Reactions of hydrogen with R3Ni8Al intermetallides (R = Y,Pr, Nd,Gd, Tb,Dy, Ho,Er) with the Ce3Co8Si type of structure

Summary X-ray phase analysis, DTA, and volumetry have been applied to the reactions with hydrogen shown by R₃Ni₈Al compounds with structures of Ce₃Co₈Si type, where R = Y, Pr, Nd, Gd, Tb, Dy, Ho and Er. The R₃Ni₈Al react with hydrogen at room temperature and pressures below 0.1 MPa. Hydrides of composition R₃Ni₈AlH_x are obtained, where x=9.3 for Y, 10.3 for Pr, 11.4 for Nd, 8.7 for Gd, 6.9 for Tb, 5.7 for Dy, 5.4 for Ho, and 2.7 for Er. Hydride formation is accompanied by an increase in unit-cell volume by 5.7–18.2% without change in the initial hexagonal symmetry. Most of the hydrides decompose on heating at moderate temperatures up to 470 K. The main interest among these hydrides is in the Y₃Ni₅AlH_8.3, which can be recommended as a low-pressure hydrogen absorber.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 3, pp. 37–44, May–June, 1992.     

Investigation on Ni3Al-Ni7Hf2 unidirectionally solidified eutectic composite

The formation of a Ni₃Al()+Ni₇Hf₂ unidirectionally solidified lamellar eutectic composite has been investigated in this paper. The results show that Ni-5.8Al-32Hf alloy, which has the Ni₃Al+Ni₇Hf₂ eutectic structure, is a suitable composition of D.S eutectic material. The melting range of this composition is 41 °C as determined by DTA. The critical ratio of G/R for Ni₃Al+Ni₇Hf₂ eutectic is found to be 5× 10⁵ °C · s · cm^–2, and the lamellar Ni₃Al+Ni₇Hf₂ eutectic aligned parallel to the direction of solidification was made with R = 5 m/s and G = 250 °C/cm. The investigation shows that the lamellar eutectic has a preferred crystallographic orientation between the Ni₃ Al and Ni₇Hf₂ lamellae, i.e., (111)_NiAl//(100)_NiHf and [110]_NiAl//[010]_NiHf. The lamellar Ni₇Hf₂ did not degrade or coarsen obviously, and no harmful phase formed in the interface of Ni₃Al/Ni₇Hf₂ after long time soaking of 1100 °C/110 h. This demonstrates that the Ni₃Al+Ni₇Hf₂ lamellar eutectic has high interface thermal stability.     

A study on a Ti52Ni47Al1 shape memory alloy

The Ti₅₂Ni₄₇Al₁ alloy has 16% volume fraction Ti₂Ni particles in the B2 matrix with Ti₂Ni particles having a higher Al content than the B2 matrix. Transformation temperatures M^* and A^* of this alloy are lower than those of the Ti₅₁Ni₄₉ alloy due to the solid solution of the Al atoms. M^* and A^* decrease with increasing aging time at 400°C because the Al atoms diffuse slightly from the Ti₂Ni to the B2 matrix. The hardness increment of this alloy is more than that of the Ti₅₁Ni₄₉ alloy under the same degree of cold rolling. At the same time, M^* and A^* of this alloy can be more depressed by thermal cycling than those of the Ti₅₁Ni₄₉ alloy, especially in the first ten cycles. All of these features result from the fact that this alloy has a higher inherent hardness due to the solid solution of the Al atoms. This also causes the R-phase transformation to be more easily promoted by both cold rolling and thermal cycling in this alloy. The strengthening effects of cold rolling and thermal cycling on the M^* (Ms) temperature of this alloy follows the expression Ms = T₀–K_y, in which K values are affected by different strengthening processes. It is found that the higher the inherent hardness of the TiNi and TiNiX alloys, the higher the K values they have.     

