Ta Crystals Research Articles

First-principles calculation of chalcogen-doped Sr2M2O7 (M=Nb and Ta) for visible light photocatalysis

Doped Sr2M2O7 (M ​= ​Nb and Ta) crystals with various chalcogen X (X ​= ​S, Se, and Te) concentrations have been studied by using first-principles calculations combined with hybrid functional (HSE06). Our results indicate that the more stable doping site of X generally induces the less distortion of the lattice. The calculated formation energies reveal that the doping becomes more difficult in the order S ​< ​Se ​< ​Te. Some localized X np states are located above the VBM of pure semiconductors, which results in the narrowing of band gaps. For Sr2Nb2O7, an increase of the X concentration has negative effect on the narrowing of band gap since the X np states moves towards the VBM of pristine semiconductor. As to Sr2Ta2O7, the higher doping level of X elements leads to the further narrowing of energy gap compared with the lower doping level. The formation of Sr2M2X7 (X ​= ​S and Se) by replacing all the O atoms in crystal structures by chalcogens extends the absorption edges to near-infrared region and loses the abilities to oxidize water. Our findings demonstrate that S-doped Sr2Nb2O7, Se-doped Sr2Nb2O7 and Se-doped Sr2Ta2O7 at a higher doping level, and Te-doped Sr2Ta2O7 at a lower doping level can catalyze overall splitting water reaction under visible light irradiation. Furthermore, they possess stronger capabilities of photo-oxidation and photo-reduction than pristine semiconductors. Therefore, these systems mentioned above are potential visible-light-driven photocatalytic materials, which are worthy of further experimental investigations.

A predictive strain-gradient model with no undetermined constants or length scales

A general meso‑scale (GM) crystal plasticity (CP) model was developed that accounts for lower-order (strain hardening) and higher-order (internal stress) effects of geometrically necessary dislocations (GNDs). It is predictive: no arbitrary parameters or length scales were invoked and no ad hoc numerical techniques were employed. It uses general stress field equations for GND content and a novel harmonization technique to enforce consistency of elastic long-range singular defect fields with applied elastic-plastic fields. The model facilitates implementation in commercial finite element programs without requiring special elements, special boundary conditions, or access to element shape functions. GM simulations confirmed, with improved accuracy, previously published predictions of the Hall-Petch effect, Bauschinger effect, and anelasticity. Previously unpredicted phenomena were also predicted: anelasticity and hysteresis for single Ta crystals and strain-hardening stagnation. The internal stresses (higher-order effect) dominate at large length scales, while at small length scales, the GND density hardening (lower-order effect) dominates. GM predicts that strain heterogeneity and consequent GND internal stresses are important factors in anelasticity.

Top-seeded solution growth and morphology change of RbTiOPO4:Ta single crystal

The RbTiOPO4:Ta single crystal with dimensions of 4 mm × 31 mm × 18 mm was successfully grown by Top Seeded Solution Growth Technique. It is concluded that the doping Ta element can strongly influence the growth and morphology of the RbTiOPO4 crystal. The evident morphology change of RbTiOPO4:Ta crystal with respect to RbTiOPO4 crystal has been observed and the (1 0 0) crystal face was more developed than any other crystal faces. The possible reasons of the morphology change were analyzed through experimental and theoretical methods. Several methods were tried to increase crystallographic a direction dimension of RbTiOPO4:Ta crystals. Finally, the RbTiOPO4:Ta single crystal with crystallographic a direction dimension up to 6 mm was obtained by using thicker seed crystal. This way makes it possible to get isometric RbTiOPO4:Ta crystals, which is beneficial for nonlinear optical applications due to larger area in x-y plane.

Planar Defect Nucleation and Annihilation Mechanisms in Nanocontact Plasticity of Metal Surfaces

The incipient contact plasticity of metallic surfaces involves nucleation of crystalline defects. The present molecular dynamics simulations and nanoindentation experiments demonstrate that the current notion of nanocontact plasticity in fcc metals does not apply to high-strength bcc metals. We show that nanocontact plasticity in Ta-a model bcc metal-is triggered by thermal and loading-rate dependent (dynamic) nucleation of planar defects such as twins and unique {011} stacking fault bands. Nucleation of different planar defects depending on surface orientation leads to distinct signatures (pop ins) in the nanoindentation curves. Nanoscale plasticity is then ruled by an outstanding dynamical mechanism governing twin annihilation and subsequent emission of linear defects (full dislocations). While this investigation concerns Ta crystals, the present are landmark findings for other model bcc metals.

