Search

Two tin dioxide samples undergone to different thermal treatments have been studied by X-ray diffraction profile analysis, using the modified Williamson-Hall and Warren-Averbach methods. Assuming {101} slip system and screw dislocations, the area and volume-weighted average particle sizes have been evaluated for both samples. The assumed log-normal particle size distribution has been calculated from these values. This distribution is in good agreement with the derived from SEM micrographs for the sample with lower particle size. Values of the dislocation densities were estimated around 10 14 m −2 for the sample with lower grain size and around 10 12 m −2 for the sample with greater grain size. The difference in the results for the analyzed samples could be explained from the difference between the treatments after synthesis.

A wide set of 3 mol% Y2O3-stabilized ZrO2 (3YSZ) powders have been synthesized by the sol–gel method without modifiers from solutions of zirconium n-propoxide and yttrium acetate prepared with different water/Zr molar ratio and zirconium concentration, and the effect of these two synthesis variables on the pressureless sinterability of the 3YSZ powders has been investigated. It was found that the sinterability increases with increasing zirconium concentration and water/Zr molar ratio, which is attributed to the smaller cluster size developed during the sol–gel synthesis. Implications for the preparation of more sinterable 3YSZ powders are discussed Fil: Mamana, Nadia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Ortiz, Angel L.. Universidad de Extremadura. Badajoz; España Fil: Sánchez Bajo, Florentino. Universidad de Extremadura. Badajoz; España Fil: Caruso, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina

ZrO 2 –3 mol% Y 2 O 3 -coated AISI 310 stainless steel was nitrided using a plasma produced by dc pulsed discharge in a mixture of N 2 and H 2 at an equilibrium temperature of 450 °C. The phase structure and surface morphology were investigated by X-ray diffractometry and atomic force microscopy, respectively. Glow discharge optical emission spectrometry was used to measure the depth composition of samples. The mechanical properties were studied by means ultramicrohardness indentation tests. Nitrogen atoms enter in the zirconia substituting to the oxygen atoms in the network. However, the nitrogen content measured in zirconia coating is greater than the allowed maximum content of nitrogen in doped-zirconia solid solution. The ion-nitriding process alters the surface topography of the zirconia coatings. The nitruration improves the mechanical responses of the ZrO 2 –3 mol% Y 2 O 3 -coated AISI 310 stainless steel.

Structural determination using X-ray powder diffraction is usually restricted to relatively simple structures, although powder diffraction data are frequently used to refine, via the Rietveld method, approximately known structures. In this work, the relatively complex crystalline structure of a compound of high pharmaceutical interest (C11H12N4S2) has been firstly ab initio determined from its X-ray powder diffraction pattern and subsequently refined by the Rietveld method. This refined structure is in excellent agreement with that obtained previously from single-crystal data. As a consequence of the severe overlapping between adjacent reflections, direct methods failed, whereas we obtained good outcomes with ‘real space’ methods. This work shows that within the current state-of-art (hardware, software, numerical methods, etc.), relatively complex structures can be afforded by means of powder diffraction.

Silicon carbide (SiC), owing to its unique combination of chemical and physical properties, is the material of choice for advanced engineering applications both at room and high temperatures. Due to the difficulty of SiC to be solid-state sintered, nowadays there has been growing interest in liquid-phase sintering (LPS) of SiC. Here, there is a critical need for developing methodologies for the accurate X-ray diffraction characterization of the microstructures of these ceramics as their microstructural evolution is not yet totally understood. Thus, the aim of the present work is to apply a procedure for the accurate line-broadening study of any given phase in LPS SiC ceramics. To that end, we have pressureless liquid-phase sintered (at 1950 °C for 0.5 h in flowing Ar atmosphere) a specimen starting from a commercially available β-SiC powder and using the simultaneous addition of Al 2 O 3 and Y 2 O 3 . The resulting material was subsequently characterized using X-ray powder diffraction; more specifically, the Rietveld method was applied for the quantitative phase-composition analysis and the Warren–Averbach method for determining the mean crystallite size, the crystallite size distribution and the root-mean-square strain along the crystallographic direction of the β-SiC polytype. The nanometric nature of the crystallite size distribution is finally related with the stacking faults in the SiC starting powder.

Accurate X-ray powder diffraction (XRD) analysis of SiC-based ceramics is a difficult task due to the significant overlap of the Bragg reflections from the different SiC polytypes. For this reason, results obtained by traditional XRD methods are, in general, unsatisfactory. Here, we have applied the Rietveld and two line-broadening (variance and integral breadth) methods to analyze a liquid phase-sintered SiC sample. The Rietveld method was used to carry out the quantitative phase-composition analysis and to resolve (as a first step) the overlapped SiC Bragg reflections. Starting from the Rietveld results, the different SiC peaks were then unambiguously obtained by a Levenberg–Marquardt non-linear least-squares fit. Subsequently, the variance and integral breadth methods were used to perform the microstructural analysis. It is shown not only that the procedure is especially useful, but also that both line-broadening methods lead to similar crystallite sizes. The co-existence of β (3C) and α (4H and 6H) SiC polytypes and the nanometric diffraction domain sizes for the SiC polytypes were also found. Finally, these results are discussed in the light of the β to α partial transformation that took place during the sintering.

