Bridgman growth and defects of Nd<Superscript>3+</Superscript> : Sr<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> laser crystals

Nd³⁺ : Sr₃Ga₂Ge₄O₁₄ crystals have been grown by the modified Bridgman method. The growth defects, such as striations, scattering particles and dislocations were investigated. Some featherlike striations were observed in as-grown crystals. EPMA analysis suggested that these inclusions were caused by the segregation of Nd₂O₃ from the melt. Chemical etching results showed that the dislocation density was in the range of 10³ ∼ 10⁵/cm².     

Laser multi-layer cladding of Zr<Subscript>65</Subscript>Al<Subscript>7.5</Subscript>Ni<Subscript>10</Subscript>Cu<Subscript>17.5</Subscript> amorphous alloy on magnesium substrates

A coating about 3-mm thick of the amorphous alloy, Zr₆₅Al_7.5Ni₁₀Cu_17.5 was fabricated on magnesium substrates using the technique of laser multi-layer cladding protected under an atmosphere of argon gas. The coating exhibited a graded microstructure, which could be generally categorized into three classes: an amorphous phase, an amorphous–nanocrystalline composite, and one which is predominantly crystalline. Formation of the latter two was due to the reheating effect of the laser cladding process. With regard to properties, the microhardness and the wear resistance of the composite material were both higher than that of the monolithic amorphous material; both materials showed excellent corrosion resistance in a 3.5% NaCl solution.     

Effect of Melt Composition on the Luminescent Properties of Czochralski-Grown Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> Single Crystals

Li₂B₄O₇ (LTB) single crystals are grown from stoichiometric and B₂O₃-enriched (1.6 and 2.4 mol %) melts. At large B₂O₃ excesses, the thermoluminescence intensity in the crystals is markedly higher, attesting to an increased density of structural defects. LTB crystals grown from B₂O₃-enriched melts may contain zones enriched in two types of defects. Defects of one type result from the breaking of bonds involving bridging oxygens, without significant changes in the position of neighboring atoms. Defects of the other type appear at large B₂O₃ excesses in the melt, which enable more significant changes in the structure of the boron-oxygen framework, leading to the formation of nanoscale regions differing in composition from LTB. The present results may be helpful in assessing the quality of LTB single crystals and in optimizing the crystal growth of LTB.     

Effect of cadmium,silver, and tellurium doping on the properties of single crystals of the layered compounds PbBi<Subscript>4</Subscript>Te<Subscript>7</Subscript> and PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript>

We have studied the effects of doping and excess Te on the thermoelectric properties of single crystals of the layered ternary compounds PbBi₄Te₇ and PbSb₂Te₄. X-ray diffraction characterization has shown that the crystals have highly disordered structures. The nature of the possible point defects in the compounds has been analyzed. The silver atoms in Ag-doped PbBi₄Te₇ seem to reside in the van der Waals gaps between the slabs. The introduction of excess Te is shown to markedly reduce the lattice thermal conductivity of both compounds.     

Preparation of B<Subscript>4</Subscript>C-ZrB<Subscript>2</Subscript>-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

The nature of optical pumping losses in low-threshold LiF(F<Subscript>2</Subscript>, F<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack>) crystal microlasers

The character of optical losses in Li(F₂, F ₃ ⁺ ) crystals were studied for the first time by methods of optical absorption spectroscopy and X-ray diffraction. It is suggested that centers absorbing at 420 nm represent quasi-metallic defects of the F₂F_L type.     

Transition from crystal-like to glass-like behavior of thermal conductivity in the Sr<Subscript>0.16</Subscript>Ba<Subscript>0.54</Subscript>La<Subscript>0.30</Subscript>F<Subscript>2.30</Subscript> solid solution

Using a steady-state axial flow technique, we have measured the thermal conductivity of single crystals of congruently melting SrF₂-BaF₂-LaF₃ solid solutions from 50 to 300 K: Ba_0.66Sr_0.34F₂ (minimum in the melting curves), Ba_0.70La_0.30F_2.30 (maximum in the melting curves), and Sr_0.16Ba_0.54La_0.30F_2.30 (saddle point on the melting surfaces). The thermal conductivity of the isovalently doped fluorite solid solution is typical of crystals containing point defects. Heterovalent substitution leads to glass-like behavior of thermal conductivity.     

