B. Palosz

Analysis of Short and Long Range Atomic Order in Nanocrystalline Diamonds with Application of Powder Diffractometry

Abstract Fundamental limitations, with respect to nanocrystalline materials, of the traditional elaboration of powder diffraction data like the Rietveld method are discussed. A tentative method of the analysis of powder diffraction patterns of nanocrystals based on the examination of the variation of lattice parameters calculated from individual Bragg lines (named the “apparent lattice parameter”, alp) is introduced. We examine the application of our methodology using theoretical diffraction patterns computed for models of nanocrystals with a perfect crystal lattice and for grains with a two-phase, core-shell structure. We use the method for the analysis of X-ray and neutron experimental diffraction data of nanocrystalline diamond powders of 4, 6 and 12 nm in diameter. The effects of an internal pressure and strain at the grain surface are discussed. The results are based on the dependence of the alp values on the diffraction vector Q and on the PDF analysis. It is shown, that the experimental results lend a strong support to the concept of a two-phase structure of nanocrystalline diamond.

High-pressure high-temperature in situ diffraction studies of nanocrystalline ceramic materials at HASYLAB

Abstract High-pressure in situ diffraction studies were performed up to 8 GPa in a cubic anvil cell MAX80 (Station F2.1) and up to 45 GPa in a Diamond Anvil Cell (DAC-Station F3 at HASYLAB, Hamburg). A series of nanocrystals of SiC with grain sizes ranging from 2 nm to several μm were examined in non-hydrostatic conditions by pressing pure powders. A new method of evaluation of powder diffraction data measured at high pressures is presented. This method is based on quantitative evaluation of asymmetry of Bragg reflections where each peak is described as a combination of two reflections of two similar crystallographic phases having different compressibilities. The measured changes of the lattice parameters calculated for split Bragg reflections were used for determination of the pressure gradient which occurs across the grain boundaries in the compressed materials. A model of the strain induced in compacts of pure powders under high pressures is proposed. The model accounts for the presence of two phases: a volume phase corresponds to cores of individual grains which are surrounded by a surface phase which is formed of free surfaces in loose powders and of grain boundaries in solids. Due to extreme hardening of the boundaries under non-hydrostatic pressure conditions, the effective pressure in the interior of the grains is much lower than the applied external pressure. It is suggested that additional ‘hardening’ of the grain boundaries results from the presence of dislocations which are generated at the surface of the grains. The actual gradient of the pressure depends on the size of the grains, and also on the method of synthesis of the materials.

High Pressure X-Ray Diffraction Studies on Nanocrystalline Materials

Application of the in situ high pressure powder diffraction technique for examination of specific structural properties of nanocrystals based on the experimental data of SiC nanocrystalline powders of 2–30 nm in diameter is presented. Limitations and capabilities of the experimental techniques themselves and methods of diffraction data elaboration applied to nanocrystals with very small dimensions (<30 nm) are discussed. It is shown that a unique value of the lattice parameter cannot be determined for such small crystals using a standard powder diffraction experiment. It is also shown that, due to the complex structure constituting a two-phase, core/surface shell system, no unique compressibility coefficient can satisfactorily describe the behaviour of nanocrystalline powders under pressure. We offer a tentative interpretation of the distribution of macro- and micro-strains in nanoparticles of different grain size.

Mechanical Properties and Microstructure of Diamond–SiC Nanocomposites

A bulk composite material close in hardness to diamond was fabricated from nanocrystalline diamond and SiC. The mechanical properties and microstructure of the composite were studied. Youngs modulus of the composite is found to be notably lower than the one following from the additivity rule, which is attributable to the influence of structural defects present in the interfacial zone between SiC and diamond. SiC consists of nanometer-scale grains near the interface and submicron grains in the “pores.”

Application of the apparent lattice parameter to determination of the core-shell structure of nanocrystals

In this review work we discuss applicability of Bragg scattering to examination of nanocrystals. We approximate the structure of nanograins by a commonly accepted core-shell model. We show that, for principal reasons, the Bragg equation is not applicable directly to nanocrystals. We use the Bragg relation through application of the apparent lattice parameter (alp) concept which we use to evaluate quantitatively the core-shell model. We also introduce a new parameter of the structure, Equivalent Cubic Lattice Parameter (EClp), which quantifies deviation of the real (trigonal) lattice from its parent fcc structure due to the lattice deformation (e.g. by the stacking faults). We show examples of an analysis of experimental X-ray and neutron diffraction data based on the alp methodology and on the theoretical patterns calculated for various core-shell models.

