Toepassing van een sterk magnetisch veld voor getextureerde technische keramieken

Texturing of materials is very important since many material properties are orientation specific. It has been reported that the value of a material property (such as electrical or mechanical properties) in one direction improves a lot by texturing. Many materials have already been textured by a variety of methods. The employed methods to achieve texturing in the current thesis were templated grain growth (TGG) and magnetic alignment, by colloidal processing in a strong magnetic field. The aim of this thesis is to investigate the alignment of materials ‘difficult to texture’ and weak magnetic materials like zirconia or BaTiO3. Thus, the role of a high magnetic field of 17.4 T that has not been used before for magnetic alignment of ceramics was assessed. The final goal is to enhance the electrical or mechanical properties of the textured ceramics. The colloidal processes used were electrophoretic deposition (EPD) and slip casting and the materials investigated were BaTiO3, α-Al2O3, 3Y-TZP and 12Ce-TZP. Highly textured BaTiO3 ceramics were developed by TGG during EPD. Two types of platelets were prepared in house by a 2-step and 3-step molten salt method, and two platelet concentrations were used. The 3-step molten salt platelets had a higher bismuth impurity content which was harmful for the electrical properties of the final ceramics. The alignment, grain growth and electrical properties of the textured BaTiO3 were influenced by the template type and concentration. The dielectric and piezoelectric properties of the textured ceramics were found to be enhanced compared to the randomly oriented BaTiO3. The piezoelectric constant of the textured BaTiO3 prepared with 1 wt% 2-step molten salt platelets was 47% higher than the non-textured BaTiO3. Another approach to obtain highly textured BaTiO3 that was explored was magnetic alignment. The degree of texture so far achieved in published work has been rather weak. Additionally, there is clear confusion in the literature regarding the crystallographic direction that will align preferentially with the magnetic field. In the current work, a strong magnetic field of 17.4 T was used and BaTiO3 was successfully aligned by slip casting and EPD, proving that such a high magnetic field intensity is essential to achieve highly textured BaTiO3 ceramics. The role of sintering temperature on the degree of texture was assessed. The influence of solids loading on the magnetic alignment was investigated, choosing α-Al2O3 as a model material. Textured α-Al2O3 ceramics were obtained by slip casting in a strong magnetic field of 17.4 T. Suspensions of 10-40 vol% solids loading were investigated revealing that the viscosity of the suspension significantly influenced the degree of texture and the microstructure development of the sintered and textured ceramics. The role of a high magnetic field of 17.4 T on the alignment of 3Y-TZP zirconia by EPD was investigated, in order to enhance the degree of texture achieved in previous earlier in a lower field. It is the first time that texture is detected in green zirconia ceramics. It was proven that this high magnetic field results in a Lotgering factor of 0.4 for m-ZrO2 and 0.6 for 3Y-TZP, versus 0.05 and 0.38, respectively, for a magnetic field of 9.4 T (results of earlier work). As a consequence, the textured 3Y-TZP exhibited an anisotropic toughness with a high value of 7.4 MPa m0.5 along the [001] direction. In order to upscale the results on textured zirconia and be able to measure the strength of the textured 3Y-TZP ceramics, it was necessary to produce thicker ceramics. Therefore, highly textured 6 mm thick 3Y-TZP ceramics were produced by slip casting in a strong magnetic field of 17.4 T, resulting in the highest Lotgering factor (f=0.8) reported for zirconia ceramics. The slip cast (001)-oriented 3Y-TZP exhibited a substantial toughness anisotropy, which was 163% higher along the [001] direction than normal to it and 125% higher than the toughness of the randomly oriented material. Additionally, the flexural strength of the textured zirconia was 26% higher than for the non-textured ceramic. Finally, ceria stabilized zirconia ceramics were successfully aligned in a strong magnetic field of 17.4 T via slip casting, with its (001) plane aligned perpendicular to the magnetic field direction. The sintering temperature was found to influence the density, the grain size, the degree of texture and the mechanical properties of the 12Ce-TZP ceramics. The higher hardness and fracture toughness values were observed for the ceramics sintered at 1450oC. All the ceramics, irrespective whether they were cast inside or outside the magnetic field, showed anisotropic toughness with almost no crack propagation along the casting direction.Table of Contents 1 Chapter 1 1 Introduction and objectives 1 1.1 General introduction 1 1.2 Goals of this research work 2 1.3 Material Selection 3 1.4 Outline of the thesis 3 1.5 References 6 2 Chapter 2 9 Literature review 9 2.1 The advantage of textured materials 10 2.2 Colloidal processing 13 2.2.1 Stability of colloids 13 2.2.2 Electrophoretic deposition (EPD) 19 2.2.3 Slip casting 21 2.3 Methods to achieve texture 22 2.3.1 Texturing by templated grain growth (TGG) 22 2.3.2 Texturing by magnetic alignment 25 2.4 Methods to characterize texture 31 2.4.1 The Lotgering factor 32 2.4.2 The pole figure 33 2.4.3 The inverse pole figure 35 2.4.4 Electron Backscattered Diffraction (EBSD) 36 2.5 Conclusions 37 2.6 References 38 3 Chapter 3 45 Textured BaTiO3 by templated grain growth and electrophoretic deposition 45 3.1 Introduction 45 3.2 Experimental Procedure 46 3.2.1 Synthesis of BaTiO3 platelets 46 3.2.2 Preparation of textured BaTiO3 ceramics 48 3.2.3 Determination of the degree of orientation 49 3.2.4 Electrical measurements 50 3.3 Results and Discussion 51 3.4 Conclusions 65 3.5 References 66 4 Chapter 4 69 Effect of powder loading on texturing alumina in a strong magnetic field 69 4.1 Introduction 69 4.2 Experimental procedure 69 4.3 Results and discussion 70 4.4 Conclusions 79 4.5 References 80 5 Chapter 5 81 Hydrothermally synthesized BaTiO3 textured in a strong magnetic field 81 5.1 Introduction 81 5.2 Experimental procedure 82 5.3 Results 85 5.4 Conclusions 89 5.5 References 90 6 Chapter 6 91 Texturing of 3Y-TZP zirconia by electrophoretic deposition in a high magnetic field of 17.4 T 91 6.1 Introduction 91 6.2 Experimental procedure 91 6.3 Results and discussion 94 6.1 Conclusion 101 6.4 References 102 7 Chapter 7 105 Texturing of 3Y-TZP zirconia by slip casting in a high magnetic field of 17.4 T 105 7.1 Introduction 105 7.2 Experimental procedure 105 7.3 Results and discussion 108 7.4 Conclusions 119 7.5 References 120 8 Chapter 8 123 Texturing of 12Ce-TZP zirconia by slip casting in a high magnetic field of 17.4 T 123 8.1 Introduction 123 8.2 Experimental procedure 124 8.3 Results and discussion 126 Conclusions 137 8.4 References 138 9 Chapter 9 141 General conclusions and future perspectives 141 9.1 Main achievements and conclusions 141 9.2 Suggestions for future work 144 10 Curriculum vitae 147nrpages: 166status: publishe