Mechanochemical synthesis and characterization of Ni<Subscript>25</Subscript>Te<Subscript>75</Subscript> nanocrystalline alloy

Ni₂₅Te₇₅ nanocrystalline alloy containing trigonal NiTe₂ and Te nanocrystals was prepared through mechanochemical processing of pure elemental tellurium and nickel powders in argon atmosphere. The Ni₂₅Te₇₅ samples processed from 3 h to 30 h milling times were characterized by X-ray powder diffraction, transmission electron microscopy, magnetization and Raman spectroscopy. Trigonal NiTe₂ crystals with average size of 16 nm can be obtained after only 3 h of processing time. For longer milling times, the trigonal NiTe₂ phase becomes majority (about 70% with 30% for nanometric Te and no pure Ni was detected) and its average crystallite size slightly increases to 20 nm. Transmission electron microscopy images and electron diffraction patterns confirm the nanometric size of the crystalline domains in the agglomerated particles. The magnetic properties of the Ni₂₅Te₇₅ powders are dependent on synthesis time, suggesting a paramagnetic behavior mainly associated with the NiTe₂ nanophase. Raman spectra showed peaks that can be associated with unreacted Te and tellurium oxides modes, but it also showed several modes that can be attributed to trigonal NiTe₂ nanophase. The high-pressure experiments showed no phase transitions for NiTe₂ up to 17 GPa and Te phase transitions from form I to forms II and III occurred simultaneously at 4.5 GPa, remaining up to 12 GPa; after that, only reflections of Te-III and the NiTe₂ were observed. All the phase transitions observed with pressure are reversible after decompression. The bulk modulus determined from the least-squares fit of first-order Murnaghan equation of states is 110 GPa for the NiTe₂ nanophase and 28 GPa for Te-I.     

X-ray Diffraction Study of the Structure and Defect System of Nominally Pure and Er<Superscript>3+</Superscript>- and Ce<Superscript>3+</Superscript>-Activated (Na<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>)MoO<Subscript>4</Subscript> Crystals

Nominally pure and activated (Er³⁺ and Er³⁺ + Ce³⁺) sodium lanthanum molybdate single crystals grown by the Czochralski technique from the (Na_0.5La_0.5)MoO₄ melt in different environments and then oxidized in air at 100°C are characterized in detail by x-ray diffraction (lattice parameters and structure refinement by the Rietveld method and single-crystal techniques). The results confirm that (Na_0.5La_0.5)MoO₄ crystallizes in a tetragonal scheelite structure (sp. gr. I₁/a). The crystals (especially unannealed crystals grown in neutral atmosphere) are shown to contain oxygen vacancies. In addition, some of the samples contain Mo vacancies. The Er³⁺ distribution over some of the activated crystals is highly inhomogeneous. As a result, the crystals contain Er-enriched zones with a distorted scheelite structure.     

Elastic properties of Li<Subscript>2</Subscript>Zn<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The complete elastic modulus matrix of Li₂Zn₂(MoO₄)₃ single crystals has been measured for the first time. The sound velocity has been measured in different directions of the crystals by a pulse-phase method. The measurement results have been used to calculate elastic moduli. The sound velocity has been calculated in the three main crystallographic planes of the crystals.     

Reactive Stresses in Ni<Subscript>49</Subscript>Fe<Subscript>18</Subscript>Ga<Subscript>27</Subscript>Co<Subscript>6</Subscript> Shape-Memory-Alloy Single Crystals

The reactive stresses induced in Ni₄₉Fe₁₈Ga₂₇Co₆-alloy single crystals during martensitic transformations with a limited possibility of shape-memory-strain recovery have been experimentally studied. The data on these crystals are compared with the results obtained previously for Cu–Al–Ni, Ni–Ti, and Ni?Fe–Ga crystals. The potential of application of the Ni₄₉Fe₁₈Ga₂₇Co₆ single crystals in designing drives and power motors is demonstrated.     

Microstructure and properties of SiC-whisker-reinforced Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics with calcium aluminate additions

Silicon-nitride-based ceramics containing Al₂O₃-CaO sintering aids and reinforced with silicon carbide whiskers have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and their microstructure, phase composition, and mechanical properties have been studied. The results indicate that the Si₃N₄ ceramic containing 15 wt % calcium aluminate additions and 10 wt % SiC fibers has a dense microstructure with a uniform distribution of skeletal and dendritic silicon carbide crystals. The observed variations in the morphology of the crystals are tentatively attributed to the secondary crystallization of silicon carbide from the eutectic calcium aluminate melt during cooling.     

Proton Conductivity and Thermal Properties of Ba(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript>

We have grown single crystals of barium dihydrogen phosphate and studied its thermal transformations during heating to 500°C and its electrotransport properties. Ba(H₂PO₄)₂ (Pccn) has been shown to undergo no phase transitions up to its dehydration temperature. The thermal decomposition of Ba(H₂PO₄)₂, accompanied by dehydration, involves two steps, with maximum rates at ~265 and 370°C, and results in the formation of barium dihydrogen pyrophosphate and barium metaphosphate, respectively. The total enthalpy of the endothermic dehydration events is–244.6 J/g. Using impedance spectroscopy, we have studied in detail the proton conductivity of polycrystalline and single-crystal Ba(H₂PO₄)₂ samples in a controlled atmosphere. Adsorbed water has been shown to have a significant effect on the proton conductivity of Ba(H₂PO₄)₂ up to 130°C. The proton conductivity of the Ba(H₂PO₄)₂ single crystals has been shown to be anisotropic. The conductivity anisotropy correlates with specific structural features of the salt. Higher conductivity values, 3 × 10^–9 to 2 × 10^–7 S/cm in the range 60–160°C, have been observed in the [100] crystallographic direction, exceeding the conductivity along [010] by an order of magnitude. The activation energy for proton conduction is 0.80 eV.     

