The search for novel superhard and incompressible materials on the basis of higher borides of <Emphasis Type="Italic">s</Emphasis>, <Emphasis Type="Italic">p</Emphasis>, <Emphasis Type="Italic">d</Emphasis> metals

The state of the art in the search for novel superhard and (or) incompressible materials on the basis of higher borides of s, p, d metals has been briefly reviewed. The information has been considered about experimental and theoretical studies of the following groups of borides: diborides of 4d, 5d heavy metals (Tc, Ru, Rh, Re, Os, and Ir), hexagonal tetraborides with the WB₄-type structure, and AMB₁₄ borides (where A, M are s, p metals) as well as of a number of related systems.     

Compounds WAl<Subscript>4</Subscript>, WAl<Subscript>3</Subscript>, W<Subscript>3</Subscript>Al<Subscript>7</Subscript>, and WAl<Subscript>2</Subscript> and Al-W-N combustion products

X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl₄ are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl₄ are monoclinic, with a ₀ = 5.272 Å, b ₀ = 17.770 Å, c ₀ = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl₄ are monoclinic, with a ₀ = 5.465 Å, b ₀ = 12.814 Å, c ₀ = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl₂ and W₃Al₇, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl ₂ ^′ : orthorhombic, a ₀ = 5.793 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. WAl ₂ ^″ : orthorhombic, a ₀ = 11.586 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. W₃Al ₇ ^′ : orthorhombic, Pmm2, a ₀ = 6.225 Å, b ₀ = 4.806 Å, c ₀ = 4.437 Å. W₃Al ₇ ^″ : orthorhombic, Pmm2, a ₀ = 12.500 Å, b ₀ = 4.806 Å, c ₀ = 8.874 Å. The new phase WAl₃: triclinic, P1, a ₀ = 8.642 Å, b ₀ = 10.872 Å, c ₀ = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.     

The Activation Energy <Emphasis Type="Italic">U</Emphasis>(<Emphasis Type="Italic">T</Emphasis>,<Emphasis Type="Italic">H</Emphasis>) in Y-based Superconductors

Based on the Arrhenius equation, a method to calculate the activation energy from the resistance transition is proposed for high temperature superconductors. This method is applied to the Y-based superconductors. The activation energy is found to be U(T,H)∼(1−T/T _c )^4.8(H/H ₀)^−3.8 of YBCO crystal, and U(T,H)∼(1−T/T _c )^3.3(H/H ₀)^−2.2 of Er doped MTG YBCO crystal, respectively. With the obtained activation energy U(T,H), the lower part of the experimental curve ρ(T,H) and its derivative can be reproduced.      

Nuclear-Physical Properties of <Superscript>105, 106<Emphasis Type="Italic">m</Emphasis>, 110<Emphasis Type="Italic">m</Emphasis></Superscript>Ag

The nuclear-physical properties of the nuclides ^{105, 106m,110m} Ag, which are concomitant nuclides in reactor production of ¹⁰³Pd and ¹⁰⁹Cd used in metrology of ionizing radiations and in nuclear medicine, were studied. The following quantities were determined by semiconductor X-ray and γ-ray spectrometry: the parameter K _α/K _β of X-ray K radiation accompanying the decay of ^110m Ag and the intensities of γ-ray quanta with the energies of 280 and 345 keV (¹⁰⁵Ag); 430, 451, and 512 keV (^106m Ag); 658, 764, 885, 938, and 1384 keV (^110m Ag). The half-lives determined using the above γ-ray lines are as follows: T _1/2(¹⁰⁵Ag) = 41.2 ± 0.1 days; T _1/2(^106m Ag) = 8.30±0.07 days. The errors are given for 95% confidence level.__________Translated from Radiokhimiya, Vol. 47, No. 1, 2005, pp. 31–34.Original Russian Text Copyright © 2005 by Popov, Zakharova, Sadulin, Andreev, Pakhomov.     

