Davor Balzar

Size-strain line-broadening analysis of the ceria round-robin sample

The results of both a line-broadening study on a ceria sample and a size–strain round robin on diffraction line-broadening methods, which was sponsored by the Commission on Powder Diffraction of the International Union of Crystallography, are presented. The sample was prepared by heating hydrated ceria at 923 K for 45 h. Another ceria sample was prepared to correct for the effects of instrumental broadening by annealing commercially obtained ceria at 1573 K for 3 h and slowly cooling it in the furnace. The diffraction measurements were carried out with two laboratory and two synchrotron X-ray sources, two constant-wavelength neutron and a time-of-flight (TOF) neutron source. Diffraction measurements were analyzed by three methods: the model assuming a lognormal size distribution of spherical crystallites, Warren–Averbach analysis and Rietveld refinement. The last two methods detected a relatively small strain in the sample, as opposed to the first method. Assuming a strain-free sample, the results from all three methods agree well. The average real crystallite size, on the assumption of a spherical crystallite shape, is 191 (5) A. The scatter of results given by different instruments is relatively small, although significantly larger than the estimated standard uncertainties. The Rietveld refinement results for this ceria sample indicate that the diffraction peaks can be successfully approximated with a pseudo-Voigt function. In a common approximation used in Rietveld refinement programs, this implies that the size-broadened profile cannot be approximated by a Lorentzian but by a full Voigt or pseudo-Voigt function. In the second part of this paper, the results of the round robin on the size–strain line-broadening analysis methods are presented, which was conducted through the participation of 18 groups from 12 countries. Participants have reported results obtained by analyzing data that were collected on the two ceria samples at seven instruments. The analysis of results received in terms of coherently diffracting, both volume-weighted and area-weighted apparent domain size are reported. Although there is a reasonable agreement, the reported results on the volume-weighted domain size show significantly higher scatter than those on the area-weighted domain size. This is most likely due to a significant number of results reporting a high value of strain. Most of those results were obtained by Rietveld refinement in which the Gaussian size parameter was not refined, thus erroneously assigning size-related broadening to other effects. A comparison of results with the average of the three-way comparative analysis from the first part shows a good agreement.

Internal stress storage in shape memory polymer nanocomposites

We examine the storage and release of internal stresses in shape memory polymers reinforced with a dispersion of nanometer-scale SiC particles. A quantitative Rietveld analysis of diffraction peaks was used to measure changes in the lattice parameter of the SiC particles after permanent deformation at 25°C, and subsequent shape recovery induced by heating to 120°C. Under 50% compression of the composite material, the nanoparticles store a finite compressive stress, which is almost completely released during heated strain recovery. The values of the stored internal stresses in the particles are compared to values based on micromechanic calculations.

Silicoboron-carbonitride ceramics: A class of high-temperature, dopable electronic materials

The structure and electronic properties of polymer-derived silicoboron–carbonitride ceramics are reported. Structural analysis using radial-distribution-function formalism showed that the local structure is comprised of Si tetrahedra with B, C, and N at the corners. Boron doping of SiCN leads to enhanced p-type conductivity (0.1 Ω−1 cm−1 at room temperature). The conductivity variation with temperature for both SiCN and SiBCN ceramics shows Mott’s variable range hopping behavior in these materials, characteristic of a highly defective semiconductor. The SiBCN ceramic has a low, positive value of thermopower, which is probably due to a compensation mechanism.

Profile fitting of X-ray diffraction lines and Fourier analysis of broadening

The extraction of pure-specimen X-ray diffraction-line broadening is described using a convolution of the instrumental profile and an exact Voigt function. Real Fourier coefficients were computed from the Cauchy and Gauss integral breadths and were input for Warren–Averbach analysis. Smooth surface-weighted and volume-weighted column-length distribution functions were obtained and errors in root-mean-square strains as well as effective domain sizes were evaluated. The method was applied to two cubic structures with average volume-weighted domain sizes up to 3600 A as well as patterns of tetragonal and orthorhombic (La, Sr)2CuO4, which exhibit weak line broadenings and highly overlapping reflections. Comparison with the integral-breadth methods is given. Reliability of the method is discussed in the case of a cluster of overlapping peaks.

Reliability of the Simplified Integral-Breadth Methods in Diffraction Line-Broadening Analysis

A comparison between different simplified integral-breadth methods, often used in the Rietveld-refinement programs to calculate coherent domain size and lattice strain, is carried out. It is shown that systematic differences exist for both domain size and strain, when they simultaneously broaden diffraction lines. Among different approximations, the values of domain size exceptionally scatter and sometimes are completely false (negative or not real). A comparison to the alternative Fourier method shows that all the simplified integral-breadth methods overestimate domain size, but especially the Cauchy-Cauchy approximation. The root-mean-square strain and the upper limit of strain are related in the general case of the Voigt strain-broadened line profile. It is shown that they should not differ much as the profile changes between the Gauss and Cauchy extremes. The pure-Gauss size-broadened profile is incompatible with the definitions of surface-weighted domain size and column-length distribution function.