Influence of different purification techniques on triamcinolone yield and particle size spectrum

To systematically investigate the common purification techniques for commercially available triamcinolone acetonide preparations and the influence of different filter parameters on final yield, reproducibility and particle size spectrum. Two non-filter techniques using either sedimentation or centrifugation, and two different filter techniques were tested. Filters with different characteristics were investigated with the pore size ranging from 0.1 to 5.0 microm, and the filter diameter ranging from 4 to 30 mm. Only sterile standard syringe filters with low adsorptive characteristics (hydrophilic cellulose filter membranes) for pharmaceutical use were employed. For quantification, triamcinolone acetonide was dissolved in 60% methanol and measured spectrophotometrically at 239 nm. The crystal size spectrum was determined using a particle size analyzer that combines electronic pulse area analysis and resistance measurement. Depending on the purification technique used the resulting triamcinolone doses differed significantly. While the centrifugation method achieved purification without relevant loss, the sedimentation method yielded only 28.7% compared to the original commercial suspension, and in addition, the predictability was low (range 8.1-17.2 mg). With filter techniques high and consistent doses with a good reproducibility were achieved, but results were highly dependent on the filter characteristics. The final triamcinolone amount inversely correlated with the filter diameter due to a uniform loss of crystalline particles. In contrast, enlarging the pore size caused a substantial shift in the particle size spectrum due to a selective loss of small crystalline particles. The most common purification techniques vary notably in regard to final triamcinolone doses, reproducibility and particle size spectrum. The appropriate choice of the filter parameters seems to be more important than assumed, as pore size and filter diameter substantially influence both the final TA doses and the particle size of the TA crystals.

First principles study of tantalum thermodynamics by the statistical moment method

The thermodynamic properties of bcc tantalum are studied using the analytic statistical moment method and the first principles PAW–LDA (projector augmented wave method with local density approximation) scheme. The equation of states at finite temperatures are derived taking into account the anharmonicity effects of thermal lattice vibrations. The calculated thermodynamic quantities of bcc Ta crystals are in good agreement with the experimental results.

Semi-epitaxial bcc Ta Growth on Metal Nitride

The crystal structure change of sputtered Ta metal according to the nonmetal underlayer film was studied. The bcc-structure Ta was grown on TaNx, NbNx and bcc Mo underlayer films, whereas tetragonal-structure Ta was grown on glass, Ta2O5 and some metals. The crystal structure of Ta varied from tetragonal to bcc according to the N composition of the TaNx and NbNx underlayer films. The bcc Ta growth on TaNx was explained in terms of crystal structure compatibility of the Ta lattice arrangement between TaNx and bcc Ta, and tetragonal Ta growth on Ta2O5 film could be explained by partial Ta lattice matching between Ta2O5 and Ta crystals. Bcc Ta on the TaNx film gave a low resistivity of 25 µΩcm, in contrast to that of conventional tetragonal Ta of 180 µΩcm. The resistivity of Ta anodic oxide was increased by five orders of magnitude by adding N compared to that of conventional Ta anodic oxide. Low-resistivity bcc Ta on TaNx and high-resistivity TaN anodic oxide are applied as gate bus lines of TFT-LCD.

Configuration effects of ortho, meta, and para linkages on liquid crystallinity during thin-film polymerization of poly(ester-amide)s

By in situ thin-film polymerization conducted on a heating stage of a polarizing microscope, we have investigated the effects of monomer structures on the formation of liquid crystallinity. Three polymerization systems studied are 2,6-acetoxynaphthoic acid (ANA)/acetoxy acetanilide (AAA)/phthalic acid (PA), ANA/AAA/isophthalic acid (IA) and ANA/AAA/terephthalic acid (TA). In the three systems, PA, IA, and TA may create an ortho, a meta, and a para linkage, respectively. The formation of liquid crystallinity was found strongly dependent on the straightness and configuration of monomeric units. For ANA/AAA/PA and ANA/AAA/IA systems, there exists the critical ANA content to yield the liquid crystalline phase. Below this critical content, either amorphous phase forms or crystallization occurs during polymerization. Experimental data also indicate that defect density in the polymerization product reduces with increasing ANA content. Surprisingly, for the first time, we have observed that the ANA/AAA/PA system has a higher tendency to yield liquid crystallinity than the ANA/AAA/IA system. For the ANA/AAA/TA system, the polycondensation reaction is incomplete if the TA content is too high because of the low reactivity and the high melting point of TA. Polymerization of the ANA/AAA/TA system does not yield totally random copolymers because the liquid crystal phase appears before all TA crystals disappear during the polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2221–2231, 2000

Properties of Ta-doped SrTiO3 crystals

The behavior of ε′ and ε″ (100 Hz—1 MHz, 5–300 K), FTIR absorption (1500–5000cm−1) and EPR (9.2 GHz) spectra were studied in SrTiO3: Ta crystals (0.02% m.f. of Ta2O5). Dielectric losses reveal dipole relaxation behavior around 230, 130 and 40 K. The cooperative hopping reorientation of bounded electronic polarons due to charge transfer as the caused of low temperature relaxation and polaronic reorientation freezing in 31–40 K region was suggested. EPR signals that observed with g| ≥ 2.21 and g‖ ≤ 1.85 are tentatively polarons. Thus, low temperature dipole relaxation was considered as a cooperative dipole-type resonance cross-relaxation in the system of Ta4+−O−-Ti3+ clusters. This model was supported by FTIR experiments, where absorption at ≈0.23 eV is suggested to be due to polarons.

