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Abstract: Mg(BH4)2 is a complex hydride with one of the highest hydrogen contents (∼ 15%) known yet. Several synthesis routes have been reported for it, all based on the metathesis reaction of MgCl2 with NaBH4 performed in a ball-mill or in suitable solvents. In the present study a new approach for synthesis of Mg(BH4)2 will be presented in which the more reactive MgBr2 is used instead of MgCl2. For this purpose a mixture of MgBr2 and NaBH4 (molar ratio: 1: 2 and 1:2.15) was ball-milled for 6, 12 and 18h, respectively. Mg(BH4)2 was extracted from the reaction product (Mg(BH4)2 + NaBr) by Soxhlet with diethylether over a day. The remaining residue after solvent evaporation was dried in vacuum at 150°C for 24h and 5h at 190°C. The intermediate and final products of the reactions were analyzed using XRD, DTA/TG, Mass and Vibrational Spectroscopy. The XRD diagrams of the mixture after ball milling showed only the characteristic reflections of NaBr and the patterns obtained after solvent extraction was in all cases consistent with β- Mg(BH4)2. The additional weak MgBr2 reflections, decreased by increasing the ball milling time from 6 to 18h. The DTA/TG coupled with MS revealed ∼11% mass loss when the product was heated up to 600°C. The result of MS detected that the exhaust gas is exclusively H2. Compared to MgCl2, the use of MgBr2 has two advantages: the reaction time is considerably shorter and the excess of MgBr2 can act as additive lowering the onset temperature for hydrogen release from 290°C - for pure Mg(BH4)2 - to ∼ 220°C. [Copyright &y& Elsevier]

Abstract: The title compound Na2[BH4][NH2]≡Na2BNH6 exists in two different modifications: the low-temperature (LT) β- and the high-temperature (HT) α-phase. The HT α-Na2BNH6 is accessible by single heating of the binaries NaNH2 and NaBH4 (molar ratio=1:1) to the melting point of 492K for 1h in evacuated Pyrex ampoules. On longer annealing (T =342K, 10 days), the HT α-phase converts completely to the LT β-phase. The β→α transition at 371K is very fast, but the conversion α→β is quite slow. No further phase formation was detected in the system up to a molar ratio of 5:1 (NaNH2:NaBH4). The cubic α-Na2BNH6 crystallizes in the K3SO4F-type of structure (space group: , Z =1) with a =4.6950(1)Å. β-Na2BNH6 crystallizes in orthorhombic symmetry (space group: Pbcm, Z =4) with a =6.5384(2)Å, b =6.4960(1)Å, c =9.8512(2)Å. Both structures can directly be derived from the perovskite type structure ABO3, with the [BH4]− and [NH2]− moieties in the A and B sites, respectively. The charge balancing Na ions are occupying 2/3 of the O positions. In both cases, the presence of the two different anions in the solids was also confirmed by vibrational spectra. According to DTA and high temperature X-ray diffraction (HT-XRD) measurements, the educts NaNH2 and NaBH4 react at 430K to form α-Na2BNH6 which remains stable until the melting point of 492K. Between 492 and 573K, no significant effects were detected in DTA and TG. Above 573K, however, a sudden decomposition takes place in two steps (659 and 689K) causing a total mass loss of 8%. The gaseous species formed during the decomposition process were followed by mass spectra (MS). The mass spectra obtained from 1:1 and 2:1 mixtures of NaNH2 and NaBH4 in the range 492K< T <773K prove that the main gaseous product evolved during the thermal decomposition is H2. NH3 and H2O were also detected, but their concentrations are quite low. These findings confirm that the mixture of the complex hydrides NaNH2–NaBH4 (molar ratio ≥1:1) – comparable to the LiNH2–LiBH4 system – are potential candidates for solid hydrogen storage materials. The composition of the solid residues after the thermal treatment depends on the molar ratio of the starting binaries NaNH2 and NaBH4. While Na2BNH6 (1:1) decomposes to a mixture of NaH, Na and an unidentified amorphous solid, mixtures ≥2:1 yielded exclusively Na3BN2. The vibrational spectra of the title compounds Na2BNH6 have been measured and interpreted based on the T d and C 2v symmetry of the relevant [BH4]− and [NH2]− groups. Both the (N–H) and (B–H) frequencies exhibit small but significant shifts with respect to the pure binaries. [Copyright &y& Elsevier]

