%40تخفیف

Development of co-extrusion process to fabricate multi-layer ceramic micro-tubes to be used as solid oxide fuel cell

تعداد203 صفحه در فایل word

Ph.D.

MATERIALS SCIENCE AND ENGINEERING

Development of co-extrusion process to fabricate multi-layer ceramic micro-tubes to be used as solid oxide fuel cell

A process for manufacturing of ceramic micro-tubes was developed with the aim of reducing the cost and improving the properties of micro-tubular solid oxide fuel cells. This was achieved by using co-extrusion to create a five layer ceramic tubular graded structure from yttria stabilised zirconia to an 80% NiO cermet. These layers were extruded around a sacrificial carbon core. Carbon was added to the cermet compositions to give porosity. The arrangement and the tool design alleviated many of the previously reported defects found in co-extrudates. The final product possessed an inner diameter of 2.5 mm and a wall thickness of 0.5 mm, with the possibility of reducing these dimensions even further.

The relationship between the extrusion pressure and solids loading was used to measure the rheological properties of the individual pastes. The rheological properties of intermediate pastes later used for paste formulation were predicted using a linear relationship developed in this thesis.

Mechanical testing is performed on the sample, with emphasize on bending test to determine modulus of rupture (MOR). The bending test performed at room temperature and weibull analysis is conducted on the samples.

Keywords: co-extrusion, paste, mechanical testing, solid oxide fuel cell.

دسته: زبان خارجه, زبان خارجه برچسب: Keywords: co-extrusion, mechanical testing, paste, solid oxide fuel cell.

توضیحات

Table of Contents

Content Page

Introduction. ………………………………………………………………………2

1.1. Aim.. 4

Literature review.. …………………………………………………………………6

2.1. Paste and paste processing. 6

2.1.1. Paste formulation. 6

2.1.2. Elastic, plastic and viscous behavior. 7

2.1.3. Flow constitutive equations. 11

2.1.4. Flow behavior. 13

2.1.5. Paste extrusion flow.. 16

2.2. Solid oxide fuel cells. 20

2.2.1. SOFCs design. 23

2.2.2. Fabrication methods. 27

2.3. Co-extrusion and co-extruders. 36

2.4. Extrusion defects. 41

2.5. Mechanical characterization. 44

2.6. Summery and aim.. 45

Experimental procedure. …………………………………………………………48

3.1. Starting materials. 48

3.2. Ceramic extrusion feasibility investigation. 49

3.3. Co-extrusion. 53

3.3.1. Rheological characterization. 53

3.3.2. Paste preparation. 54

3.3.3. Paste rheological characterization. 59

3.3.4. Sintering and micro-structural characterization. 61

3.3.5. Co-extrusion. 65

3.4. Micro-tubes characterization. 66

3.4.1. Structural characterization. 66

3.4.2. Bending test and strength characterization. 66

3.4.3. Thermal shock resistance. 69

3.4.4. Indentation test 71

Micro-tube manufacture. …………………………………………………………74

4.1. Co-extrusion. 75

4.1.1. Flow field design considerations and features. 76

4.1.2. Co-extruder design. 82

4.2. Pressure modelling. 92

4.2.1. Single layer tube extrusion die modelling. 92

4.2.2. Complex multi-layer micro-tube co-extrusion die pressure modelling. 95

Results and discussion. …………………………………………………………106

5.1. Paste rheological characterization and unification for co-extrusion. 106

5.1.1. Rheological behaviour. 106

5.1.2. Paste flow characterization. 114

5.1.3. Slip flow characterisation. 116

5.1.4. Sintering behaviour. 125

5.1.5. Prediction of intermediate paste. 131

5.2. Fabricated micro-tubes. 135

5.3. Pressure modelling results and discussion. 140

5.3.1. Complex die tube extrusion. 140

5.3.2. Co-extrusion pressure modelling. 144

5.4. Mechanical testing results and discussion. 150

5.4.1. Indentation tests. 150

5.4.2. 3-point bending test 154

Conclusions and future works. …………………………………………………167

6.1. Raw materials. 167

6.2. Extrusion rheometry. 168

6.3. Complex die extrusion (single layer tube extrusion). 168

6.4. Co-extruder. 169

6.5. Co-extrusion pressure modelling. 170

6.6. Co-extrudates. 171

6.7. Properties of sintered materials. 171

6.8. Future works. 172

References…………………………………………………………………174

Abstract and Title Page in Persian

List of Figures

Content Page

Figure ‎2‑1 Simple shear of an elastic body [5]. 7

Figure ‎2‑2 Rheological diagram of an elastic body [5]. 8

Figure ‎2‑3 Rheological diagram of a plastic body [5]. 9

Figure ‎2‑4 Rheological diagram of a viscous material [5]. 10

Figure ‎2‑5 Simple shear of a viscous body [5]. 11

Figure ‎2‑6 Time dependent rheological behavior at constant shear rate [5]. 14

Figure ‎2‑7 The main flow behavior types on logarithmic and linear plots [5]. 15

Figure ‎2‑8 Ram extrusion of a paste. 17

Figure ‎2‑9 General schematic diagram of a fuel cell [38]. 21

Figure ‎2‑10 Schematic diagram of the cathode triple-phase boundary [46]. 23

Figure ‎2‑11 Schematic diagram of the SOFC single cell configurations [49]. 24

Figure ‎2‑12 Schematic diagram of the planar SOFC design [50]. 25

Figure ‎2‑13 Schematic diagram of the tubular SOFC design [50]. 26

Figure ‎2‑14 Volumetric power density and pressure loss of tubular SOFCs [52]. 27