Effect of heating rate on the microstructural and magnetic properties of nanocrystalline Fe<Subscript>81</Subscript>Si<Subscript>4</Subscript>B<Subscript>12</Subscript>P<Subscript>2</Subscript>Cu<Subscript>1</Subscript> alloys

The effect of heating rate on the structural and magnetic properties of the nanocrystalline Fe₈₁Si₄B₁₂P₂Cu₁ alloy has been investigated. Amorphous Fe₈₁Si₄B₁₂P₂Cu₁ alloy was annealed at 753 K for 180 s at different heating rates ranging from 0.05 to 5 K/s in protective argon atmosphere. The structural and magnetic properties of the as-quenched and annealed alloys were studied using X-ray diffractometer (XRD), differential scanning calorimeter (DSC), vibrating sample magnetometer (VSM), and B–H loop tracer, respectively. Amorphous precursor prepared by industry-grade raw materials is obtained. The increase of heating rate is found to be significantly effective in decreasing the grain size of α-Fe(Si) phase, but the grain size increases at higher heating rate. The volume fraction of α-Fe(Si) phase shows a monotonic decrease with the increase of the heating rate. The coercivity H _c markedly decreases with increasing heating rate and exhibits a minimum at the heating rate of 0.5 K/s, while the saturation magnetization, M _s, shows a slight decrease. These results suggest that the effect of heating rate on H _c and M _s is originated from the changes of grain size and the volume fraction of α-Fe(Si) phase.     

Physicochemical properties of La<Subscript>3</Subscript>Ga<Subscript>5.5</Subscript>Ta<Subscript>0.5</Subscript>O<Subscript>14</Subscript>

We report the optical and dielectric properties and microhardness of La₃Ga_5.5Ta_0.5O₁₄ lanthanum gallium tantalate (langatate) crystals. Analysis of the optical transmission spectra of the crystals in relation to their refined compositions indicates that the bands at 34000–35000 and 27000–28000 cm⁻¹ are due to lanthanum and oxygen vacancies, respectively, and that the band at 20000–21000 cm⁻¹ is responsible for the yellow (orange) coloration of the crystals. Their resistivity and microhardness decrease with increasing oxygen vacancy concentration.     

Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Thermal conductivity of LaB<Subscript>6</Subscript> and SmB<Subscript>6</Subscript> in the range 6–300 K

The thermal conductivity of LaB₆ and SmB₆ crystals has been measured from 6 to 300 K. The electronic and lattice contributions to the thermal conductivity of SmB₆ and the temperature-dependent phonon mean free path in this hexaboride have been evaluated. The results indicate strong phonon scattering by structural defects, presumably those typical of hexaborides.     

Photoluminescence and raman spectra of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Sm<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> crystals

We have measured the photoluminescence and Raman spectra of (Ga₂S₃)_0.95(Sm₂O₃)_0.05 crystals and identified the mechanism of the energy transfer from the host to the rare-earth ion and the vibrational modes of the constituent atoms.     

Preparation and properties of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> partially stabilized ZrO<Subscript>2</Subscript> crystals

We have studied the effect of composition and growth conditions on the structure and properties of 2.5–5 mol % Y₂O₃ partially stabilized ZrO₂ crystals grown by directional solidification in a cold-wall crucible. The phase composition and density of the crystals have been determined. The crystals are shown to be uniform in composition, with local changes in Y₂O₃ content within ±0.5 mol %. The dimensions and quality of the single crystals are influenced by the growth conditions.     

Anisotropic thermoelectric properties of the layered compounds PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript> and PbBi<Subscript>4</Subscript>Te<Subscript>7</Subscript>

Single crystals of the ternary layered compounds PbSb₂Te₄ (p-type) and PbBi₄Te₇ (n-type) have been grown by Czochralski pulling with melt supply through a floating crucible. The in-plane and out-of-plane thermoelectric power, electrical conductivity, and thermal conductivity of the PbSb₂Te₄ and PbBi₄Te₇ crystals and related alloys have been measured in the temperature range 85–340 K. The results attest to a significant thermoelectric anisotropy in the crystals, especially in the p-type material PbSb₂Te₄.