Recombination luminescence of CaI<Subscript>2</Subscript> crystals activated with Eu<Superscript>2+</Superscript>, Gd<Superscript>2+</Superscript>, Tl<Superscript>+</Superscript>, Pb<Superscript>2+</Superscript>, and Mn<Superscript>2+</Superscript>

The spectral characteristics of thermostimulated luminescence, steady-state roentgenoluminescence and photostimulated luminescence (PSL) buildup and decay kinetics, and the effect of IR irradiation on the roentgenoluminescence yield and glow curves of CaI₂:Eu²⁺, CaI₂:Gd²⁺, CaI₂:Tl⁺, CaI₂:Pb²⁺, CaI₂:Mn²⁺, and CaI₂: Pb²⁺, Mn²⁺ crystals grown by the Bridgman-Stockbarger method have been studied in the temperature range 90–295 K. Coupled with earlier data, the present results on the influence of oxygen and hydrogen impurities on the spectral characteristics of CaI₂ indicate that the activation of calcium iodide with Eu²⁺, Gd²⁺, Tl⁺, Pb²⁺, and Mn²⁺ leads to the formation of cation impurity-native defect complexes, which act as carrier traps and are responsible for the thermostimulated luminescence in the range 150–295 K. IR exposure after 90-K x-ray excitation gives rise to flash PSL and influences the thermostimulated luminescence light sum. The nature of the emission and trapping centers involved and the mechanisms of recombination luminescence excitation in the crystals are discussed.     

Solubility of UF<Subscript>4</Subscript>, ThF<Subscript>4</Subscript>, and CeF<Subscript>3</Subscript> in a LiF-NaF-KF melt

The solubility of UF₄, ThF₄, and CeF₃ in fluoride melt of the composition 45 mol % LiF-12 mol % NaF-43 mol % KF in the temperature interval 773–973 K was determined. The solubility of the fluorides increases with an increase in the melt temperature. The CeF₃ solubility in the LiF-NaF-KF system is high: 19.9 mol % at 923 K and 23.3 mol % at 973 K. Experimental data on the solubility of CeF₃ (PuF₃ imitator) were compared to the calculated data on the PuF₃ solubility in the LiF-NaF-KF melt. The results showed that CeF3 can be considered as PuF3 imitator in FLINAK melt.     

Ultrasound velocity and absorption in BeO,Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, ZrO<Subscript>2</Subscript>, and SiO<Subscript>2</Subscript> ceramics

We have measured the ultrasound velocity and absorption in BeO, Al₂O₃, ZrO₂, and SiO₂ ceramics. The results indicate that the ultrasound velocity in oxide ceramics depends on the nature of the basic oxide component, the density of the material, and the preferential alignment of the grains. The ultrasound velocity in ceramics is shown to correlate with their thermal conductivity: with increasing thermal conductivity, the ultrasound velocity increases. The ultrasound absorption in oxide ceramics decreases with decreasing temperature, and vice versa, with increasing temperature, the ultrasound attenuation coefficient increases.     

Electrical Conductivity of Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> Crystals Doped with Ca<Superscript>2+</Superscript>, Pb<Superscript>2+</Superscript>, Sr<Superscript>2+</Superscript>, and Ba<Superscript>2+</Superscript>

Single crystals of Bi₂WO₆ (a layered perovskite-like compound) doped with Ca²⁺, Pb²⁺, Sr²⁺, and Ba²⁺ on the Bi³⁺ site are grown, and their oxygen ionic conductivity is measured along the polar axis. The intrinsic conductivity of the doped crystals differs insignificantly from the conductivity of undoped Bi₂WO₆, indicating that the oxygen ions in the Bi₂O₂ layers contribute little to the oxygen ionic conductivity of the crystals. The sharp change in the activation energy for conduction at 600°C attests to a transition from one conduction mechanism to another in going from low to high temperatures.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 863–865.Original Russian Text Copyright © 2005 by Kharitonova, Voronkova, Yanovskii.     

Thermochemical and luminescent properties of RbVO<Subscript>3</Subscript>, CsVO<Subscript>3</Subscript>, and Rb<Subscript>0.5</Subscript>Cs<Subscript>0.5</Subscript>VO<Subscript>3</Subscript>

RbVO₃, CsVO₃, and Rb_0.5Cs_0.5VO₃ have been synthesized by the Pechini process. The vanadates have an orthorhombic structure (sp. gr. Pbcm), melt congruently in the range 650–530°C, and undergo a reversible phase transition in the range 520–340°C. We have determined the onset temperatures and end points of the transformations at a temperature scan rate of 3°C/min and their enthalpies, and measured the photo-, roentgeno-, and cathodoluminescence and diffuse reflectance spectra of the vanadates. The luminescence spectra each are well fitted with three pseudo-Voigt functions. CsVO₃ has the highest integrated emission intensity. The emission intensity of the Rb_0.5Cs_0.5VO₃ solid solution is lower than that of the simple vanadates because of the optical absorption around its intrinsic luminescence band. This may be due to the presence of stable vacancy-type structural defects in Rb_0.5Cs_0.5VO₃.     