Simulation and modelling of forest hardening in body centre cubic crystals at low temperature

In body centred cubic (bcc) crystals at low temperatures, the thermally activated motion of screw dislocations by the kink-pair mechanism governs the yield properties and also affects the strain hardening properties. In this work, the average strength of dislocation junctions is derived and numerically estimated in the case of Nb and Ta crystals. This allows us to extend an existing simulation of dislocation dynamics in bcc crystals to the case of the motion of a screw dislocation line through a random distribution of forest obstacles. Numerical results are presented in the case of Ta crystals and at two temperatures, 160 K and 215 K. They are complemented by a simple model that applies quite generally to bcc metals at low temperatures. It is shown that forest hardening is made up of two contributions, a free-length effect that depends on the length of the mobile screw segments and whose dependence on forest obstacle density is logarithmic and a line tension effect linearly proportional to the obstacle density. As a result of the thermally activated character of screw dislocation mobility, the relative weight of the two contributions to forest hardening depends on the temperature and strain rate.

Synthesis and Crystal Structure of Ternary SulfidesA3MS4withA=Na, Rb andM=Nb, Ta

The new ternary sulfides Na3MS4and Rb3MS4(M=Nb, Ta) were prepared from tantalum nitride or niobium powder in the respective alkali metal polysulfide melt. Both sodium sulfides crystallize in form of yellow (Nb) or yellow-orange (Ta) needles in the orthorhombic space groupFdd2 withZ=32 anda=27.3034(4) Å,b=28.4662(1) Å, andc=7.9642(1) Å,R/Rw=0.024/0.048 forM=Nb anda=27.2322(2) Å,b=28.4264(2) Å, andc=7.9634(6) Å,R/Rw=0.066/0.177 forM=Ta. The rubidium sulfides were obtained as yellow (Nb) and orange (Ta) crystals with irregular shape. They crystallize in the orthorhombic space groupPnmawithZ=4 anda=9.6109(6) Å,b=11.1013(7) Å, andc=9.6390(6) Å,R/Rw=0.030/0.072 forM=Nb anda=9.6084(5) Å,b=11.1067(6) Å, andc=9.6426(5) Å,R/Rw=0.031/0.070 forM=Ta. The structures of all four compounds are built by stacking isolatedMS3−4units with the alkali metal ions.

Measuring the Depth Profile of C in Ta Thin Films by SIMS.

Factors influencing the secondary ion yield and depth resolution in the SIMS depth profiling of carbon in the tantalum thin films have been investigated. The highest ion yield of C has been achieved with 193TaC- by using Cs+ primary ion beam. However some changes in surface topography during Cs+ ion sputtering were observed. We found that the extent of the changes differed depending on implanted C+ doses and that the depth resolution became lower as the quantity of dosed C increased. These phenomena are supposed to be due to that Ta crystals became amorphous and carbonized.

Frequency doubling of Nd:YAG- and GaAs-lasers by KNbO3: Ta crystals

Abstract For KNb1-xTaxO3 crystals the influence of the Ta-concentration on the phase-matching properties for optical second harmonic generation (SHG) was measured. For non-critical phase matched SHG of the Nd:YAG-laser (1064nm) the coefficient d31 of the tensor of the nonlinear susceptibility was applied, while for the GaAs-laser (905 nm) the coefficient d32 was used. For both laser wavelengths the phase-matching temperature decreases with increasing Ta-concentration. Non-critical phase-matching at room temperature can be reached with the GaAs-laser for a Ta-concentration of ≈9%. The corresponding value for the Nd:YAG-laser is ≈14%.

Phase matching for second harmonic generation in KNbO3:Ta crystals with Nd:YAG- and GaAs-laser wavelengths

Noncritical phase-matched second harmonic generation was measured in optically biaxial KNb1−xTaxO3 mixed crystals in the orthorhombic phase. For Nd:YAG-(1064 nm) the coefficient d31 of the tensor of the nonlinear susceptibility was applied, while for the GaAs-laser (905 nm) the coefficient d32 was used. For both laser wavelengths the phase-matching temperature decreases with increasing Ta-concentration. Noncritical phase-matching at room temperature can be reached with the GaAs-laser for a Ta concentration of ≊9%. The corresponding value for the Nd:YAG laser is ≊14%.