Abstract: Despite much effort spent by various research groups, there remain many aspects of nanoparticle silicalite-1 crystallization from clear solutions which require further investigation. In order to shed light, especially on the nucleation of silicalite-1, particle growth at 100°C from several starting compositions known to yield colloidal silicalite-1, which have been studied previously by other researchers using various techniques, was followed in this study by laser light scattering using scattering angles of 90° and 173°, and zeta potential and pH measurements. Crystallinity was monitored by X-ray diffraction, Fourier transform infrared analysis and transmission electron microscopy. Thermogravimetric analyses and density measurements were also used to characterize the products obtained at various times during the syntheses. The results demonstrate that the distinct time of sudden jump in the effective diameter of the nanoparticles in solution, as observed more clearly by using the back-scattering device, and which marks the beginning of the constant linear growth rate of the particles, corresponds to the nucleation of the silicalite-1 crystal structure. This time was also shown to coincide with the exo–endo thermal switch time of the reaction mechanism, which has been observed previously by another research group. Nucleation was accompanied by an aggregation of a population of smaller particles, as indicated by the broadening of the particle size distribution, and the variation of the pH and zeta potential values during synthesis. [Copyright &y& Elsevier]

Abstract: The phase stability of the two TiO2 modifications (anatase and rutile) in fumed SiO2/TiO2 nano-composites (0–24.8 wt-% silica) under thermal and hydrothermal conditions was investigated by X-ray powder diffraction, transmission electron microscopy (TEM) and gas adsorption methods (BET). The results show that the phase transformation from anatase to rutile type of structure and the growth of anatase crystallites are significantly retarded by mixing small amounts of SiO2 into TiO2, while the specific surface area is maintained. The SiO2/TiO2-composites reveal a remarkable shift in the anatase to rutile transformation temperature from approx. 500 °C (pure TiO2) to approx. 1000 °C (samples with SiO2 contents of more than 10%). The rate of phase transformation from anatase to rutile is enhanced under hydrothermal conditions compared to conventional thermal treatment, e.g. pure titania (AEROXIDE® TiO2 P25) annealed under hydrothermal conditions (100 g/m3 absolute humidity, 4 h at 600 °C) had a rutile content of 85%, while the same specimens annealed in absence of humidity contained only 46% rutile. However, the difference in rate of phase transformation became less pronounced when the silica content in SiO2/TiO2-composites was further increased. TEM results showed that the surface of the anatase crystallites was covered with silica. This averts coalescence of anatase crystallites and keeps them under a critical size during the annealing process. When the crystal domains grew larger, a rapid conversion to rutile took place. The critical size of anatase crystallites for the phase transformation was estimated to be 15–20 nm. [Copyright &y& Elsevier]

Abstract: A set of Fe3+-modified PbTiO3 nanopowders has been synthesized according to the combined polymerization and pyrolysis (CPP) route of metallorganic precursors [Erdem E, Böttcher R, Semmelhack H-C, Gläsel H-J, Hartmann E, Hirsch D. Preparation of lead titanate ultrafine powders from combined polymerisation and pyrolysis route. J. Mater. Sci. 2003;38:3211–7] with subsequent calcination at various temperatures. X-ray diffraction verifies the formation of a PbTiO3 perovskitic phase and Raman-spectroscopy proves the existence of ferroelectricity. Furthermore, the prevailing defect structure has been investigated by means of electron paramagnetic resonance (EPR) spectroscopy. The EPR results clearly indicate marked size effects by approaching to the critical grain size ( nm) at which a size-driven tetragonal-to-cubic phase transition is observed at room temperature. As a function of mean grain size, either defect dipoles or ‘isolated’ defects (, ) are formed. These results are analyzed in terms of a core–shell model. Accordingly, the obtained Fe3+-modified PbTiO3 nanoparticles consist of a ferroelectric core, a distorted interface region, and a cubic dead layer which is paraelectric. [Copyright &y& Elsevier]