Hydrothermal Crystallization in the Na2CO3–ZrO2–SiO2–H2O System at 500°C and 0.1 GPa: Na2ZrSi4O11, Na2ZrSi2O7, and Na4Zr2Si3O12 Phase Relations

The phases crystallizing in the Na₂CO₃–ZrO₂(cr)–SiO₂(silica gel)–H₂O system at 500°C, 0.1 GPa, ZrO₂ : SiO₂ = 1 : 2 to 1 : 6, and ZrO₂ : Na₂CO₃ = 1 : 1 to 1 : 3 are ZrSiO₄ (zircon), Na₂ZrSi₄O₁₁ (vlasovite), Na₂ZrSi₂O₇ (parakeldyshite), and Na₄Zr₂Si₃O₁₂ (NASICON). The structures of the three Na–Zr silicates are built up of the same polyhedral structural units—M octahedra and T tetrahedra—connected so as to form a three-dimensional MT framework. The crystallization behavior of these phases can be understood in terms of the matrix assembly of crystal structures from suprapolyhedral structural units in the form of cyclic clusters. The structures of Na₂ZrSi₄O₁₁ and Na₂ZrSi₂O₇ contain invariant six-polyhedron precursors of composition Na₂ M ₂ T ₄, each made up of two ZrO₆ octahedra and four SiO₄ tetrahedra. The formation of Na₄Zr₂Si₃O₁₂ involves four-polyhedron Na-containing clusters of composition Na₂ M ₂ T ₂.     

Low temperature specific heat of intercalation compounds M x TiS2 (M = 3d transition metals)

Specific heats of intercalation compounds Fe_x TiS₂ (x = 0–1/3) and M_1/4 TiS₂ (M = Mn, Fe, Co and Ni) have been measured in the temperature range 0.35–5 K using an ac calorimetry method. The temperature dependence of the specific heats for these materials is written as the sum of electronic, lattice and anomalous terms. The anomalous specific heats show a Schottky type behavior, which is due to a very small amount of localized 3d atoms with crystal-field splittings, but the remaining majority of the intercalated guest 3d atoms are regarded as having itinerant electron character that contributes to the enhancement of the density of states at the Fermi energy or of electronic specific heat coefficient.     

Plastic flow instabilities of L12 Ni3Al alloys at intermediate temperatures

The serrated plastic flow of L1₂ Ni₃Al alloys at intermediate temperatures was investigated using tensile tests. The effects of temperature, strain rate and composition were examined. The serrated plastic flow accompanied by the lowest (negative) strain-rate sensitivity was observed most strongly at 673 K and at a strain rate of 3.2 × 10^–4 s^–1. The serrated plastic flow became more significant as the alloy departed from a stoichiometric composition. The static strain aging at 673 K resulted in a reduced flow strength. The activation energy of the serrated plastic flow was estimated to be about 66 kJ/mol, which suggests that it is smaller than that for lattice diffusion of solutes in L1₂ lattices. The serrated plastic flow behavior of the Ni₃Al alloys was compared with that of the L1₂ Co₃Ti and Ni₃(Si,Ti) alloys, and is qualitatively explained on the basis of the dynamics of solutes in the core of dissociated screw dislocations.     

Fast Li+ ion conduction in Li2O–(Al2O3 Ga22O3)–TiO2–P2O5 glass–ceramics

Fast lithium ionic conducting glass-ceramics have been obtained by heat-treatment of glasses in the systems Li₂O–M₂O₃–TiO₂–P₂O₅ (M = Al and Ga). The glass–ceramics were mainly composed of LiTi₂(PO₄)₃ in which Ti⁴⁺ ions were partially replaced by M³⁺ ions. Considerable enhancement of the conductivity with the substitution of M³⁺ ions for Ti⁴⁺ ions was observed. The maximum conductivity obtained at room temperature was 1.3 × 10^–3 S cm^–1 for the aluminium system and 9 × 10^–4 S cm^–1 for the gallium system.     