電子線衝撃溶解法によって作製したＴａ単結晶の機械的性質におよぼす結晶方位の影響

Effects of cold roll and compression on Ta single crystals grown by electron beam melting methods were systematically studied.The rolling effect was determined using X-ray techniques and microphotography at successive stages of roll-ing, and the operating slip systems were determind from observations of slip traces on the side and front surfaces rolled. The mean grain size and the mean strain were determined by using the Hall method. The rolling texture was determined by the X-ray pole figure method at successive stages of rolling.Stress-strain curves of various Ta single crystals were obtained in order to clarify the effect of compressive deformation. The following results were obtained: According to the X-ray investigations and surface observa-tions, the mechanical structure of Ta single crystals subject to cold rolling is destroyed in the order, {110}- and , {111}- , and {100}- and .The work-hardening effect of Ta crystals, examined by using compression tests, was determined to be, small and in the order, of directions , and . The cold-rolled texture of Ta single crystals is grouped in {100}- and {111}- orientations, and it is the same as that of Fe single crystals.

Angular momentum dependence of 200Pb fission studied by comparison of 19F on 181Ta with 16O on 184W

The time development of fission in highly excited Pb nuclei has been studied by the crystal blocking technique. Thin Ta crystals were bombarded with 19F ions in the energy range 90–120 MeV and the yield of fission fragments was measured for emission directions close to a strong axis. The experimental blocking dips are compared with calculated dips containing a superposition of two components, corresponding to short- and long-lived compound nuclei. The information extracted is the energy dependence of the relative amount of fission which comes from compound nuclei with lifetimes of τ≳3 × 10 −17s. The total fission cross section and angular distribution of fission fragments were also measured for 19F bombardment of 181Ta in the energy range 84.3–114.7 MeV and for 16O bombardment of 184W in the energy range 83.4–107.9 MeV. The results of the three types of measurements have been interpreted through comparison with statistical model calculations that follow the spin and energy distribution of compound nuclei through the neutron evaporation cascade. The 19F + 181Ta measurements, when compared with the present 16O + 184W cross section and angular distribution measurements and earlier lifetime measurements for 16O + W, yield information on the spin distribution for the compound nucleus and its influence on the fission process.

Axial dechanneling: II. Point defects

The model of axial dechanneling presented in part I is applied to crystals with point defects. The additional change in transverse energy due to scattering by point defects is calculated with the same procedure as used for scattering by thermally displaced atoms. Point defects with fixed and randomly distributed positions in the channel are considered. For heavily damaged (Si) and weakly damaged (Si, Ge, Ta) crystals the magnitude and temperature dependence of the calculated dechanneling is in good agreement with the experimental data. The different temperature dependence of dechanneling in〈111〉 and 〈110〉 silicon measured by Howe et al. may be explained by 〈110〉-splits found by Götz et al.

Hyperfine interactions of interstitial12B in Fe, V and Ta studied by use of the asymmetric β decay

Magnetic hyperfine fields and spin-lattice relaxation times of interstitial and substitutional12B nuclei in Fe are studied in a temperature range, 100 K<T<1200 K, by use of the asymmetricβ decay from the spin polarized nuclei and NMR detection. In order to infer information regarding the location of12B nuclei and expansion in the nearest Fe surrounding following recoil implantation, hyperfine interactions of12B in non-magnetic bcc V and Ta crystals are studied.

Hyperfine interactions of the β-emitter12B in bcc crystals

The location of12B in bcc V and Ta crystals following implantation is determined by use of NMR detection and asymmetric β-decay. The expansion in the immediate surrounding of V is Δa/a=(21∓4) %. Magnetic hyperfine interactions of12B in bcc Fe are also discussed, as the parameters regarding the location and expansion as inferred from the results on nonmagnetic bcc metals.

Blocking experiments with positive muons and positrons arising from the decay of pions and muons implanted in single crystals

We report on the first observation of channeling phenomena in a blocking experiment with 4.12 MeV positive muons (μ +) resulting from the decay of thermalized pions (π +) implanted into Ta single crystals. The channeling manifests itself as a peak in the μ + intensity (relative height 1.25, fwhm 0.3°) along the 〈100〉 direction of the Ta crystals. In addition, first results of a similar experiment involving the blocking of relativistic positrons from the decay of muons (μ +) implanted into an Au crystal are presented. We describe the principles of the method together with the experimental set-up and discuss conclusions to be drawn from the experimental findings as well as potential applications of the technique.