects of FS: lattice defects would increase the diffusion rate, offering fast sintering and the electron-pairs would explain both the high electrical conductivity observed at the Flash event and electroluminescence. The idea of the needed nucleation is also consistent with the so-called “incubation time” for FS. This refers to the observation of a time delay after the application of an electric field greater than the critical value at a given furnace temperature before the Flash event takes place.

ZnO based ceramics

30

iii) Thermal Runaway caused by Joule heating

The thermal runaway always occurs during the Flash event and allows an abrupt heating of the ceramics due to internal heat generation. It was calculated that during the Flash event the heating rate could approach 104 °C/min [13]; nevertheless, this value depends on the maximum specific power dissipation set during the experiments, the material specific heat and other process parameters (sample shape, mass, etc).

The fact that thermal runaway is the trigger of the Flash event is accepted by the majority of the scientific community. Although the thermal runaway explains why and when the Flash event takes place, it is still not clear if it is the only reason leading to densification, photoemission and resistivity drop observed during FS.

2.4.3. Flash sintering of ZnO

In this subchapter the results of the works published so far on FS of ZnO are compiled in in Table 3 and analysed to understand what can be done to improve the knowledge attained on this subject.

First, comparing single crystals (5 mm x 5mm x 0.5mm) with powders of ZnO it was found that ZnO powders Flash sinter at substantially lower temperature, indicating the important role of surfaces and grain boundaries [63]. This temperature reduction was attributed to the enhanced conduction along the surfaces of ZnO particles. Another intriguing discovery was the enhanced grain coarsening in the anode side during FS and the observation of a discontinuous transitions between small and large grains. It was explained to be due to an electric-potential-induced accumulation of electrons and an associated oxidation reaction to form excess cation vacancies at ZnO grain boundaries that promote interfacial diffusion. The introduction of bismuth oxide (Bi2O3) increased

the onset flash temperature by forming a double Schottky barrier at the grain boundaries, however its introduction homogenised the microstructure by eliminating the anode-side abnormal grain growth via a liquid-phase sintering effect.

Then Y. Zhang et al. [74] found a strong dependence of the onset flash temperature of ZnO on sintering atmosphere demonstrating that it can be significantly decreased by using reducing atmosphere. An explanation behind this behaviour is the increase in the conductivity of the material in reduced conditions. The samples sintered did not present

31 differences in the grain size between the anode and the cathode, revealing the grain size of 1 ± 0.3 µm.

Moreover, J. Nie et al. [75] found that water can trigger FS of ZnO at room temperature to achieve ~98% of densification, because of the electrical conductivity enhanced >1000 times via absorption of water vapour. No microstructural asymmetries were reported between the anode and cathode, while the samples were possible to sinter only under 200 V/cm. Furthermore, it was stated that flash in ZnO starts as a coupled electric and thermal runaway, being a consequence of an exponential increase of the specimen conductivity with increasing temperature, which leads to rapid densification during FS [76]. It is also found possible that the fast heating rate (dT/dt ~ 200 °C/s) in conjunction with additional external stimuli (electric fields/currents for flash sintering or intense infrared radiation for rapid thermal annealing) can help generating non-equilibrium defects thus enhancing the sintering and other kinetic processes.

ZnO based ceramics

32 Table 3- State of the art of FS of ZnO.

Purity / dopant Mean particle size Power source Heating rate (ºC/min) FS conditions Atmosphere/ Furnace temperature (ºC) Dwell (s) Relative density (%) Ref. Green Sintered >99.99 100 nm --- --- 1. E = 300 V/cm, I = 1 A 2. E = 500 V/cm, I = 4 A 3. E = 1000 V/cm, I = 4 A 1. • air / 565 • Ar + 5 mol.% H2 / 186 • Air / 599 • O2 / 631 2. • Ar + 5 mol.% H2 / 108 3. • Ar + 5 mol.% H2 / 116 30 62 1. • 97.5 • 64 • 69 • 94.8 2. • 97.4 3. • 97.7 [74] >99.99 120 nm DC 20 1. E = 100 V/cm, I = 2.4 A 2. E = 150 V/cm, I = 2.4 A 3. E = 200 V/cm, I = 2.4 A

Ar + 5 mol. % H2 with water

vapour / room temperature

1. 5 2. 10 3. 30 55 1. 55 2. 55 3. ~ 98 4. [75]

33 Purity / dopant Mean Particle size Power source Heating rate (ºC/min) FS conditions Atmosphere/ Furnace temperature (ºC) Dwell (s) Relative density (%) Ref. Green Atmosphere/Sintered >99.99 120 ---- 5 E= 300 V/cm 1. I = 0.5 A 2. I = 0.75 A 3. I = 1 A 4. I = 0.5 A 5. I = 0.5 A 6. I = 0.75 A 7. I = 0.75 A air / 564 1. 30 2. 30 3. 30 4. 5 5. 20 6. 5 7. 20 58.3 – 61.2 1. 91.2 2. 94.4 3. 96.6 4. 63.4 5. 76.6 6. 93.5 7. 86.6 [76] > 99.99 / 99.8 Bi2O3 (0.5 % mol) 90-210 nm DC 5 E= 300 V/cm ZnO single crystal

1. I = 1 A 2. I = 1.45 A Pure ZnO 3. I = 1 A 4. I = 4 A ZnO + 0.5 mol % Bi2O3 5. I = 1 A 6. I = 4 A air

ZnO single crystal 1. 870 2. 877 Pure ZnO 3. 565 4. 553 ZnO + 0.5 mol % Bi2O3 5. 621 6. 620 30 63.5 -6 5 3. 90.8 4. 87.3 5. 91.5 6. 88.2 [63]

CHAPTER 3