Vertical Bridgman growth and characterization of large ZnGeP2 single crystals

The growth of ZnGeP₂ crystals by seeded Vertical Bridgman method was studied. High-quality near-stoichiometric ZnGeP₂ single crystals obtained were of 20–30 mm in diameter and 90–120 mm in length. By selection of the seed crystallographic orientation the single crystal ingots without cracks and twins were grown, as shown by X-ray diffraction. The infrared transmission property of the ZnGeP₂ crystals was studied by the calculated optical absorption coefficient spectra. The results showed that after thermal annealing of the crystals the optical absorption coefficient was ～0.10 cm^?1 at 2.05 μm, and ～0.01 cm^?1 at 3–8 μm. The rocking curves patterns of the (4 0 0) reflection demonstrated that the as-grown single crystals possessed a good structural quality. The composition of the crystals was close to the ideal stoichiometry ratio of 1:1:2. The low-loss typical ZnGeP₂ samples of 6 mm×6 mm×15 mm in sizes were cut from the annealed ingots for optical parametric oscillation experiments. The output power of 3.2 W was obtained at 3–5 μm when the incident pumping power of 2.05 μm laser was 9.4 W, and the corresponding slope efficiency and the conversion efficiency were 44% and 34%, respectively.     

Growth and thermodynamic characterization of pure and Er-doped KPb2Cl5

Centimeter-sized single crystals of pure and Er-doped KPb₂Cl₅ were successfully grown by the Bridgman–Stockbarger method using silica ampoules sealed under HCl gas. The phase transition that occurs at 255 °C (528.15 K) was characterized by differential scanning calorimetry and found not to entail mechanical breakdown of the crystals. The KPb₂Cl₅ volumic mass and specific heat functions were measured from ∼700 to 760 K and ∼120 to 760 K, respectively. An updated Ellingham diagram for chlorides aimed at choosing the crucible material, synthesis and crystal growth atmosphere, and starting products is proposed.     

Production and properties of high purity TeO2− WO3−(La2O3, Bi2O3) and TeO2− ZnO−Na2O−Bi2O3 glasses

TeO₂–WO₃ (TW), TeO₂–WO₃–La₂O₃ (TWL), TeO₂–WO₃–La₂O₃–Bi₂O₃ (TWLB) and TeO₂–ZnO–Na₂O–Bi₂O₃ (TZNB) glasses were produced by high-purity oxide mixtures melting in platinum or gold crucible at 800 °C in the atmosphere of purified oxygen. The total content of Cu, Mn, Fe, Co and Ni impurities was not more than 0.1–0.5 ppm wt in the initial oxides and glasses. The stability of TZNB glasses to crystallization, characterized by (T_x ? T_g) value more than 150 °C, was demonstrated by DSC measurements at heating rate 10 K/min. In the case of La₂O₃-containing glasses the thermal effects of both crystallization and fusion of the crystallized phases were not observed. The hydroxyl groups absorption coefficients of pure tellurite glasses at the maximum of the absorption band (λ ~ 3 μm) were in the region of 0.012–0.001 cm^?1. The optical absorption losses, measured by the laser calorimetry method at λ = 1.56 and 1.97 μm, did not exceed 100 dB/km.     

Flux growth of β-Mn2V2O7 single crystals

Large single crystals of β-Mn₂V₂O₇ are successfully grown at a slow cooling rate using flux method in a closed crucible. The grown crystals exhibit a characteristic morphology with natural (1 1 0) and (3 5 0) facets. X-ray diffraction and chemical analyses show that the grown crystals have high quality.     

Optical investigations of germanium nanoclusters – Rich SiO2 layers produced by ion beam synthesis

In this work, refractive index and extinction coefficient spectra of germanium nanoclusters – rich SiO₂ layers have been determined using variable angle spectroscopic ellipsometry (VASE) in the 250–1000 nm range. The samples were produced by Ge⁺ ion implantation into SiO₂ layers on Si substrates and subsequent annealing at temperatures from 700 to 1100 °C. It is known from previous investigations of similar samples that the Ge nanoclusterization process starts already at 800 °C and spherical Ge nanocrystallites 5–8 nm in diameter are observed in the SiO₂ layers after annealing for 1 h at even higher temperatures of 1000–1100 °C. Rutherford backscattering spectrometry (RBS) was employed to measure the Ge atom concentration depth profiles in the studied samples. The RBS results helped us choose realistic models for the VASE analysis which were necessary for a proper interpretation of the VASE data. It has been found that the refraction index value for the SiO₂/Si layer increases after Ge implantation. This effect can be explained by a defect-dependent compaction of ion-bombarded layers. A band’s tail in the extinction coefficient spectra for all the samples is observed which originates from a strong ultraviolet absorption band at 6.8 eV due to a Germanium Oxygen-Deficiency Center (GeODC) and/or a Ge-E’center in SiO₂. The annealing process results in the emergence of weaker extinction coefficient bands in the 400–600 nm region, associated with direct band-to-band transitions in Ge nanostructures. Transformation of these bands, including their blue-shift with the increasing annealing temperature could be explained via a quantum-confinement mechanism, by size and structural changes in Ge nanostructures.     