Abstract: This paper provides a detailed description of the preparation and characterization of diffusion-doped Cr2+:ZnSe and Fe2+:ZnSe. In the experiments, Cr2+:ZnSe samples with peak absorption coefficients (1775nm) of as high as 74cm−1 and fairly good spatial uniformity were obtained. In the case of Fe2+:ZnSe, samples with a maximum absorption coefficient of 11.6cm−1 at 2400nm were synthesized. A three-dimensional analytical diffusion model was further developed to determine the diffusion coefficient D from absorption data. At 1000°C, D was determined to be 5.4×10−10 cm2/s and 7.95×10−10 cm2/s for Cr2+:ZnSe and Fe2+:ZnSe, respectively. In the case of Cr2+:ZnSe, we employed an alternative technique to determine the diffusion coefficient from position-dependent absorption data taken with a Cr4+:YAG laser. With this method, the temperature variation of D was further measured in the 800–1100°C temperature ranges. [Copyright &y& Elsevier]

Yellow, transparent single crystals of Ba3[BN2]2 (P212121 (No. 19), a=424.73(2), b=1105.60(4) and c=1475.72(6) pm, Z=4) are obtained by the reaction of Ba2N and BN in arc-welded Nb ampoules at 1300 K. The title compound crystallizes, despite its familiar 3:2:4 stoichiometry, in a new structure type. The vibrational spectra of orthorhombic Ba3[BN2]2 have been measured and assigned. The IR spectrum reveals strong band splitting arising from the low site symmetry and the presence of two different sets of crystallographically independent [BN2]3−groups. The new structure and the vibrational data are compared with those previously reported for the cubic form of Ba3[BN2]2. [Copyright &y& Elsevier]

• Single-step synthesis of cobalt-titanium-boron based composite nanostructures. • Investigation of different reaction conditions on phase formation. • Investigation of microstructural and thermal properties of powders. • The effect of binary or composite boride reinforcements on mechanical properties. • Enhancement of hardness and strength by the use of composite reinforcement. Due to promising mechanical and chemical properties, transition metal borides have attracted attention, and numerous studies have investigated various combinations of transition elements in hopes of acquiring a final product with desired properties combined. In this study, novel low-temperature approach was adopted for the synthesis of cobalt-titanium-boron based crystalline powders. The method was based on the single-step direct reaction of CoCl 2(s) , TiCl 4(l) and NaBH 4(s) in a sealed reactor under autogenic pressure. After the reaction of the precursors at 850 °C by using the molar ratios of metal chlorides to NaBH 4 as 1:3, CoB and TiB 2 phases were formed in-situ. The subsequent annealing process at 1100 °C achieved a full conversion of metal chlorides to CoB-TiB 2 composite nanostructures. It was concluded that the binary forms of the borides tend to form as separate phases, which is illustrated in the SEM/EDS analyses with different morphologies. Amorphous boron layer surrounded TiB 2 particles with an average particle size of 60 nm, whereas the CoB particles formed agglomerates with an average size of 450 nm. The use of synthesized composite powders as reinforcement in metal matrices resulted in enhanced hardness (506 HV) and compressive strength (1682 MPa) of the Ti6Al4V bulk samples. [ABSTRACT FROM AUTHOR]

