Crystallization kinetics of Zr-rich FexZr0−x0−x0−x(20 ⩽ x ⩽ 40) metallic glasses

The crystallization kinetics of the melt-spun Fe-Zr metallic glasses in the iron-rich region has been investigated by means of DSC and X-ray diffraction. The crystallization mode changes with iron concentration. In the lower iron region, 20 x 25, the Fe _x Zr100–x glasses crystallize into -Zr and Ti₂Ni-type FeZr₂ with an accompanying sharp and large exotherm at the first crystallization step and immediately after this step, they transform into orthorhombic FeZr₃. On the other hand, the alloys with 35 x 40 exhibit a gradual exotherm which initiates from a temperature far below the definite crystallization temperature (T _x). The Fe-Zr metallic glasses in this concentration region crystallize polymorphously into the oxygenstabilized Ti₂Ni-type FeZr₂ with accompanying relatively small and composite exotherms. The annealing at a temperature where the gradual exotherm occurs for the alloys with 30 x 40 does not cause any changes of X-ray halo pattern but results in the reduction of the heat of exotherm due to the crystallization.     

Some properties of Sb2Te3−xSex for nonvolatile memory based on phase transition

Composition dependence of properties of Sb₂Te_3–x Se _x in the range 0x<3 were studied using differential thermal analysis and X-ray diffraction. Sb₂Te_3–x Se _x form solid solution for 0<x1.25 and 2.75x<3. A systematic study of crystallization temperature in Sb₂Te_3–x Se _x (0x2.75) thin films prepared by flash evaporation was carried out. In preliminary experiments for some compositions, more than 10³ repetitions between amorphous and crystalline states were attained by the application of electric pulses.     

Lead zinc niobate pyrochlore: Structure and dielectric properties

In Pb(B _x B_1–x )O₃ ceramic compositions, it is customary to find a mixture of cubic pyrochlore and perovskite phases after calcination. Based on X-ray diffraction analysis, we concluded that both phases are made up of the same structural unit of BO₆ octahedra. The B and B ions occupy the B sites randomly to the extent that local charge equilibrium is maintained. Thus a general formula for the pyrochlore phase can be expressed as Pb(B _x B_1–x )O₃ · O_p where O p 0.5. An extensive study of the Pb(Zn _x Nb_1–x )O_3.5–1.5x pyrochlore system was made by varying the zinc concentration. We interpret changes in the X-ray diffraction pattern and the lattice constant as indicative of the changing occupancy of the seventh oxygen sites in order to maintain local charge balance. The best combination of the dielectric properties, with a dielectric constant of130 and a factor greater than 1000 at10 MHz, is achieved at a composition of 0.3 x 0.4 and a sintering temperature of 980° C. The temperature coefficient of the dielectric constant measures –0.75 × 10^–3° C^–1. It decreases to –0.54 × 10^–3° C^–1 when5 mol% of PbTiO₃ was mixed with the nominal pyrochlore compositions and sintered. Thus, it may be possible to effect a larger change in the temperature coefficient by judiciously including selective amounts of a second phase which has the best compensating temperature coefficient.     

Photoelectric properties of undoped and lithium-doped Zn1−xMgxTe alloys

The room temperature photoelectric response of undoped and lithium-doped Zn_1–xMg_xTe (0 x 0.50) alloys has been measured in the wavelength range 0.50 3.0 m. The response curve for undoped samples is characterized by a single peak in the band edge region. The peak shifts with composition in accordance with the expected shift in the energy band gap. Lithium-doped samples show an additional peak centred at 1.04 eV for all compositions. This peak is attributed to photo-generated holes in the split-off band created as the result of electronic transitions to shallow acceptor impurities.     

Soft magnetic properties of NiFeTa thin films for biasing film applications in magnetoresistive elements

The effects of Ta addition on the magnetic properties of permalloy thin films have been investigated. The alloy compositions on a weight basis are (Ni₈₁Fe₁₉)_1-x Ta _x with 0 x 0.105, and the films are sputtered onto a glass substrate at between room temperature and 300 C. The saturation magnetization and anisotropic magnetic field decrease with increasing Ta content. The saturation magnetization is 0.75 T at 5 wt % Ta. The coercivity remains constant at 125 Am^-1. The electrical resistivity increases linearly with increasing Ta content, then saturates at approximately 7.5 wt% Ta. The saturation resistivity is approximately 1.00 m. The magnetoresistivity ratio (/) decreases with increasing Ta content, mainly due to increased electrical resistivity (). The magnetostriction changes from negative to positive with increasing Ta content and reaches nearly zero at 2 wt% Ta. The NiFeTa films containing 5–6 wt% Ta have potential for use as the soft-biasing film in magnetoresistive elements.     