Nonlinear ultrasonic parameter in quenched martensitic steels

We have determined the nonlinear ultrasonic parameter β and the ultrasonic longitudinal phase velocity vL for a series of martensitic steel specimens which varied in carbon content. The specimens were measured in the as-quenched state to ensure that the carbon was present primarily as an interstitial in the martensite. β increased monotonically with carbon content and hardness over the range 0.10–0.40 mass % C (39.0–57.5 Rockwell C hardness). However, vL remained virtually the same for all specimens. Therefore we conclude that β is sensitive to microstructural variations between the specimens, but vL is not. X-ray diffraction experiments indicate that the dislocation density in the specimens is large (∼1011–1012/cm2) and increases with increasing carbon content. These results support the hypothesis that the observed increase in β can be attributed to dislocation-related effects in the specimens.

Nonlinear Ultrasonic Assessment of Precipitation Hardening in ASTM A710 Steel

We investigated several specimens of ASTM A710 steel containing copper-rich precipitates with variations in the final aging treatment. X-ray diffraction line-broadening and small-angle neutron-scattering experiments revealed the existence of the precipitates and associated coherency strain. We determined the nonlinear ultrasonic parameter β for each specimen by harmonic-generation experiments and measured the ultrasonic longitudinal velocity v L and attenuation α L . Although v L and a L showed no consistent trends, β increased with increasing strain. This correlation is compared to a microstructural model for harmonic generation that includes a contribution from precipitate-inned dislocations.

Fundamental studies on large area Cu(In,Ga)Se2 films for high efficiency solar cells

This work reports the growth and characterization of thin films of Cu(In,Ga)Se2 (CIGS), which were grown by sequential sputtering, electrodeposition, and physical vapor deposition. Photovoltaic cells have been fabricated using these films with CdS heterojunction partners and the performance has been characterized. The effect of annealing conditions (temperature and duration) on the CIGS film microstructure and corresponding device performance has been investigated. Structure-property correlations were made using diffraction studies and Rietveld analysis. SEM studies were carried out to understand the effect of microstructure of the CIGS films on the solar cell efficiency. Cell efficiencies in excess of 10% have been achieved by using optimized annealing conditions. In addition, the optical properties of the sputtered CIGS films were characterized using variable angle spectroscopic ellipsometry and sputtered CIGS films were found to have optimum band gap. r 2001 Elsevier Science B.V. All rights reserved.

Tuning of tungsten thin film superconducting transition temperature for fabrication of photon number resolving detectors

Tungsten thin films can form in one of two crystal structures: alpha (bcc), with a superconducting transition temperature (T/sub c/) of 15 mK, and beta (A15), with a T/sub c/ between 1 and 4 K. Films with intermediate T/sub c/s are composed of both alpha and beta phases. We have investigated how to tune the film T/sub c/ in order to obtain certain values (T/sub c/ /spl sim/ 100 mK) suitable for the fabrication of photon number resolving transition-edge sensor (TES) and arrays of TES detectors for astronomical and quantum information applications. Variation of deposition conditions, and also the choice of the underlayer/coating for equal deposition conditions, affect the T/sub c/s of tungsten films. We have used x-ray diffraction to determine the structure of tungsten thin films and film stress. The results indicates that the film stress state depends on the underlying substrate and coating. To understand the variation of T/sub c/ values and to allow precise tuning of these values, we have investigated substrates and coatings for tungsten film multilayer stacks and determined tungsten film stress by x-ray diffraction at both room temperature and 8 K.

Influence of Strains and Defects on Ferroelectric and Dielectric Properties of Thin-Film Barium–Strontium Titanates

Pristine, W and Mn 1% doped Ba0.6Sr0.4TiO3 epitaxial thin films grown on the LaAlO3 substrate were deposited by pulsed laser deposition (PLD). Dielectric and ferroelectric properties were determined by the capacitance measurements and X-ray diffraction was used to determine both residual elastic strains and defect-related inhomogeneous strains by analyzing diffraction line shifts and line broadening, respectively. We found that both elastic and inhomogeneous strains are affected by doping. This strain correlates with the change in Curie-Weiss temperature and can qualitatively explain changes in dielectric loss. To explain the experimental findings, we model the dielectric and ferroelectric properties of interest in the framework of the Landau-Ginzburg-Devonshire thermodynamic theory. As expected, an elastic-strain contribution due to the epilayer-substrate misfit has an important influence on the free-energy. However, additional terms that correspond to the defect-related inhomogeneous strain had to be introduced to fully explain the measurements.