Electron transport behaviors across single grain boundaries in <Emphasis Type="Italic">n</Emphasis>-type BaTiO<Subscript>3</Subscript>, SrTiO<Subscript>3</Subscript> and ZnO

In some electroceramic materials, their unique electrical properties are due to potential barriers, i.e., double Schottky barriers (DSBs), formed at grain boundaries. So far, some researchers have revealed that the electrical properties of DSB are closely related to grain boundary characters, especially grain boundary coherency. For example, highly coherent boundary does not give PTCR or varistic property, while random types exhibit clear resistivity jump or abrupt current increment. Therefore, a concept of grain boundary design will be required for future device manufacturing, even in bulk materials. But it has not been clarified yet why the electron transport behaviors depend on them. In order to address this question, it is necessary to carry out a systematic experiment focusing on single grain boundaries using well-defined bicrystals.In the present study, we have summarized our studies with a special interest in electron transport behavior across single grain boundaries for n-type BaTiO₃, SrTiO₃ and ZnO.     

Valence-band offsets of InGaZnO<Subscript>4</Subscript>, LaAlO<Subscript>3</Subscript>, and SrTiO<Subscript>3</Subscript> heterostructures explained by interface-induced gap states

The intrinsic interface-induced gap states (IFIGS) which derive from the virtual gap states of the complex band structure are the fundamental mechanism that determines the band-structure lineup at semiconductor interfaces. The valence-band offsets of heterostructures are composed of a zero-charge-transfer term and an electrostatic-dipole contribution which are given by the difference of the p-type branch-point energies of the IFIGS and of the electronegativities, respectively, of the two semiconductors involved. The valence-band offsets of InGaZnO₄, LaAlO₃, and SrTiO₃ heterostructures are quantitatively and consistently explained by the IFIGS-and-electronegativity concept. The analysis of the experimental InGaZnO₄, LaAlO₃, and SrTiO₃ data yields the p-type branch-point energies as 2.37?±?0.18 eV, 2.59?±?0.13 eV, and 2.86?±?0.14 eV, respectively.     

Toroidal spin ordering in BiFeO<Subscript>3</Subscript>, GaFeO<Subscript>3</Subscript>, and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in the Faddeev model with a magnetic field

The Fadeev model is used for describing the recently discovered toroidal spin ordering in piezoelectric and ferrimagnetic GaFeO₃ and piezo- and magnetoelectric Cr₂O₃ and BiFeO₃. A stable toroidal solution of the Faddeev model with the topological charge Q= 1 in an external homogeneous magnetic field was obtained using the trial function method. The energy of a toroid as a function of its radius (R) was determined at various values of the external magnetic field (H). It was shown that the energy minimum is shifted toward smaller R’s with an increase in H. At a critical field value, the torus collapses so that the local spin structure disappears. It is suggested to use magnetic field for controlling the torus size in multiferroics, promising materials of spintronics.     

Programmed water-induced shape-memory of bioabsorbable poly(<Emphasis Type="SmallCaps">d</Emphasis>,<Emphasis Type="SmallCaps">l</Emphasis>-lactide): activation and properties in physiological temperature

