High-pressure synthesis, structure and properties of cubic zirconium(IV)- and hafnium(IV) nitrides.
[Ph.D. Thesis], (2009)
Dzivenko Dissertation -
Available under Creative Commons Attribution Non-commercial No Derivatives, 2.5.
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|Item Type:||Ph.D. Thesis|
|Title:||High-pressure synthesis, structure and properties of cubic zirconium(IV)- and hafnium(IV) nitrides|
This thesis is concerned with recently discovered high-pressure (HP) zirconium- and hafnium nitrides having cubic Th3P4-type structure, c-M3N4 (M=Zr or Hf). These compounds belong to a rapidly growing group of new hard HP nitrides exhibiting advanced properties. The research was focused on (i) synthesis of macroscopic amounts of c-M3N4, (ii) investigation of their solid state structure, composition and morphology and (iii) measurement of properties related to potential industrial applications of these compounds as hard wear-resistant materials. Nitrogen-rich starting materials for high-pressure high-temperature (HP/HT) synthesis of c-M3N4, namely nanocrystalline powders of M3N4+x with distorted NaCl-type structure, had to be prepared in this work because they were not commercially available. They were obtained via HT ammonolysis of the corresponding metal dialkylamides, M[N(C2H5)2]4, at moderate temperatures up to 873 K. Both, c-Zr3N4 and c-Hf3N4, were synthesized from the nanocrystalline M3N4+x powders applying a pressure of 12 GPa and a temperature of 1873 K using a multi-anvil HP-apparatus. The products were characterized using various techniques including powder XRD (Rietveld refinement), TEM, EPMA, SEM/EDX and Raman spectroscopy. In the case of zirconium nitride, formation of a single phase crystalline material was verified by both XRD and TEM. The presence of a small amount of oxygen in the sample, revealed by EPMA, suggested the formation of oxygen-bearing zirconium(IV) nitride (or oxynitride) having Th3P4-type structure, c-Zr2.86(N0.88O0.12)4. The measured composition was found to correspond to the general formula Zr3-u(N1-uOu)4 which fulfils the electrical neutrality condition. A high quality of the Rietveld structure refinement of the powder XRD data for c-Zr2.86(N0.88O0.12)4 supported the above findings. Further, the assignment of the structure of the obtained oxygen-bearing zirconium nitride to the Th3P4-type was confirmed by Raman spectroscopic measurements. In contrast to zirconium nitride, a minor oxidation of the hafnium nitride sample led to formation of a mixture of oxygen-poor c-Hf3N4 and an oxidic material. The latter was evident from the XRD, EPMA, and SEM/EDX measurements. A detailed analysis of the powder XRD patterns and Rietveld refinement suggested that the oxidic material is comprised of a mixture of the known γ-Hf2N2O and oI-HfO2. The last part of the thesis concerned the investigation of the properties of the synthesized materials. The bulk moduli of c-Zr2.86(N0.88O0.12)4 and c-Hf3N4 were determined via their quasi-hydrostatic compression in a diamond anvil cell up to 45 GPa. The obtained values are B0=219 GPa (B'0=4.4) and B0=227 GPa (B'0=5.3) for c-Zr2.86(N0.88O0.12)4 and c-Hf3N4, respectively. The reduced elastic modulus, Er=224 GPa, of porous c-Zr2.86(N0.88O0.12)4 (volume fraction porosity of 0.3) was measured using nanoindentation techniques. Combining B0 and Er values, the lower limits of the shear modulus G0=96 GPa, and of the Young’s modulus E0=252 GPa were determined for the oxygen-bearing c-Zr3N4. The nanoindentation hardness and Vickers microhardness, HV(1), of the porous zirconium nitride sample were measured to be 18 GPa and 12 GPa, respectively. Using a relation between hardness and volume fraction porosity, suggested in the literature, the HV(1) of the fully dense c-Zr3N4 was estimated to exceed 25 GPa. The indentation fracture toughness of 3.2 MPa m^1/2 for porous c-Zr2.86(N0.88O0.12)4 was evaluated from the Vickers indentation cracks. The linear thermal expansion coefficient of the oxygen-bearing c-Zr3N4 was found to increase from 6.6×10^6 1/K at room temperature to about 14×10^6 1/K at 873 K. Onset of the material oxidation in air was observed at 773 K. Finally, the obtained results were compared with existing experimental and theoretical data for c-M3N4 and for other related technological materials and discussed with respect to potential industrial applications of c-M3N4.
|Uncontrolled Keywords:||high pressure, zirconium nitride, hafnium nitride, Th3P4-type structure, elastic moduli, hardness, multi-anvil, diamond anvil cell|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 500 Naturwissenschaften
600 Technik, Medizin, angewandte Wissenschaften > 620 Ingenieurwissenschaften
|Divisions:||Fachbereich Material- und Geowissenschaften
Fachbereich Material- und Geowissenschaften > Materialwissenschaften
|Date Deposited:||09 Sep 2009 11:26|
|Last Modified:||16 Sep 2015 07:57|
|Referees:||Riedel, Prof. Dr. Ralf and Fueß, Prof. Dr.- Hartmut|
|Refereed:||17 December 2008|