List of frequently used symbols and abbreviations ............... I
Introduction .................................................... 1
1 Boron Nitride .............................................. 3
1.1 Overview ................................................... 3
1.2 Crystalline phases of Boron Nitride ........................ 4
1.2.1 Low pressure phases: sp2 hybridisation .............. 4
1.2.2 High pressure phases: sp3 hybridisation ............. 6
1.2.3 Are there other BN 'phases'? ........................ 7
1.2.4 General remarks on BN modifications ................. 8
1.3 Microstructure defects in the BN phases .................... 9
1.3.1 Puckering ........................................... 9
1.3.2 Buckling ........................................... 10
1.3.3 Turbostratic Disorder .............................. 10
1.3.4 Dislocations ....................................... 11
1.3.5 Stacking faults & partial dislocations ............. 12
1.3.6 Twins .............................................. 12
1.4 Phase transition paths between BN phases .................. 13
1.5 Phase diagram and kinetics of transitions in BN ........... 17
1.5.1 The (p - T) diagram(s) of BN ....................... 17
1.5.2 Models of phase transition kinetics applicable at
HP/HT .............................................. 19
1.5.3 Impact of microstructure defects on the
transition kinetics ................................ 20
1.6 Concept of superhard nanocomposites ....................... 22
2 X-ray diffraction ......................................... 23
2.1 Revisiting 'whole powder pattern fitting' methods ......... 23
2.1.1 Overview and progress .............................. 23
2.1.2 Important equations, quantification and sources
of errors .......................................... 24
2.1.3 Inclusion of real structure models in the
Rietveld method .................................... 25
2.2 X-ray line profile analysis and β(q) plots ................ 30
2.2.1 Lattice parameter .................................. 31
2.2.2 Microstructure information enclosed in integral
line widths ........................................ 31
2.3 Pattern modelling using DIFFaX ............................ 33
2.4 Energy dispersive HP/HT X-ray diffraction ................. 34
3 Principles of the HP/HT synthesis ......................... 37
3.1 Quasi-static generation of high pressure .................. 37
3.2 Pressure cells for multi-anvil devices (MAP) .............. 41
3.2.1 General laboratory set-up .......................... 41
3.2.2 Adaption to in situ experiments .................... 43
3.3 Pressure and temperature determination ............... 43
4 Details of the experiments ................................ 45
4.1 Starting materials for the HP/HT synthesis ................ 45
4.2 Laboratory HP/HT experiments .............................. 46
4.3 Investigations using laboratory XRD and ТЕМ ............... 46
4.3.1 X-ray diffractometer ............................... 46
4.3.2 Transmission electron microscope ................... 48
4.4 Peculiarities of HP/HT synchrotron experiments ............ 49
4.4.1 Preliminary considerations on pressure cell setup .. 49
4.4.2 Improvements of the high pressure cell setup ....... 50
4.4.3 Characterisation of the experimental
peculiarities ...................................... 52
4.4.4 Primary data reduction ............................. 55
5 Characterisation of the precursors ........................ 59
5.1 'Whole powder pattern fitting' ............................ 59
5.2 Analysis of diffraction line widths ....................... 63
5.2.1 Parametrisation of puckering defects ............... 65
5.2.2 Method of line width analysis ...................... 66
5.2.3 Refinement of β(q) plots of h-BN ................... 70
5.3 Confirmation of the results from β(q) plots ............... 73
5.3.1 Parameters from β(q) plots in context to other
methods ............................................ 73
5.3.2 Verification of β(q) parameters .................... 73
5.4 Concluding remarks ........................................ 80
5.4.1 Quantifying microstructure defects in β-BN from
β(q) plots ......................................... 80
5.4.2 Microstructure of h-BN precursors .................. 81
6 HP/HT synthesis of BN composites .......................... 83
6.1 Covered pressure and temperature region ................... 83
6.2 Conversion of HD BN into ω-BN and c-BN .................... 84
6.2.1 Synthesis product: ω-BN ............................ 84
6.2.2 Synthesis product: c-BN ........................... 102
6.2.3 Concluding remarks: HDBN precursor ................ 104
6.3 Conversion of defect-rich h-BN into c-BN ................. 105
6.3.1 Conversion of pyrolytic h-BN to c-BN at 12 GPa .... 105
6.3.2 Conversion of CVD-h-BN to c-BN .................... 128
6.3.3 Concluding remarks: pBN & CVD-BN as c-BN
precursor ......................................... 137
6.4 Microstructure of c-BN ................................... 138
7 Piecing the puzzle together .............................. 141
7.1 Puckering and the role of compression .................... 141
7.2 Nucleation scenarios for c-BN and го-BN .................. 144
7.2.1 Nucleation of c-BN from pBN ....................... 144
7.2.2 Nucleation of c-BN from CVD-BN .................... 152
7.2.3 Nucleation of w-BN from HDBN ...................... 153
7.2.4 Microstructure defects determine phase
transition paths .................................. 155
7.3 Growth of c-BN into a defect-rich h-BN matrix ............ 157
7.3.1 Growth of c-BN crystals and their microstructure .. 157
7.3.2 Confining residual 'compressed' h-BN .............. 159
8 Conclusions .............................................. 161
Appendices .................................................... 163
A Calculation of elastic modulus bodies ...................... 163
В Equation of State (EoS) .................................... 164
С High pressure reference materials .......................... 167
C.l Materials for EoS and in situ experiments ............. 167
С.2 Bi as electrical ex situ pressure reference ........... 169
D Overview on formulas for β1 ................................ 170
E Indexation of FFT patterns ................................. 172
F Dislocation contrast factors ............................... 173
G XRD patterns of in-situ experiments ........................ 174
Bibliography .................................................. 177
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