Constitution,structure and magnetic properties of some rare-earth-cobaltaluminium alloys

The constitution and structure of the alloys represented by the formulae Ce_1–x Al _x Co₅ and Pr_1–x Al _x Co₅ (where 0x0.6) have been studied by X-ray diffraction and metallographic techniques. The substitution of between 1 and 3 at % Al (0.06x0.18) for Ce in CeCo₅ produces a mixture of the 15 and 217 phases based on CeCo₅ and Ce₂Co₁₇; there are two variations of the 217 phase which are isostructural with the hexagonal Th₂Ni₁₇-type and rhombohedral Th₂Zn₁₇-type phases. At the composition Ce_0.76Al_0.24Co₅ (4 at% Al) the alloy consists only of the 217-type phases and metallographically the alloy is one phase in appearance. Further substitution of Al results in the precipitation of an fcc phase, based on the Co-Al solid solution, in the 217 matrix. The crystal structures of the Pr_1–x Al _x Co₅ alloys are very similar to those of the equivalent cerium alloys, although the ternary 217-type phase has only the rhombohedral structure, as does the binary Pr₂Co₁₇ phase. The metallographic structures of the Pr alloys in the composition range 1 to 3 at% Al show significant differences from the corresponding Ce alloys. Determination of the Curie temperatures of theR _1}-xAl _x Co₅ alloys (R= Ce and Pr) in the composition range 0x0.24 showed good agreement between the measured and published values for both the CeCo₅ and PrCo₅ phases. The Curie temperatures of the ternary 217-type phases were approximately 50 K and 70 K lower than those of the binary, Ce₂Co₁₇ and Pr₂Co₁₇ alloys, respectively.     

Electrical properties and phase transition of Ge1−x Sn x Se2.5 thin films

The Ge_1–x Sn _x Se_2.5 system was prepared by melting the correct ratio of high purity elements in quartz evacuated ampoules followed by quenching in ice. It was found that, within the Ge_1–x Sn _x Se_2.5 system, a glassy state can be formed when 0 x 0.4. On increasing x to 0.6 a glassy state could not be obtained, as is confirmed by X-ray diffraction. Differential thermal analysis (DTA) was carried out to study the effect of composition on the stability of amorphous phase. Ge_1–x Sn _x Se_2.5 (where 0 x 0.6) thin films have been prepared by the thermal evaporation technique. The electrical conductivity of the thin films have been studied as a function of composition and film thickness.     

Transmission electron microscopy analysis of a TiN x substoichiometric CVD coating

The TiN _{x 1–x} substoichiometric chemical vapour deposited titanium nitride coatings have been studied in an early work by means of high-resolution X-ray emission spectroscopy. It was found that a strong vacancy-induced peak was present in the Ti K emission band. Its intensity can be correlated to the 1–x vacancy concentration deduced from nuclear reaction spectroscopy and X-ray diffraction. This relationship is linear if 01–x0.3. If 1–x is higher than 0.3, an anomalous behaviour occurs which is expected to be due to microstructure change. For this purpose, transmission electronic studies of a TiN_{0.57 0.43} layer have been developed. The most striking result of this work is the existence of many stacking faults. These defects are extrinsic ones and the stacking fault energy is about 3.5 mJ m^–2. Their density seems to depend on their distance from the substrate-coating interface. Further investigations are needed to confirm this assumption.     

Electrical properties of (Ni1−x )Li x )O (x=0.1 and 0.2) under various relative humidities

Rocksalt-type (Ni_1–x Li _x )O (x=0.1 and 0.2) was synthesized at 1350° C in air and its electrical resistivity (R) was measured under various relative humidities (H). R increases with increasing H in the range 0H79%, reaches a maximum value, then decreases in the range 79%<H100%. The increase in R is explained by an electron boundary layer model. On the other hand, the decrease in R is explained by ionic conductivity.     

Superconducting properties of the lithium-substituted YBa2Cu4O8 prepared by the sol-gel method

The effects of the lithium substitution for copper on the properties of the superconducting YBa₂Cu₄O₈ were studied. Single-phase YBa₂Cu_4–x LixO₈ was successfully prepared by the sol-gel method under ambient pressure over a composition range of 0x 0.08, while impurity phases appeared at x=0.10 and 0.20. The lattice constants of YBa₂Cu_4–x Li _x O₈ were almost invariant with increasing lithium content x. The superconducting transition temperature decreased monotonically with increasing x in the range, 0x0.08. The suppression of superconductivity was discussed in terms of the impurity effect in the CuO₂ planes.     