The deconvolution of X-ray diffraction profiles is a basic step in order to obtain reliable results on the microstructure of crystalline powder (crystallite size, lattice microstrain,etc.). A procedure for unfolding the linear integral equationh=g finvolved in the kinematical theory of X-ray diffraction is proposed. This technique is based on the series expansion of the `pure' profile,f. The method has been tested with a simulated instrument-broadened profile overlaid with random noise by using Hermite polynomials and Fourier series, and applied to the deconvolution of the (111) peak of a sample of 9-YSZ. In both cases, the effects of the `ill-posed' nature of this deconvolution problem were minimized, especially when using the zero-order regularization combined with the series expansion.

In recent years, several profile-shape functions have been successfully used in X-ray powder diffraction studies. Here, a new profile function for approximating the X-ray diffraction peaks is proposed. This model, based on a Gaussian function multiplied by a correction factor in the form of a series expansion in Hermite polynomials, can be employed in the cases where there are peak asymmetries. The function has been tested by using samples of α-Al2O3and 9-YSZ (yttria-stabilized zirconia), yielding generally satisfactory results.

A worked example of polytypism is presented, aimed at assisting undergraduates in the learning and instructors in the teaching of this topic. In particular, this crystallography concept, not necessarily obvious for beginners, is illustrated pedagogically using to that end the model case of the prolific polytypism of silicon carbide (SiC). On the basis of concepts that are easily assimilated by students (i.e. simple topological constraints) this article first presents a unified description of the polytypism phenomenon in SiC that allows one to understand without difficulty the existence of its numerous polytypic variants and how they develop. Then the various notations used to designate these different polytypes are described, and finally the crystal structures of the most common are discussed. This worked example is thus expected to contribute to motivating undergraduates in the study of a crystallography topic that often is not treated in sufficient depth in class. Ministerio de Ciencia y Tecnología MAT2010-16848

The present work is concerned with the classical Johnson-Mehl-Avrami-Yerofeev-Kolmogorov (JMAYK) equation. Some critical points are presented because this kinetic relation has been extensively misused. Various aspects regarding its failure when it is applied to recrystallization of plastically deformed metals are also considered. Recent conceptual improvements, such as the new approach where the transformation is described by the superposition of single JMAYK processes, have resulted in the resurgence of this approach.

During the past few years, several studies have been carried out of the structural characterization of amorphous materials using the Monte Carlo method. It has been shown that a variety of errors gives rise to pronounced artifacts in the R-space correlation functions, which may hinder the accurate interpretation of the Monte Carlo results. The elimination of these ambiguities, particularly for heteroatomic systems, demands very careful experiments in association with careful error analysis. Recently, Kaszkur [J. Appl. Cryst. (1990), 23, 180–185] presented a theory describing some convolutional properties of the reduced interference function. This procedure enables an estimation of the normalization constant (in all probability, nowadays, one of the more serious limitations in obtaining significant and reproducible radial distribution functions) with a high degree of accuracy. Since the Kaszkur formulation is limited to monoatomic substances, the present work recalls his basic arguments with the aim of extending them to the most general case of heteroatomic materials. In addition, the proposed procedure allows an overall insight into the quality of the measured data and determination of the macroscopic density on the basis of the scattered intensities. The success of the procedure is proved by its application to two experimental data sets and one simulated example.

A quantitative determination of the phase abundance on Y-PSZ (4 mol%) polycrystals is performed by X-ray diffraction (XRD). Our procedure takes account of the strong peak overlapping, the presence of t′ phase and preferred orientation artifacts. The individual reflections of each phase are fitted to Pearson VII functions. The influence of the preferred orientation is handled by a procedure in which an expansion in spherical harmonics allows us to relate the measured integrated intensities to the “randomoriented” ones. The method is used to study the microstructural evolution of two sintered samples (1550°C and 1650°C) heat treated in air at 1600°C and subsequently air cooled.

Accurate quantitative X-ray diffraction analysis of SiC-based ceramics is difficult because of the significant overlap of the Bragg reflections from the different SiC polytypes. In this regard, the Rietveld method is a powerful tool for the accurate and precise analysis of the phase/polytype compositions in these materials. In this study, we have used two different types of Rietveld codes for the quantitative phase/polytype analysis of a liquid-phase-sintered SiC specimen: FULLPROF, which is based on the classical Rietveld approach, and BGMN, which is based on the new fundamental parameter approach. In both cases, the effect of texture corrections on the precision of the analysis also was studied. The accuracy of the analysis, in terms of the weight percentage of SiC (all polytypes) and yttrium aluminum garnet liquid phase, as determined from the starting powder composition, is within the standard deviation of the analysis in both cases (FULLPROF and BGMN), with and without the texture corrections. In addition, in the case of the classical code (FULLPROF), inclusion of the texture corrections has been shown to improve the precision. In contrast, the precision of the analysis using the BGMN code without the texture corrections is better. However, inclusion of the texture corrections is expected to improve the accuracy of the analysis.