Abstract This study reports the fabrication of high-density TiB 2 ceramics at reduced temperatures (1500 °C) by using field assisted sintering technology/spark plasma sintering (FAST/SPS) technique. In-house processed TiB 2 powders with high purity were employed as starting materials. The effects of SPS conditions on the sintering behavior and microstructure of TiB 2 were investigated. Sub-micron character of TiB 2 particles has a strong influence on the densification during SPS. Microstructural analyses revealed that the high pressure affected both the grain size and the shape of TiB 2 crystallites. Elongated grains with average size of 6 μm were perceived in TiB 2 sample fabricated by SPS at 1500 °C under 60 MPa pressure with a relative density of 96.7%. The microhardness and elastic modulus were measured as 27.4 GPa and 685.2 GPa, respectively. This sample revealed an improved oxidation resistance due to the formation of a thin oxidation layer on the surface when heated up to 1200 °C. Highlights • Nearly full densification of in-house processed TiB 2 ceramic powders at 1500 °C. • Microstructures revealing the size and shape of TiB 2 grains after SPS. • The effect of sub-micron particles on the densification during SPS. • The effect of sintering pressure on the microstructure. • Microhardness and nano-indentation measurements along with oxidation studies. [ABSTRACT FROM AUTHOR]

Sub-micron sized TiB 2 ceramic powders were prepared via self-propagating high-temperature synthesis (SHS) followed by HCl leaching at different temperatures. Purified powders obtained using optimum process parameters were consolidated by field assisted sintering technology/spark plasma sintering (FAST/SPS) technique. Phase and microstructural analyses of both the powder and sintered samples were carried out by X-ray diffractometer (XRD) and scanning electron microscope (SEM). The chemical analyses and particle size measurements of the specimen were conducted by inductively coupled plasma-mass spectrometry (ICP-MS) and dynamic light scattering (DLS) techniques. The final properties of the sintered sample were determined in terms of density and microhardness. The effects of different HCl leaching temperatures on the formation, microstructure, particle size, purity and sintering behavior of the SHS-produced TiB 2 powders were investigated. The SHS reaction of TiO 2 -B 2 O 3 -Mg powders as a starting mixture yielded MgO, Mg 3 (BO 3 ) 2 and Mg beside the desired phase TiB 2 . All three magnesium containing by-products were completely removed by performing hot HCl leaching. TiB 2 powders after SHS reaction and leaching with 9.3 M HCl for 30 min at 80 °C revealed a minimum purity of 98.4% and a homogenous particle size distribution with an average particle size of 536 nm. In the ultimate SPS experiment which was conducted at 1500 °C for 5 min under a pressure of 50 MPa, a relative density of 94.9% and a micro-hardness value of 24.56 GPa were achieved. [ABSTRACT FROM AUTHOR]

The unavailability of high quality precursor is encouraging researchers to seek effective ways to fabricate magnesium diboride (MgB 2 ) wire. Herein, cost-effective amorphous boron powder produced through a diborane (B 2 H 6 ) gas process is investigated for the possibility of further industrial application. A thin carbon layer to encapsulate the boron particles is simultaneously deposited by pyrolysis of hydrocarbon. We found that the carbon-encapsulated amorphous boron has a high upper critical field due to impurity scattering, and thereby, enhanced high-field critical current density. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the effect of Cr2+ concentration on the fluorescence lifetime, fluorescence efficiency, and lasing performance of Cr2+:ZnSe lasers. In the experiments, polycrystalline Cr2+:ZnSe samples with concentrations of 0.34×1018–66×1018 ion/cm3 were prepared by diffusion doping. Spectroscopic measurements performed with Cr4+:YAG and Tm3+:fiber lasers show that the fluorescence efficiency decreases monotonically with increasing active ion concentration. Lifetime measurements and lasing characterization were also performed with a pulsed 1570-nm KTP optical parametric oscillator. In gain-switched operation, the highest output energy was obtained with the Cr2+:ZnSe sample having a Cr2+ ion concentration of 14×1018 cm−3. The power performance degrades with increasing concentration due to larger passive losses at the lasing wavelength. Furthermore, the pulse build-up time and the output pulsewidth decrease with pump energy and ion concentration. Finally, with a single set of optics, tunable output could be obtained in the 2520–3050nm wavelength range. [Copyright &y& Elsevier]