Svetlana Stelmakh

Nanocrystals: Breaking limitations of data analysis

Abstract A series of “virtual powder diffraction experiments” was made on models of small single crystals and nanocrystals with the core-shell structure. The results of those experiments were elaborated with application of standard methods of data analysis routinely used for reciprocal and real space analyses of polycrystalline materials. It is shown that the assumption of a uniform crystal structure of nano-materials is not justified and, therefore, application of routine procedures of collection and elaboration of diffraction data may lead to misinterpretation of the experiments and to incorrect conclusions about their structure. Tentative ways of using powder diffraction data to learn about the structure of nanocrystals with different atomic architecture of the core and of the surface of the grains are discussed. A need for elaboration of a model of the atomic structure of an individual nanograin with a non-uniform structure is discussed. An alternative approach to diffraction studies of nanocrystals by presenting the “footprints” of materials under study in the form of plots showing distribution of the experimental apparent lattice parameters as a function of diffraction vector Q, or bond length distribution as a function of r-distances derived from PDF function is suggested.

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.

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.

SiC – Zn Nanocomposites Obtained Using the High – Pressure Infiltration Technique

Two-nanophase SiC-Zn composites were synthesized under pressure up to 8 GPa at up to 1000oC using an high-pressure infiltration method. The advantage of this technique is that in a single, continuous process the ceramic nanopowder is compressed to form the matrix with nanopores; the nanopores are filled with a liquid secondary phase, (here Zn), which crystallizes as nano-scale grains. The key limitation is that the pores in the infiltrated preform have to stay open during the entire process. For this reason only powders of very hard ceramic materials can be used as a matrix. Two types of SiC nanopowders with average crystallite size of 10 nm and 60 nm and average particle size of 30 nm and 100 nm, respectively were used. The measurements of porosity of the green compacts prepared from these powders, pressed at 2.5 GPa and 8 GPa at room temperature, indicated that open porosity was maintained. The nanocomposites obtained show a “nano-nano” type microstructure with a uniform mixture of SiC and Zn phases. The volume fraction of Zn is 20 % independent of the process conditions and initial powder morphology. The process parameters and powder granularity influenced the crystal size of the secondary phase. The average grain size of Zn varied from 20 to 85 nm and was smaller in the composites obtained with the finer matrix, under higher pressure and at lower temperature. The microhardness HV02 of SiCZn nanocomposites varied from 6 to 22 GPa and increased with an increase of pressure and temperature of the infiltration process, and was significantly larger for the finer grained composites.

Qualitative and Quantitative Analysis of Stacking Disorder in α-and β-SiC by X-ray Diffraction and Structure Modeling

A method of analysis of disordering in {alpha}- and {beta}-SiC polycrystals by numerical modeling, and a simulation of X-ray diffraction profiles are presented. This diffraction patterns of non-periodic structures were simulated for models of 2,000 layer fragments of the structure. Computer generation of the models was based on the Poisson function describing the size distribution of the domains of basic polytypes and faults. The models were quantified by a set of input probability parameters describing relative frequencies of the occurrence of the domains of polytypes and faults. Implementation of a correlation parameter that characterizes coherence of sequential domains of a given polytype assures a good reproducibility of the simulated diffraction profiles obtained for the same set of the model parameters. Based on this method, a quantitative analysis of disordering in polycrystals of SiC annealed in the temperature range 1,100--2,200 C was performed.

Powder Precursors for Nanoceramics: Cleaning and Compaction

Thermal surface purification in an inert gas flow and densification processes of SiC and diamond nanocrystalline powders with specific surface in the range of 60 – 300 m2/g and average grain sizes from 5 to 15 nm in diameter were examined. Termogravimetric Analysis (TGA) linked with mass spectrometry of outgassing products show that surface impurities desorb at up to 450°C. Further heating above 450°C leads to oxidation of the powder surface. Small Angle X-Ray Scattering (SAXS) and gas porosimetry (ASAP) was applied to investigate densification of the nanocrystalline powders. Compaction under 1GPa or higher pressure was found necessary for obtaining the ceramic matrix with porosity in the nanometer range.

Investigation of the Microstructure of SiC-Zn Nanocomposites by Microscopic Methods: SEM, AFM and TEM

SiC-Zn nanocomposites with about 20% volume fraction of metal were fabricated by infiltration process under the pressure of 2-8 GPa and at the temperature of 400_1000oC. SiC nanopowders used in the experiments formed loosely agglomerated chains of single crystal nanoparticles. The dimension of the agglomerates was in the micrometer range, the mean grain size was up to tens of nanometers. Microstructural investigations of the nanocomposites were performed at a different resolution levels using scanning, transmission electron microscopy and atomic force microscopy techniques (SEM, TEM, AFM, respectively). SEM observations indicate a presence of nano-dispersed, uniform (on the micrometer scale) mixture of two phases. TEM observations show that distribution of SiC and Zn nanocrystallites is uniform on the nanometer scale. High-resolution TEM images demonstrate an existence of thin (on the order of tens of Angstroms) Zn layers separating SiC grains. AFM images of the mechanically polished samples show a smooth surface with the roughness on the order of the SiC grain size (10-30 nm). After ion etching of some samples the AFM topographs show surface irregularities: periodically spaced hillocks 50-100 nm in height. The size of the SiC grains remains equal to that of the initial powder crystallites. The size of the Zn grains varies in the range of 20-100 nm depending on the initial SiC grain size and the composite fabrication conditions.

Preparation of SiC-Diamond Nanocomposites

Compacts of composites SiC-diamond were made by infiltration of Si into nanocrystalline diamond powders in a toroid-type press under the pressure of 7.7 GPa at 1300 °C. In-situ high pressure diffraction studies of these processes were performed in MAX80 cubic anvil press at a pressure of 8.5 GPa in temperatures up to 1800°C in HASYLAB at DESY, Hamburg, Germany. Sintering was performed for (i) pure nanocrystalline diamond powders, (ii) a mixture of nanocrystalline powders of diamond and nanocrystalline SiC, (iii) a mixture of nanocrystalline diamond with microcrystalline Si powders and (iv) compacts of nanocrystalline diamond infiltrated by Si. The SiC-diamond composites obtained by infiltration of Si have best physical properties: hardness similar to conventional diamond compacts (approximately 50 GPa), highest density 3.35 g cm -3 and uniform nanocrystalline microstructure.