Structural,electrical and optical properties of in-situ phosphorous-doped Ge layers

We have studied the impact of temperature and pressure on the structural and electronic properties of Ge:P layers grown with GeH₄+PH₃ on thick Ge buffers, themselves on Si(0 0 1). The maximum phosphorous atomic concentration [P] exponentially decreased as the growth temperature increased, irrespective of pressure (20 Torr, 100 Torr or 250 Torr). The highest values were however achieved at 100 Torr (3.6×10²⁰ cm^?3 at 400 °C, 2.5×10¹⁹ cm^?3 at 600 °C and 10¹⁹ cm^?3 at 750 °C). P atomic depth profiles, “box-like” at 400 °C, became trapezoidal at 600 °C and 750 °C, most likely because of surface segregation. The increase at 100 Torr of [P] with the PH₃ mass-flow, almost linear at 400 °C, saturated quite rapidly at much lower values at 600 °C and 750 °C. Adding PH₃ had however almost no impact on the Ge growth rate (be it at 400 °C or 750 °C). A growth temperature of 400 °C yielded Ge:P layers tensily-strained on the Ge buffers underneath, with a very high concentration of substitutional P atoms (5.4×10²⁰ cm^?3). Such layers were however rough and of rather low crystalline quality in X-ray Diffraction. Ge:P layers grown at 600 °C and 750 °C had the same lattice parameter and smooth surface morphology as the Ge:B buffers underneath, most likely because of lower P atomic concentrations (2.5×10¹⁹ cm^?3 and 10¹⁹ cm^?3, respectively). Four point probe measurements showed that almost all P atoms were electrically active at 600 °C and 750 °C (1/4th at 400 °C). Finally, room temperature photoluminescence measurements confirmed that high temperature Ge:P layers were of high optical quality, with a direct bandgap peak either slightly less intense (750 °C) or more intense (600 °C) than similar thickness intrinsic Ge layers. In contrast, highly phosphorous-doped Ge layers grown at 400 °C were of poor optical quality, in line with structural and electrical results.     

Synthesis of BaCeO3 powders by a fast aqueous citrate–nitrate process

A fast aqueous citrate–nitrate process has been successfully used to prepare the stoichiometric BaCeO₃ powders. It was found that an optimal amount of water added in the mixing process of barium nitrate, cerium nitrate, and citric acid is helpful on the formation of a clear gel that was subsequently formed by the condensation of added diethylene glycol monomers at room temperature. The gel was heated to an easily handled precursor in a powder form at 400 °C for 1 h. The precursors were characterized by X-ray powder diffraction (XRD), differential thermal analysis (DTA), thermal gravimetric analysis (TGA), thermal mechanical analysis (TMA), Fourier transform infrared spectrometry (FTIR), and scanning electron microscopy (SEM). The phase evolution from the precursor to the desired BaCeO₃ powders by XRD and FTIR revealed that the presence of BaCO₃ is a complex problem in this process. A high-temperature trigonal BaCO₃ intermediate was found between 800 °C and 1000 °C by the in-situ X-ray powder diffraction experiments. The phase purity of the formed BaCeO₃ powders may be more properly identified by FTIR instead of XRD because of the amorphous feature of the carbonate phase. The TMA revealed that at 1100 °C part of the precursor crystallized to needle-shaped BaCeO₃ crystals accompanied by a small expansion, which is also related to the presence of BaCO₃ in the precursor, and this can be removed by a preheating treatment of the precursor at 900 °C for 4 h.     

PbO–B2O3–SiO2 glass powders with spherical shape prepared by spray pyrolysis

PbO–B₂O₃–SiO₂ glass powders were directly prepared using spray pyrolysis. The powders were spherical and fine-sized. One glass particle was obtained from one droplet by melting the precursors inside a hot wall tubular reactor. The characteristics of these powders were compared with those of the commercial product, which was prepared using the conventional melting process. The spherical powders, which were prepared using spray pyrolysis at 1000 °C, had broad peaks at around 28° in the XRD spectrum. The glass phase was formed during the spray pyrolysis process even within a short residence time of the powders inside the hot wall tubular reactor. The mean size of the prepared glass powders was 1 μm. The dielectric layers formed from the spherical, fine-sized glass powders had a high transparency at firing temperatures above 520 °C. The maximum transparency of the dielectric layer formed from the glass powders obtained from spray pyrolysis was 95.3% at the firing temperature of 560 °C. The dielectric layer formed from the spherical, fine-sized glass powders had a smooth surface and no void inside the dielectric layer.     