Figure ‎2‑15 Light optical photos of the YSZ electrolyte surfaces [58]. 30

Figure ‎2‑16 Photographs of C-shaped plastic tube holders [61]. 31

Figure ‎2‑17 Plastic mass ram extrusion process [59]. 32

Figure ‎2‑18 Cross-sectional SEM images of the 0.8 mm tube after sintering [59]. 33

Figure ‎2‑19 Recent development of single MT-SOFCs using extrusion [60]. 35

Figure ‎2‑20 multi billet extrusion methods [25,68,69]. 37

Figure ‎2‑21 Lay-up extrusion methods [70,72]. 38

Figure ‎2‑22 Initial feed rod made from pressed rods of extrudate [82]. 41

Figure ‎2‑23 Second feed rod made from pressed rods of co-extrudate [82]. 41

Figure ‎3‑1 Parts of the extrusion die. 50

Figure ‎3‑2 The schematic of extrusion die and extrusion process. 50

Figure ‎3‑3 Extruded and sintered YSZ rods. 52

Figure ‎3‑4 The SEM and particle size distribution of YSZ powder. 54

Figure ‎3‑5 Jar mill machine used for mixing paste constituents. 55

Figure ‎3‑6 YSZ grinding media with radius of 5mm used in this study. 55

Figure ‎3‑7 Shear mixer used in paste preparation. 56

Figure ‎3‑8 Flow chart of electrolyte paste preparation. 56

Figure ‎3‑9 Wet attrition container used in this study. 57

Figure ‎3‑10 Owen used to dry the powders after wet attrition. 58

Figure ‎3‑11 Flow chart for anode paste preparation steps. 58

Figure ‎3‑12 Dies for rheological characterization of pastes. 59

Figure ‎3‑13 Typical extrusion pressure versus L/D ratios. 60

Figure ‎3‑14 Furnaces used for sintering process of the extruded tubes. 62

Figure ‎3‑15 Typical sintering cycle used in this study. 63

Figure ‎3‑16 Thermal gravimetery and DTA analysis of pore formers. 64

Figure ‎3‑17 Assembled co-extrusion die seated on the press machine. 65

Figure ‎3‑18 Disassembled co-extrusion die designed in this project. 66

Figure ‎3‑19 Bending test die. 67

Figure ‎3‑20 The effect of m on the shape of the Weibull distribution [109]. 68

Figure ‎3‑21 Painting brushes used to introduce YSZ layer by painting method. 71

Figure ‎3‑22 SEM image of a indentation with the characteristic dimensions. 72

Figure ‎4‑1 The conventional ceramic extrusion die. 76

Figure ‎4‑2 Compression of paste during extrusion the formation of dead zones. 77

Figure ‎4‑3 Flow of paste around a mandrel and entering a tubular die. 78

Figure ‎4‑4 Preferential flow of the extruded paste due to design of flow fields.. 79

Figure ‎4‑5 Formation a flow lamination indicated by the white line. 79

Figure ‎4‑6 Reduced deformation of structure using sacrificial tube core. 81

Figure ‎4‑7 Sectioned view of the co-extruder designed for this study. 84

Figure ‎4‑8 barrel unit of the designed die. 85

Figure ‎4‑9 Flow fields of the six co-extruded pastes. 86

Figure ‎4‑10 The annulus formed between two tube die. 86

Figure ‎4‑11 Flow of the paste from barrels to the center of the cone. 87

Figure ‎4‑12 The feeding cone and the inserted paste spreader part. 88

Figure ‎4‑13 Flow of the Play Doh paste in the feeding cone. 88

Figure ‎4‑14 Nozzle which combines the separate paste flow streams. 89

Figure ‎4‑15 Conical end die forms final co-extrudate. 90

Figure ‎4‑16 Assembled co-extrusion die. 91

Figure ‎4‑17 Disassembled co-extrusion die. 91

Figure ‎4‑18 Schematic of conical, multi-holed and tubular dies. 93

Figure ‎4‑19 Schematic of the numerical model of a cone. 93

Figure ‎4‑20 Simplified model of the co-extruder defined different sections. 96

Figure ‎4‑21 Schematic diagram of paste flow in a conical annulus. 98

Figure ‎4‑22 Schematic of the numerical model for flow along a conical annulus. 102

