Earle Ryba

A study of positional disorder in strontium barium niobate

The first systematic examination of trends in the site occupancies of strontium and barium ions in strontium barium niobate (SBN) at three compositions across the phase range is presented. X-ray powder diffraction and Rietveld structure refinement were used to obtain refined structures for SBN. Some problems were encountered in the structure refinement due to the domain structure in the material. Barium was found only at the A2 site for each composition. Ba/Sr occupancy for the A2 site varied from 91% for SBN (72-28) to 99% for SBN (50-50) in the unannealed state, while strontium occupancy of the A1 site varied from 70% to 54%. In annealed SBN (50-50), the A2 site occupancy decreased to 95%, while the A1 site occupancy increased to 62%. It is proposed that a twisting of the structure in the unannealed state decreases the sizes of the cages at the A1, A2 and C sites. After annealing, the structure becomes more regular, enlarging the A1, A2 and C sites and allowing more strontium atoms to occupy the A1 site. Cation distribution is controlled by the twisting of the octahedral framework. The overall properties of the material are modified by this distortion of the structure.

Calculation and evaluation of the gibbs energies of formation of Cr3C2, Cr7C3, and Cr23C6

A thermodynamic analysis of chemical composition data for a series of iron alloys containing chromium and carbon has been completed. These data were obtained from literature compilations for alloys equilibrated for extended times at 700 °C under neutral atmospheres. The results of this analysis, when supplemented with thermochemical data from the literature, permitted the calculation of the standard Gibbs energies of formation for the chromium carbides Cr7C3 and Cr23C6 over the range 600 to 1000 °C. These standard Gibbs energies were compared to data for these carbides from other sources. Available Gibbs energy data for the third pure chromium carbide, Cr3C2, were also evaluated. For each of these three compounds, a separation of the values for the Gibbs energy of formation into two distinct groups was observed. Each of these groups can be classed according to the nature of the experimental study used, whether it be a high temperature solid-gas equilibration involving a system of a carbide-chromic oxide-carbon (or chromium) with carbon monoxide, on one hand, or a series of investigations concerned mainly with electrolytic cell measurements, plus the work on which the present study is based. It is suggested that the differences in the Gibbs energies of formation for the respective carbides are associated with 1) the nonstoichiometric nature of these carbides and 2) possible dissolution of oxygen in the carbides during the equilibration studies.

A combined single crystal X-ray diffraction and electron diffraction study of the T2phase in Al-Li-Cu alloys

Both single crystal X-ray diffraction techniques and convergent beam electron diffraction have been employed to examine the structure of theT2 (Al6CuLi3) phase in three particular Al-Li-Cu alloys. It is shown thatT2 displays icosahedral symmetry both in a high purity laboratory melt and in two “impure” alloys which had been processed industrially. Possible reasons for the five-fold symmetry ofT2 are discussed.

Twinning in the structure of the ‘decagonal’ phase Al65Cu20Co15

Abstract The existence of twinning in the pseudo-decagonal phase Al65Cu20Co15 has been established through TEM studies. From electron diffraction patterns, a rhombic crystalline lattice with a very large unit cell is easily identified. Diffraction patterns for microtwinned regions exhibit splitting and distortion of the reflections which is a result of the overlap of lattices which are rotated by 36° with respect to one another. While patterns from single components of the twins exhibit only pseudo-tenfold symmetry, those from the microtwinned regions exhibit nearly perfect tenfold symmetry.

Decagonal and decagonal approximant formation as a function of composition in ternary Al-Co-Cu alloys

Abstract As-cast alloy ingots of nominal compositions Al65Co20Cu15, A160Co20Cu20, Al65Co15Cu20, and Al70Co15Cu15 were studied with transmission electron microscopy and energy-dispersive X-ray spectroscopy. A compositional variation among the grains, in each alloy, allowed an examination of phase formation as a function of the ternary composition near the nominal compositions. Alloys of composition Al65Co20Cu15, often taken as a stable decagonal phase, instead form a large (about 100 A) unit-cell crystal phase that approximates the decagonal phase. No decagonal phase forms near this composition. True decagonal phase formation is only observed in a narrow phase field centred near Al65Co15Cu20.

Structural repeat units and repeat schemes for the Al-Cu-Co decagonal phase and its crystalline approximants

Abstract Several repeat units and repeat schemes for the Al-Cu-Co decagonal phase and its crystalline approximants are proposed. An inherent strain is introduced into the structure through repetition of these units. The transition from the crystalline to the quasicrystalline state appears to be due to an increase in twinning, which reduces this strain.

Phenomenological theory of 1–3 type multiferroic composite thin film: thickness effect

The effect of thickness on the para–ferro-phase transition temperatures, the spontaneous polarization and magnetization and hysteresis loops of 1–3 type multiferroic composite thin films was studied in the framework of Landau phenomenological theory. We took into account the electrostrictive and magnetostrictive effects, misfit strains induced from the interfaces of ferroelectric/ferromagnetic portions and film/substrate. Butterfly loops under external fields were also simulated.

Coupling interaction in 1-3-type multiferroic composite thin films

Using the time-dependent Ginzburg-Landau equation, the coupling interaction of the ferroelectric (FE) and ferromagnetic (FM) phases in epitaxial 1-3-type multiferroic thin films was investigated considering the effect of elastic stress arising from the FE/FM and film/substrate interfaces. The result of the authors shows that the maximum polarization and magnetization appear with the FM fractions of 70% and 30%, respectively. The significant changes of the FE and FM properties are caused by the special structure in which the induced misfit strain greatly affects the anisotropy of the crystals and the properties of the materials.

Grazing incidence X-ray diffraction studies on the structures of polyurethane films and their effects on adhesion to Al substrates

Grazing incidence X-ray diffraction was carried out to analyze the structure of polyurethane films as a function of X-ray penetration depth by varying the angle of incidence. Coherence lengths, interplanar spacings, and crystallinities were obtained for non-aged and aged films of OH numbers of 120, 375, and 600 bonded to an aluminum substrate. Aging led to the improvement of bulk crystallinity of all the samples, particularly in the case of the aged PU-375 film, for which a dramatic increase of the bond strength was observed. The crystallinity of non-aged samples varied from the air/polymer surface down to the polymer/aluminum interface. Invariance of coherence length from air surface to interface was observed for PU-120 and PU-375 samples, implying that substrate-induced ordering has little effect on the average crystallite size. As the X-ray penetration increases near the polymer/Al interface, interplanar spacing of (021) reflection in all the films approach the value found for a bulk polyurethane-urea by Ishihara et al. The bond strength of the polyurethane film to the aluminum was exponentially proportional to the crystallinity including the crystalline interphase formed near the substrate. It is also found that the polymer film containing more (100) planes provided higher bond strength.

Magnetic Susceptibilities of Some Rare‐Earth‐Zinc (RZn2) Intermetallic Compounds

Data on the paramagnetic susceptibilities of the rare‐earth‐zinc intermetallic compounds, NdZn2, SmZn2, GdZn2, TbZn2, HoZn2, and YbZn2, in the 77° to 300° K region, are presented. The susceptibility obeys the Curie—Weiss law for all the compounds except YbZn2. YbZn2 has a susceptibility which is negative at room temperature but positive at lower temperatures. The data obtained from the paramagnetic region for the other compounds, except GdZn2, yield effective magnetic moments in agreement with the theoretical moments for the corresponding free tripositive ions. The reason for the anomalous value for GdZn2 is not understood.