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (Cr_xFe_1–x)₂O₃, 0 x 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, -(Cr_xFe_1–x)₂O₃, which is isostructural with -Fe₂O₃ and phase C, which is closely related to Cr₂O₃. Phase F exists in samples heated up to 900°C, for 0 x 0.95. Phase C exists from x0.27 to x=1 for samples heated up to 900°C and from x0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr₂O₃ in -Fe₂O₃ and 4.0 ± 0.5 mol % of -Fe₂O₃ in Cr₂O₃. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. ⁵⁷Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at 460 and 370 nm (22 000 and 27 000cm^–1) for x 0.5, 500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the ⁶A₁ 4T₁ Fe³⁺ ions in all spectra and the development of the absorption at 13000 cm^–1 due to a metal-metal charge transfer (Cr³⁺ Fe³⁺) transition, might be explained by exchange coupling which has been observed in some spinel compounds.     

Crystal structure and hydrogen occupation in H x V2O5(x=0.0–3.9)

Changes in the crystal structure and the lattice constants of powdered crystalline V₂O₅ after absorption of hydrogen were measured, as a function of the concentration, by X-ray diffractometry. The diffraction spectra obtained showed that three kinds of phases, (x<0.4 in H _x V₂O₅), (co-existing with either terminal phase in the range of 0.4x<3.2) and (3.2x3.9), appear up tox=3.9, which was found to be the upper limit of the absorption at 70C under a hydrogen pressure of 5 MPa. The crystal system of the three phases was determined to be orthorhombic, although the lattice constants differed. The diffraction lines were broadened, and the background of the X-ray spectra grew with hydrogenation, especially in the -region, which indicates the growth of the lattice distortion and a quasi-amorphous state. The occupation of hydrogen atoms in each phase is discussed on the basis of the experimental results.     

Properties of Va metal-B films prepared by r.f.-sputtering

Ta_100-x B _x alloy films were prepared by r.f.-sputtering in the chemical composition range 45 x 77. Ta_100-x B _x (45 x 58) films consist of the amorphous phase, while the TaB₂ crystal phase was observed in Ta_100-x B _x (66 x 77) films. A remarkable preferred orientation with the (001) plane of TaB₂ parallel to the film surface was observed in Ta₃₄B₆₆. The d.c. electrical conductivity of Ta_100-x B _x (45 x 77) films decreases with increasing boron content in the range 6.7 × 10³ to 1.3 × 10^3–1 cm^–1. The micro-Vickers hardness of Ta_100-x B _x (45 x 77) films was in the range 2200 to 2600 kg mm^–2.     

Magnetic and Electrotransport Properties of the Anion-Deficient Manganites with Perovskite Structure

The chemical phase content and microstructure as well as magnetic and magnetotransport properties for the doped anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 orthomanganites have been experimentally studied. The high-quality anion-deficient ceramic with controlled chemical phase content and microstructure has been obtained from the parent stoichiometric one at the 800 C in vacuum. The average grain size is ~5 m. It is established that the samples in the region of 0 x 0.125 are O-orthorhombic perovskites whereas in the 0.175 x 0.30—rhombohedric. As doping level increases along with the oxygen vacancies number the samples in ground state undergo a transition from the antiferromagnet A-type (x=0) through the inhomogeneous magnetic state (0 < x 0.175 to the cluster spin glass one (0.175 < x 0.30. The temperature of magnetic moment freezing is ~ 45 K. All the samples are semiconductors and show considerable magnetoresistance over a wide temperature range with peak for x = 0.175 only. Fitting of the electrical resistivity to T^–1 and T^–1/4 realized. The concentration dependences of the activation energy, spontaneous magnetization, coercivity as well as magnetic transition temperatures for the anion-deficient La³⁺_1-xSr²⁺_xMn³⁺O^2–_3-x/2 ortomanganites have been established. Obtained experimental results are interpreted in terms of the oxygen vacancies, isotropic superexchange Mn³⁺-O–Mn³⁺ interactions and phase separation models.     

Investigation of structural transport and magnetic properties of the system Li0.25Cu0.5Fe2.25- x Al x O4

Experimental data on X-ray electrical conductivity, thermoelectric coefficient, magnetic hysteresis and infrared absorption spectra of the system Li_0.25Cu_0.5Fe_2.25–x Al _x O₄ are presented. All the compounds, 0x2.25, showed cubic symmetry. Lattice constant values progressively decreased on increasing the Al³⁺ content. X-ray intensity calculations, magnetic hysteresis and infrared spectroscopy studies indicated the presence of Li⁺, Al³⁺ and Fe³⁺ at tetrahedral and octahedral sites, while Cu²⁺ is present only at the octahedral site. The activation energy and threshold frequency increased with increasing values ofx. The compounds withx1.50 aren-type, and those withx2.0 arep-type semiconductors. Magnetic hysteresis indicated that compounds withx1.50 are ferrimagnetic, and those withx2.0 are antiferrimagnetic. High coercive force,H _c, values and remanence ratios (J _R/J_S) showed that all the compounds exceptx2.0 exhibit single-domain behaviour. The probable ionic configuration for the system is suggested as Li _0.15 ⁺ Al _0.5 ³⁺ Fe _0.35 ³⁺ [Li _0.1 ⁺ Cu _0.5 ²⁺ Fe _0.4 ³⁺ Al³⁺]O ₄ ^2– .     