Figure ‎5‑1 Typical extrusion load versus displacement curves [113]. 107

Figure ‎5‑2 Typical extrusion pressure versus L/D ratios [113]. 108

Figure ‎5‑3 The reciprocal convergent flow yield stress [113]. 109

Figure ‎5‑4 The reciprocal convergent velocity factor [113]. 109

Figure ‎5‑5 The reciprocal wall shear yield stress [113]. 110

Figure ‎5‑6 Surface condition on extruded anode tube [113]. 111

Figure ‎5‑7 Pore structure of anode tubes [113]. 112

Figure ‎5‑8 The reciprocal wall shear velocity factor [113]. 114

Figure ‎5‑9 YSZ Mooney plot. 121

Figure ‎5‑10 YSZ paste plug and slip velocities for different die diameters. 121

Figure ‎5‑11 YSZ paste extrusion wall shear stress versus bulk velocity. 124

Figure ‎5‑12 Microstructure of electrolyte tube sintered for 2 hours. 128

Figure ‎5‑13 Modulus of rupture (MOR) of electrolyte vs. temperatures. 128

Figure ‎5‑14 Microstructure of electrolyte tube sintered at 1450°C. 130

Figure ‎5‑15 Modulus of rupture (MOR) of electrolyte tube vs. time. 130

Figure ‎5‑16 Microstructure of anode tubes sintered at 1450°C for 2 hours. 131

Figure ‎5‑17 Relationship between liquid content and NiO content. 132

Figure ‎5‑18 Cross section of the co-extruded tube. 136

Figure ‎5‑19 Back scatter image of sintered micro-tube. 136

Figure ‎5‑20 Back scattered image of longitudinal cross section of a micro-tube. 137

Figure ‎5‑21 SEM and EDX images on the layers. Red -YSZ, green-Ni.. 138

Figure ‎5‑22 Fuel cell paste experimental and model co-extrusion pressure data. 147

Figure ‎5‑23 Correlation between the mean error and extrusion geometries. 150

Figure ‎5‑24 Vickers indentation impression versus load in pure YSZ. 151

Figure ‎5‑25 Vickers indentation impression versus load in YSZ-NiO. 152

Figure ‎5‑26 Schematic cross sections of the co-extruded sample. 153

Figure ‎5‑27 Vickers hardness versus NiO content. 154

Figure ‎5‑28 Weibull plots of anode tubes with different processing routs. 157

Figure ‎5‑29 Weibull plots of half-cell tubes with different processing routs. 159

Figure ‎5‑30 Mechanical strength versus temperature gradient. 161

Figure ‎5‑31 Cross section of tubes after exposure to ultimate thermal shock. 162

Figure ‎5‑32 Interface between anode and electrolyte layers. 164

List of Tables

Content Page

Table ‎3‑1 Water based binder system used in this study. 51

Table ‎3‑2 Cyclohexanone based binder system used in this study. 52

Table ‎3‑3 Materials and their weight in preparation of YSZ slurry. 70

Table ‎4‑1 Different co-extrusion pastes. 75

Table ‎4‑2 Co-extrusion model pressure equations. 97

Table ‎5‑1 Modulus of rupture of different anode compositions [113]. 113

Table ‎5‑2 YSZ die entry Benbow-Bridgwater rheological parameters. 119

Table ‎5‑3 YSZ die land Benbow-Bridgwater rheological parameters. 119

Table ‎5‑4 Benbow-Bridgwater model predictions for apparent shear rate. 120

Table ‎5‑5 Apparent and bulk shear rates for YSZ paste. 123

Table ‎5‑6 Corrected Benbow-bridgwater parameters. 124

Table ‎5‑7 Liquid content and rheological properties of unified pastes. 125

Table ‎5‑8 Relative density of electrolyte tubes sintered at different conditions. 127

Table ‎5‑9 Liquid content and rheological properties of pastes. 133

Table ‎5‑10 Different co-extrusion paste. 133

Table ‎5‑11 Rheometry extrusion pressures for co-extrusion pastes, D = 4 mm. 134

Table ‎5‑12 Layer thickness of co-extruded micro-tube. 139

Table ‎5‑13 Rheological properties of Play Doh paste. 140

Table ‎5‑14 Rheological properties of alumina paste. 141

Table ‎5‑15 Rheological properties of YSZ paste. 141

Table ‎5‑16 Complex die extrusion pressures (MPa). 142

Table ‎5‑17 Effect of step number on the numerical solution of cone die. 143

Table ‎5‑18 Experimental and model co-extrusion pressures. 145

Table ‎5‑19 Rheological properties of co-extrusion pastes. 146

Table ‎5‑20 Power law parameters fitted to co-extrusion pressure data. 147

Table ‎5‑21 Experimental and model pressures through the separate flow fields. 149

Table ‎5‑22 hardness value and fracture toughness of different samples. 153

Table ‎5‑23 Average strength of the anode tubes. 155

Table ‎5‑24 Weibull properties of anode tubes. 156

Table ‎5‑25 Average strength of the half-cell tubes. 158

Table ‎5‑26 Weibull properties of different half-cell tubes. 160

Table ‎5‑27 Critical temperature difference of anode tubes. 165

Table ‎5‑28 Critical temperature difference of half-cell tubes. 165

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