TiC/Ni3Al Composites Manufactured by Self-Propagating High-Temperature Synthesis and Hot Isostatic Pressing

TiC–20 wt% Ni₃Al and TiC–40 wt% Ni₃Al composite materials were produced by self-propagating high-temperature synthesis (SHS) and hot isostatic pressing (HIP). In the SHS method the reacted powders were compacted by uniaxial pressing immediately after the reaction. The microstructure of the materials produced by SHS consisted of spherical carbides embedded in the Ni₃Al matrix, whereas the microstructure of the materials produced by HIPing was more irregular. A maximum hardness of 2010 HV₁ was measured for the material produced by HIP and a maximum fracture toughness of 10.5 MPa m^1/2 was measured for materials produced by SHS. High-temperature resistance was investigated by exposing the materials to 800°C in air for 110 h. The results obtained showed that the TiC + Ni₃Al composite materials can be recommended for use in environments consisting of oxidizing atmosphere at temperatures around 800°C where high wear resistance is required.     

VUV photoluminescence of green-emitting GdPO4:Tb,M (M: Al,Zn) synthesized by ultrasonic spray pyrolysis

We successfully synthesized (Gd_0.94−xTb_0.06M_x)PO₄ (M: Al and Zn; 0 ≤ x ≤ 0.06) green phosphors by ultrasonic spray pyrolysis and investigated their photoluminescence characteristics. The synthesized (Gd_0.94−xTb_0.06M_x)PO₄ phosphor powders showed a spherical morphology and a smooth surface. The phosphors emitted a green light due to the transition from ⁵D₄ to ⁷F_j of Tb³⁺ (j = 3–6) at 489, 544, 585, and 621 nm, respectively. The partial incorporation of Al or Zn for Gd up to x = 0.45 in (Gd_0.94Tb_0.06)PO₄ phosphors yielded a significant rise in the emission intensity. The (Gd_0.895Tb_0.06Al_0.045)PO₄ and (Gd_0.895Tb_0.06Zn_0.045)PO₄ phosphors showed 31 and 13% stronger emission intensity at 544 nm, respectively, compared to (Gd_0.94Tb_0.06)PO₄ phosphor.     

Solute diffusion in the γ′ phase of Ni based alloys

We systematically investigate the diffusion mechanisms of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition metal solutes in γ′-Ni₃Al phase using first-principles calculations. The results reveal that the diffusion of Ni-substituting solute is mainly controlled by the sublattice diffusion mechanism via Ni vacancies and the diffusion of Al-substituting solute is mostly governed by the formation of the anti-structure defects on the Ni sublattice with negligible contribution of the anti-structure bridge mechanism. The elements which occupy both the Al and Ni sites show a diffusion behavior similar to that of the Ni-substituting solutes. Our calculations show that larger atoms can move much faster than smaller atoms, which disprove the traditional view that larger atoms move slower than smaller atoms.     

Ab initio calculations of Fe–Ni clusters

The clusters of Fe, Ni, and Fe–Ni are investigated computationally using a density functional approach. The geometries of clusters are optimized under the constraint of well-defined point group symmetries at the UB3LYP/LanL2DZ level. The equilibrium geometries and binding energies are presented and discussed, together with natural populations and natural electron configurations. In addition, the binding energies of Fe_n−xNi_x clusters are found to generally decrease by successive substitutions of Ni atoms for Fe atoms. For Fe_n−xNi_x clusters, the comparisons on total energies between isomers indicate that Ni atoms energetically prefer clustering in the mixed Fe–Ni clusters. The calculations for Fe_n−xNi_x clusters show that the clustering leads to a segregation of Ni atoms from Fe atoms.     

Effects of B+ and Cr+ ion implantation on the oxidation of Ni3Al

Ni₃Al samples were implanted with different doses of 150keV B⁺ and Cr⁺ ions to modify the surface region and the high-temperature oxidation behaviour was tested. The surface layer structure was investigated by Auger electron spectroscopy, and transmission electron microscopy, secondary ion mass spectroscopy and optical microscopy before and after testing. The experimental results show that boron atoms exist in the form of interstitial atoms. No evidence was found that any new phase existed in boron implanted Ni₃Al. Implanted Ni₃Al alloy has better oxidation resistance than the unimplanted ones at 900°C. For B⁺-implanted Ni₃Al, the oxide layer is basically composed of fine-grained NiO inner layer and an a-Al₂O₃ outer layer. Boron is oxidized into B₂0₃ of comparatively larger grain size. B₂0₃ particles are enriched at grain boundaries and defects. This curtails the short-circuit transportation of oxygen and improves the oxidation resistance of Ni₃Al. Implantation with Cr⁺ and B⁺ combines the good effects of both elements and produces a remarkable improvement on the oxidation resistance. The effects of implanted elements and the possible reaction mechanisms are discussed.     