X-ray,transport, magnetic and catalytic studies of the system Cd1–x Cu x FeCrO4

The compounds of the system Cd_1–x Cu_xFeCrO₄, where 0.0 x 1.0, synthesized by the co-precipitation technique have been studied with a view to investigating the cation distribution and the transport and magnetic properties, and to correlating the physical properties of oxidic spinels with their catalytic behaviour in the decomposition of benzyl alcohol. All the compounds of the system were crystallized with the cubic-spinel structure. The activatio-energy values of electronic conduction varied between 0.69 and 0.16 eV. Thermoelectric power measurements indicated a p-type semiconducting nature for all the compounds. Magnetic hysteresis studies indicated that the compounds with x 0.4 were ferrimagnetic. The Curie temperatures increased with increases in the Cu²⁺ -ion concentration. A good correlation could be established between the physical properties and the catalytic behaviour of the system.     

Nuclear magnetic resonance of adsorbed 3He films on Grafoil and argon-coated grafoil. II. Intermediate and near-monolayer coverages

Continuous wave (CW) nuclear magnetic resonance measurements have been made at 5.5 MHz of ³He adsorbed on Grafoil and on argon (monolayer)-coated Grafoil at coverages 0.55 x 1.35 (x = 1 corresponds to one monolayer), in the temperature range 0.35–4.21 K. The results indicate that the near-monolayer coverages (0.9 x 1.0) behave as 2D solids with dipole-dipole coupling modulated by quantum exchange between nearest neighbors. The exchange frequency for this solid phase is evaluated and is seen to be comparable to that observed in bulk, solid ³He for similar atomic spacing. The intermediate coverages, 0.55 x 0.80, showed no evidence of line broadening in the entire temperature range investigated, while the relative magnetic susceptibilities indicated no deviation from Curie's law. No evidence for the registered phase was found at x = 0.60 either for ³He on Grafoil or for ³He on argon-coated Grafoil. A sharp decrease was observed in linewidth after completion of the monolayer, making NMR an excellent probe for the determination of monolayer completion. We have also made linewidth measurements for a coverage x = 0.95 of ³He on graphitized carbon black (Sterling FT grade) at 4.2 and 1.2 K. The results are consistent, as in the case of ³He on Grafoil, with a 2D solid with significant quantum tunneling between nearest neighbors. This result, however, is in disagreement with the measurements of Rollefson, also for ³He on graphitized carbon black of the same grade, who observed a rigid lattice behavior with negligible tunneling between neighbors, for a coverage x = 0.90. This discrepancy is not yet understood.Supported by a grant from the National Science Foundation.     

Modified YBCO superconductor-ferroelectric composites: Bulk materials and thick films

Bulk samples and 50–125 m thick films of (1-x)YBa₂Cu₃O_7–-xBaTiO₃ (0x0.12 by weight) composites have been synthesized, the films being obtained by Stokes sedimentation on SrTiO₃ ceramic plates and firing in oxygen by Melt Textured Growth techniques.The phase composition and texture have been studied by X-ray diffraction and microstructure. Resistance and magnetic susceptibility of prepared samples have been measured.     

Influence of preparation method on catalytic properties of mixed calcium-cobalt orthophosphates

Mixed calcium-cobalt orthophosphates of the formula Ca_3–x CO_x(PO₄)₂, with 0 x 1.1, were prepared by coprecipitation and solid-state reaction. Structural characterization showed that, in both cases, the solids obtained resulted from substitution of some Ca²⁺ ions by Co²⁺ ions in the -Ca₃(PO₄)₂ structure. The solubility limit of cobalt in this structure corresponded tox=0.31. The morphology of these compounds was investigated by surface-area measurements and scanning electron microscopy. When tested in the 2-propanol decomposition reaction, the maximum activity in dehydrogenation was obtained with the compound corresponding to cobalt maximum solubility in -Ca₃(PO₄)₂ structure. Higher activities were obtained for samples prepared by coprecipitation.     

The orthorhombic-to-tetragonal phase transition in YBa2(Cu1−xFex)3O7−y

The phase transition YBa₂(Cu_1–x Fe _x )₃O_7–y (0x0.05) prepared through solid-state reaction was studied with X-ray diffraction analysis and optical microscope. It was found that the orthorhombic-to-tetragonal phase change was induced between x=0.02 and x=0.03 by X-ray diffraction. The phase transition was also identified by the disappearance of striations in plate-like grains under polarized light. The microstructures of the specimens with compositions of x=0.01 and x=0.02 showed faint striation images. It is considered that a mixture of the orthorhombic and tetragonal phase exists in these compositions.