This study reports of the novel water-induced shape-memory of bioabsorbable poly(d,l-lactide). We have developed an orientation-based programming process that generates an ability for poly(d,l-lactide) to transform its shape at 37°C in an aqueous environment without external energy and to adapt to a predefined stress level by stress generation or relaxation. In this orientation-programming process, polymer material is deformed and oriented at an elevated temperature and subsequently cooled down while retaining its deformed shape, tension, and polymer chain entanglements. At body temperature and in an aqueous environment, the shape-memory is activated by the plasticizing effect of water molecules diffused into the polymer matrix causing an entropy-driven directed relaxation of oriented and preloaded polymer chains. This plasticizing effect is clearly seen as a decrease of the onset glass transition temperature by 10–13°C. We found that γ-irradiation used for sterilizing the orientation-programmed materials strongly affected the shape-recovery rate, but not the recovery ratio. Both non-γ-irradiated and γ-irradiated sample materials showed excellent shape-recovery ratios during a ten-week test period: 94 and 97%, respectively. The orientation-programmed materials generated a predefined load in a 37°C aqueous environment when their shape-recovery was restricted, but when external tension was applied to them, they adapted to the predefined level by stress relaxation. Our results show that functionality in terms of shape-memory can be generated in bioabsorbable polymers without tailoring the polymer chain structure thus shortening the time from development of technology to its utilization in medical devices.     

Poly (<Emphasis Type="SmallCaps">d</Emphasis>,<Emphasis Type="SmallCaps">l</Emphasis>-lactide)/nano-hydroxyapatite composite scaffolds for bone tissue engineering and biocompatibility evaluation

Biodegradable polymer/bioceramic composite scaffolds can overcome the limitations of conventional ceramic bone substitutes such as brittleness and difficulty in shaping. However, conventional methods for fabricating polymer/bioceramic composite scaffolds often use organic solvents (e.g., the solvent casting and particulate leaching (SC/PL) method), which might be harmful to cells or tissues. In this study, Poly (d,l-lactide)/nano-hydroxyapatite (PDLLA/NHA) composites were prepared by in-situ polymerization, and highly porous scaffolds were fabricated using a novel method, supercritical CO₂/salt-leaching method (SC CO₂/SL). The materials and scaffolds were investigated by scanning electronic microscopy (SEM), transmission electronic microscopy (TEM) and gel permeation chromatography (GPC). GPC showed that the molecular weight of composites decreased with increase of NHA content. However, the water absorption and compressive strength increased dramatically. The SEM micrographs showed that the scaffolds with pore size about 250 μm were obtained by controlling parameters of SC CO₂/SL. The biocompatibility of PDLLA/NHA porous scaffolds were evaluated in vitro and in vivo. The evaluation on the cytotoxicity were carried out by cell relative growth rate (RGR) method and cell direct contact method. The cytotoxicity of these scaffolds was in grade I according to ISO 10993-1. There was no toxicosis and death cases observed in acute systemic toxicity test. And histological observation of the tissue response (1 and 9 weeks after the implantation) showed that there are still some slight inflammation responses.     

Reactions of V<Subscript>2</Subscript>O<Subscript>5</Subscript>, Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>, and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with AlN

Reactions of vanadium, niobium, and tantalum pentoxides with aluminum nitride have been studied using X-ray diffraction. At temperatures from 1000 to 1600°C, we have identified various V, Nb, and Ta nitrides. The composition of the niobium and tantalum nitrides depends on the reaction temperature. The tendency toward nitride formation becomes stronger in the order V₂O₅ < Ta₂O₅ < Nb₂O₅.     

Determination of <Emphasis Type="Italic">P</Emphasis>,<Emphasis Type="Italic">T</Emphasis>-conditions developed at high-temperature shock compression of silicon nitride in planar recovery ampoules

We performed experiments on shock compression up to pressures of 36 and 50 GPa of mixed samples of silicon nitride and potassium bromide, placed between copper plates that serve as walls of a recovery ampoule. For comparison, similar experiments were carried out by the conventional compression of a mixture of silicon nitride and copper powder. The loading of the samples was fulfilled by means of aluminum flyers accelerated by products of explosion to a few kilometers per second. The pressure profiles prior to a shock wave entering the sample and after its runout were measured with the use of manganin sensors. It is found that for the configurations of the experimental assembly used, the pressure in the samples, accumulated by circulating the shock wave, reaches the desired value before unloading. Based on estimates of the rate of heat transfer between the components, it is shown that thermal equilibrium can be set during the existence of high pressure in the mixed samples. Within the framework of the single-temperature medium model, the equations of state of the samples are derived, and the temperatures of their shock compression are calculated. Using these equations, we performed numerical simulations that showed good agreement with the experimental data.     