Crystal Structure of Ni5P2

It is shown by x-ray diffraction, energy-dispersive x-ray spectrometry, and electron probe microanalysis that, at 800°C, the Ni–P system contains four phosphides in the composition range 25–33 at % P: Ni₃P, Ni₅P₂, Ni₁₂P₅, and Ni₂P. The structure of Ni₅P₂ is determined by single-crystal x-ray diffraction: sp. gr. P3¯c1, new structure type, a = 0.6613(3) nm, c = 1.2311(6) nm, 347 independent hklreflections with F _hkl > 4(F _hkl), R _F = 0.0346. The key structural features of Ni₅P₂ are discussed.     

Up-hill diffusion of hafnium in Ni3Al intermetallic alloys

An up-hill diffusion of hafnium was observed during the interdiffusion of Ni-Ni₃Al(Hf, B) couples. The Hf atoms diffused backward from the original interface to the Ni₃Al end, instead of moving forward to the Ni end under the concentration gradient. The observation confirms that the Hf atoms partition preferentially to the Ni₃Al phase by substituting for Al atoms in their sublattice sites.     

VUV–UV spectroscopic properties of RE (RE = Ce, Eu and Tb)-doped KMLn(PO4)2 (M = Ca, Sr; Ln = Y, La, Lu)

The VUV excited luminescent properties of Ce³⁺, Eu³⁺ and Tb³⁺ in the matrices of KMLn(PO₄)₂ (M²⁺ = Ca, Sr; Ln³⁺ = Y, La, Lu) were investigated. The bands at about 155 nm in the VUV–UV excitation spectra are attributed to the host lattice absorption, which indicates that the optical band gap of KMLn(PO₄)₂ is about 8.0 eV. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 300–450 nm, and the energy transfer from host lattice to Ce³⁺ is efficient. For Eu³⁺-doped samples, the O²⁻–Eu³⁺ CTBs are observed to be at about 228 nm except KSrLu(PO₄)₂:Eu³⁺ (247 nm). As for Tb³⁺-doped samples, typical 4f → 5d absorption bands in the region of 175–250 nm were observed.     

Mechanical properties of Ni3(Si,Ti) polycrystals alloyed with substitutional additions

The mechanical properties of the L1₂-type Ni₃(Si, Ti) polycrystals, which were alloyed with 1–2 at% of various transition metals and also doped with boron, were investigated over a wide range of temperatures. The addition of Hf enhanced the levels of yield stress whereas the addition of Cr, Mn and Fe reduced the levels of the yield stress over a wide range of temperatures. Ni₃(Si, Ti) alloyed with Cr, Mn and Fe showed a shallow minimum in the yield stress-temperature curves. This result was correlated with the modification of the micro-cross-slip process by the additives. At low temperatures, the addition of Hf and Nb slightly reduced the elongation, while the addition of Cr, Mn and Fe improved elongation. This elongation behaviour was interpreted as the alloying effect on the intergranular cohesive strength of L1₂ ordered alloys. At high temperatures, the elongation of Ni₃(Si, Ti) alloyed with Hf showed a particularly high value. This elongation behaviour is discussed based on the alloying effect on the competition between dynamic recrystallization and cavitation at grain boundaries. The fracture surfaces exhibited a variety of fracture patterns, depending on temperature and the alloy, and were primarily well correlated with the elongation behaviour. The ductilities of most of the alloys at high temperatures were reduced by the tests in air.     