Microwave – Hydrothermal Synthesis of Nanocrystalline Pr - Doped Zirconia Powders at Pressures up to 8 MPa

Nanocrystalline praseodymium doped zirconia powders were produced using a microwave driven hydrothermal process under pressures up to 8 GPa. The ai m of the work was to evaluate the effect of synthesis conditions on the phase composition and g rain size of nanopowders of zirconia with Pr in solid solutions having Pr contents of: 0, 0.5, 1, 5 and 10 mol %. Introduction Application of microwaves (MW) in chemical synthesis has attrac ted onsiderable attention [1]. The advantage using MW as an energy source is primarily the possi bility of carrying out the processes at a much higher rate than during conventional heating. Zirc onia is an important ceramic material with useful mechanical, thermal, optical and electrica l properties. Recently much research effort has been dedicated to study nanocrystalline ceramics, since they may display a range of novel properties, including enhanced plasticity [2,3]. Therefore it is of gr eat importance to develop versatile synthesis methods for producing nanocrystalline ceramic powders. The aim of this work was to explore the possibility of synthesizing nanopowders in a MW driven reaction in aqueous solution under pressures of up to 8 MPa. The tests we re performed with nanocrystalline ZrO2 powders doped with Pr, which might be an interesting pigment [4,5] or oxyg en storage material [6]. It’s production by hydrothermal methods has been extensively studied [4,5,7,8]. Experimental methods Powders containing praseodymium in the 0.5–10 mol% range were obtained by addi ng praseodymium(III) nitrate (Pr(NO3)3*H2O, Carlo Erba) to a 0.5M ZrOCl 2 aqueous solution. The solutions were neutralized with NaOH 1 M to pH 10. 40 ml of the solution was poured into the Teflon vessel of the microwave reactor of volume 110 ml. The reacti ons were carried out using a MW reactor from Plazmatronika Ltd. The system operates at 2.45 GH z and can deliver up to 250 W of unpulsed MW power to the reaction fluid. The power level was automati cally adjusted to the maximum pressure, which pre-set for each experiment. The maximum accessible pressure is 10 MPa. The typical ramp time to a pressure of 4 MPa was 5 min and the cooling down time was 10 min. When the reaction was completed the solid phase was separated from the solution by filtering and the solids were washed free of salts with distilled water and isopropanol. One run produced approximately 0.5 g of powder which was annealed in air at 200 C for 0.5h after synthesis. Solid State Phenomena Online: 2003-06-20 ISSN: 1662-9779, Vol. 94, pp 193-196 doi:10.4028/www.scientific.net/SSP.94.193

Synthesis of Metal-Ceramic Nanocomposites by High-Pressure Infiltration

A technique of preparation of novel nanocrystalline composites by infiltration of a liquid metal under high-pressure is presented. A porous nanocrystalline body is obtained by compacting nanosized powder of a high-hardness ceramic material under the pressure of 2-8 GPa. The molten metal penetrates into the open pores and crystallizes there upon cooling. As a result the second nano-phase is obtained. Practical aspects of the technique and some properties of the composites are discussed.

Sintering of Compacts from Nanocrystalline Diamonds Without Sintering Agent

Compacts of polycrystalline diamond were made in toroid-type high-pressure camera under the pressure of 8 GPa using temperatures between 800 to 2150°C without the use of additive components. Nanocrystalline commercial DALAN, and microcrystalline ASM diamond powders were used. The compacts were characterized by helium pycnometry, Vickers hardness measurements, X-ray diffraction and SEM methods. The starting and sintered nanocrystalline grain compacts were found to have strongly one-dimensionally disordered cubic modification. The nanocrystalline powder had a bimodal grain size distribution function as determined from X-ray diffraction data and ab initio intensity calculations performed with the use of Debye functions. It was found that neither the grain size nor one-dimensional disordering change under high-pressure high-temperature conditions. There is a general tendency in a decrease of density of compacts with increase in the sintering temperature what resulting partly from graphitization above 1000–1200°C. The main factor which determines the density of the diamond compacts is closed porosity. Typically, the nanocrystalline diamond compacts sintered from 30 sec. to 6 min. have densities around 90% of the theoretical value. Their Vickers microhardness is 24 GPa and less.

Neutron diffraction studies of the atomic thermal vibrations in complex materials: application of the Wilson method to examination of micro- and nano-crystalline SiC

The Wilson method was applied for determination of the thermal atomic motions in micro- and nano-crystalline SiC. Limitations of application of this method to examination of complex materials with atoms vibrating with more that one amplitude were discussed. It is shown that a unique interpretation of Wilson plots for crystals with more than one type of atoms and weak vibration component(s) requires measurements performed up to a very large diffraction vector Q (>25 Å–1). Atomic vibrations in microcrystalline SiC were evaluated based on the diffractograms calculated for models built assuming different mean square atomic displacements (vibration amplitudes) of the component atoms. For nanocrystalline SiC two different temperature atomic factors which describe vibrations of the atoms in the grain interior (Bcore) and at its surface (Bshell) were determined.

Looking beyond Limitations of Diffraction Methods of Structural Analysis of Nanocrystalline Materials

In this work we discuss how to learn about the real atomic structure of nanocrystalline materials without misinterpreting the results of powder diffraction experiments. We discuss implications of nano-size on powder diffractograms based on some theoretical models of nanograins. Examples of experimental studies on nanocrystalline diamond and SiC are demonstrated.