Body distribution of poly(<Emphasis Type="SmallCaps">d</Emphasis>,<Emphasis Type="SmallCaps">l</Emphasis>-lactide-<Emphasis Type="Italic">co</Emphasis>-glycolide) copolymer degradation products in rats

Poly (d,l-lactide-co-glycolide) (PLGA) copolymers are among the few synthetic polymers approved for human use, but the biocompatibility of PLGA-derived oligomers and particles remains questionable. Here, high molecular weight PLGA (Mw = 32,000) was radiolabeled with ¹²⁵I in chloroform solution, and the body distribution of PLGA copolymer degradation products was examined following subcutaneous implantation of round ¹²⁵I-PLGA films on the back of Sprague Dawley rats. Autoradiographic images of the PLGA implant taken at 2, 4, 6, 8, 10, and 12 weeks revealed that the central portion of the film degraded much more rapidly than the marginal portions. Examination of the body compartment distribution at these time points revealed that over one-half of the radioactivity was recovered from skin. The remaining radioactivity was concentrated in the blood, liver, and kidneys. Radioactivity steadily appeared in the blood and remained elevated up to 12 weeks after implantation, while the liver to kidney distribution began to decrease after 6 weeks. Cumulatively, these results indicate that the clearance of degraded particles and fragments from the implantation site is extremely delayed. Moreover, the degraded particles and fragments were selectively concentrated in the liver and kidneys, following release of degraded products into the bloodstream from the implantation site.     

Microemulsion-based synthesis and luminescence of nanoparticulate CaWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, CaWO<Subscript>4</Subscript>:Tb,and CaWO<Subscript>4</Subscript>:Eu

Blue emitting CaWO₄ and greenish blue emitting ZnWO₄ nanoparticles are synthesized via microemulsion techniques applying a cationic (CTAB) as well as a non-ionic surfactant (TritonX-100). The influence of the surfactant on particle size and shape is studied. Scanning electron microscopy and dynamic light scattering confirm the presence of uniform and non-agglomerated nanoparticles, 60–80 nm in diameter. Photoluminescence confirms [WO₄]²⁻-related broad-band emission with its maximum at 440 nm (CaWO₄) and 420 nm (ZnWO₄). The highest quantum yield (QY) is observed for nanoscaled CaWO₄ with a value of 23–25%. Doping of CaWO₄ and ZnWO₄ with Tb³⁺ and Eu³⁺ was performed and in the case of CaWO₄:Tb and CaWO₄:Eu and results in the emission of green and red light, again with comparably high QYs (17–19%).     

Histological study of surface modified three dimensional poly (<Emphasis Type="SmallCaps">d</Emphasis>,<Emphasis Type="SmallCaps"> l</Emphasis>-lactic acid) scaffolds with chitosan in vivo

Biocompatibility and tissue regenerating capacity are essential for biomaterials that used in tissue engineering. The aim of this study was to histologically assess the tissue reactions and bone conductivities of surface modified three dimensional (3-D) poly (d, l-lactic acid) (PDLLA) scaffolds, which were coated with chitosan via a physical entrapment method. The native PDLLA scaffold was prepared via thermally induced phrase separation technique and was characterized by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). Osteocalcin assay, a method to evaluate the bone formation potential, has shown that the osteocalcin production in chitosan-modified 3-D PDLLA scaffold group was significantly higher (p < 0.05) than that of in control. The tissue reactions and bone conductivities between surface modified PDLLA and native PDLLA scaffolds were evaluated using a rabbit radialis defect model in vivo and compared at different implantation intervals (2, 4, 8 and 12 weeks). The histological results have shown a higher bone formation potential and better biocompatibility of chitosan-modified 3-D PDLLA scaffolds as compared with the control group scaffolds.     

Synthesis and properties of AgTi<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>-based NASICON-type phosphates doped with Nb<Superscript>5+</Superscript>, Zr<Superscript>4+</Superscript>, and Ga<Superscript>3+</Superscript>

We have synthesized materials based on a silver titanium phosphate with partial substitution of tri-, tetra-, or pentavalent cations for titanium: Ag_1±x Ti_2−x M _x (PO₄)₃ (M = Nb⁵⁺, Ga³⁺) and AgTi_2−x Zr _x (PO₄)₃. The materials have been characterized by X-ray diffraction and impedance spectroscopy and have been shown to have small thermal expansion coefficients. Their ionic conductivity has been determined. Silver ions in these materials are difficult to replace with protons.