Core and valence excitations in Ni2Si

We present an electron energy loss analysis of Ni₂Si around the NiM_2,3 and NiL_2,3 edges and in the range 0–30 eV (valence excitations). An intensity reduction of the Ni L_2,3 edge of the silicide is observed and interpreted in terms of a depletion of d metal electrons at the Fermi level E_F. The reduction in the Ni M_2,3 Fano line shape with respect to that for pure nickel is also described in a similar way. The near-edge region gives information on the partial density of states in the region above E_F and is related to the structures observed in the valence band range. Particular attention was paid to the study of the Ni₂Si local structure obtained from the analysis of the extended energy loss fine structures beyond the Ni M_2,3 level.     

Methanation of CO2 with the oxygen-deficient Ni(II)-ferrite under dynamic conditions

The continuous methanation of CO₂ has been accomplished over hydrogen-reduced Ni(II)-bearing ferrite (Ni_xFe_3–xO_4–; x=0.39, > 0) in a mixed gas flow of CO₂ and H₂ at 250–375 °C. The yield and the selectivity for the methanation were larger than 50% and 95%, respectively, at the initial stage of the process. They decreased to 31% and 89%, respectively, after 6 h methanation. The innovative results can be ascribed to the use of the new material; hydrogen-reduced Ni(II)-bearing ferrite. Its formation was evinced by chemical analyses and the increase in the lattice constant; the lattice constant of the Ni(II)-bearing ferrite (a₀ 0.8375 nm) was enlarged to 0.8379 nm by hydrogen reduction. The enlarged lattice constant was not changed during the methanation. These findings suggest that the methanation occurs at the oxygen-deficient site of the hydrogen-reduced Ni(II)-bearing ferrite, as well as the formation of water by combination of the incorporated oxygens with hydrogen. The methanation consists of three steps of the elementary reaction. First, the oxygen-deficient sites are formed by hydrogen reduction; second, CO₂ is reduced to elementary carbon and two oxygen ions which are incorporated into the oxygen-deficient sites; and third, the carbon deposited on the surface of the reduced ferrite is selectively hydrogenated to CH₄.     

A Bent Tb2C2 Cluster Encaged in a CS (6)-C82 Cage: Synthesis,Isolation and X-ray Crystallographic Study

Terbium (Tb)-based dimetallofullerene, Tb₂C₈₄, has been synthesized and isolated by multistep HPLC. According to the UV-vis-NIR spectroscopic characterization and comparison with the reported analogous M₂C₈₄ (M = Sc, Y) metallofullerenes, the molecular structure of Tb₂C₈₄ is proposed to be Tb₂C₂@C_s (6)-C₈₂ featuring a metal carbide clusterfullerene (CCF) structure, which is unambiguously confirmed by X-ray crystallographic study. A detailed analysis of the crystal structure of a cocrystal of Tb₂C₂@C_s (6)-C₈₂·Ni^II(OEP)·2C₆H₆ (Ni^II(OEP) = nickel (II) octaethylporphyrin) reveals that a bent Tb₂C₂ carbide cluster is encaged rigidly inside a C_s (6)-C₈₂ cage. The C-C bond within the encaged Tb₂C₂ carbide cluster behaves as a typical C-C triple bond. The asymmetric unit in Tb₂C₂@C_s (6)-C₈₂·Ni^II(OEP)·2C₆H₆ contains two fullerene sites, for which the encaged Tb₂C₂ carbide cluster however locates at nearly the same position with one Tb atom being beneath a hexagon and another Tb atom pointing to the conjunction of two adjacent hexagons.     

Lumine scence of Ce3+ and Tb3+ in M3(PO4)2 (M = Sr,Ba)

The luminescence properties of Ce³⁺- and Tb³⁺-activated M₃(PO₄)₂ (M = Sr,Ba) are reported. It is shown that only a small part (⩽ 0.3%) of the M2+ ions can be replaced by Ln³⁺ ions. At higher concentrations a second phase, M₃Ln(PO₄)₃, is formed. The luminescence properties are due to these two different phases.