Glass formation in and structural investigation of Li2S + GeS2 + GeO2 composition using Raman and IR spectroscopy

Li₂S + GeS₂ + GeO₂ ternary glasses have been prepared and a wide glass-forming range was obtained. The glass transition temperatures increase with the GeO₂ concentration in the glasses. The vibrational modes of both bridging (Ge–S–Ge) and non-bridging (Ge–S⁻) sulfurs are observed in Raman and IR spectra of binary Li₂S + GeS₂ glasses. Additions of GeO₂ to this binary glass increase the bridging oxygen band (Ge–O–Ge) at the expense of decreasing the bridging sulfur band (Ge–S–Ge), whereas the bands associated with the non-bridging sulfurs (Ge–S⁻) remain constant in intensity up to high GeO₂ concentrations. At higher concentrations of GeO₂ (⩾60%), the non-bridging oxygen band, which is not observed at low and intermediate GeO₂ concentrations, appears and grows stronger. From these observations, it is suggested that the added lithium ions favor the non-bridging sulfur sites over the oxygen sites to form non-bridging sulfurs, whereas the added oxygen prefers the higher field strength Ge⁴⁺ cation to form bridging Ge–O–Ge bonds. The structural groups in the Li₂S + GeS₂ + GeO₂ glasses that are consistent with results of Raman and IR spectra are described and are used to develop a structural model of these glasses.     

Single crystal growth and physical properties of superconducting ferro-pnictides Ba(Fe,Co)2As2 grown using self-flux and Bridgman techniques

We have employed the high temperature solutions growth technique and the Bridgman technique to grow cm-size high-quality single crystals of pristine BaFe₂As₂ (Ba122) and its Co-doped superconducting variants. In the first approach, self-flux (Fe, Co)As was used to achieve a homogeneous melt of composition Ba(Fe, Co)_3.1As_3.1 at T=1463 K. The melt was then cooled slowly under a temperature gradient in a double-wall crucible assembly to obtain large and flux-free single crystals of Ba(Fe, Co)₂As₂. In the second approach, single crystals were grown directly from a stoichiometric Ba(Fe, Co)₂As₂ melt at T=1723 K employing the Bridgman technique associated with a vertical tube furnace. Using both techniques single crystals with lateral dimensions up to 25×10 mm² and thickness up to 1 mm were obtained. Details of the two methods are given and a comparative study of the magnetic and transport properties of the single crystals obtained using the two methods is presented.     

Growth of CdSiP2 single crystals by self-seeding vertical Bridgman method

Using a high purity CdSiP₂ polycrystalline charge synthesized in a single-temperature zone furnace, a CdSiP₂ single crystal with dimensions of 8 mm in diameter and 40 mm in length was successfully grown by the vertical Bridgman method. The quality of the crystal was characterized by high resolution X-ray diffraction and the full width at half maximum (FWHM) of the rocking curve for the (200) face is 33″. Thermal property measurements show that: the mean specific heat of CdSiP₂ between 300 and 773 K is 0.476 J g^?1 K^?1; the thermal conductivity of the crystal along the a- and c-axes is 13.6 W m^?1 K^?1 and 13.7 W m^?1 K^?1 at 295 K, respectively; and the thermal expansion coefficient measured along the a- and c-axes is 8.4×10^?6 K^?1 and ?2.4×10^?6 K^?1, respectively. The optical transparency range of the crystal is 578–10,000 nm, and there is no absorption loss in the spectrum from 0.7 to 2.5 μm, as often exists with ZnGeP₂ crystals grown from the melt.     

Non-isothermal crystallization kinetic studies on MgO–Al2O3–SiO2–TiO2 glass

Thermal stability and crystallization kinetics of the glass 21% MgO, 21.36% Al₂O₃, 53.32% SiO₂ and 4.11% TiO₂ (mol%) has been studied using differential thermal analysis (DTA), dilatometry and X-ray diffraction (XRD). Glass in both bulk and frit forms were produced by melting in platinum crucible at 1600 °C for 1–2 h. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 340 kJ mol⁻¹ and 498 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Crystallization of bulk glass was carried out at various temperatures and for different time durations in the range of 850–1000 °C. The influence of the addition of TiO₂ on the crystallization sequence of the glass was experimentally determined and discussed.     

Ultrafine Co1−xZnxFe2O4 particles synthesized by hydrolysis: Effect of thermal treatment and its relationship with magnetic properties

Co_1−xZn_xFe₂O₄ (x = 0, 0.2 and 0.4) fine powders with particles size of 3 nm were prepared by hydrolysis method. The powders were annealed at 500 °C for 3 h. With heat treatment, the average particles size increased to 12 nm with corresponding increase in blocking temperature, saturation magnetization and reduced remanence. A significant increase in coercive field was found only for the pure CoFe₂O₄.     

Crystal growth of ZrW2O8 and its optical and mechanical characterization

ZrW₂O₈ is known for its isotropic negative thermal expansion over a wide of range of temperature from ?272 to 777 °C. However, ZrW₂O₈ melts incongruently at 1257 °C and is stable only over a short temperature interval between 1105 and 1257 °C. This makes the growth of single crystals a formidable challenge. In order to study the intrinsic properties of this compound, a repeatable, viable single crystal growth strategy is required. Here we report a simple, self-seeding, self-fluxing single crystal growth process which resulted in single crystals of ZrW₂O₈ up to about 4 mm in size. Grown crystals were characterized by X-ray diffraction (XRD), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. Mechanical properties of the crystals were studied using nanoindentation.     

Phase evolution on heat treatment of sodium silicate water glass

Heat treatment of sodium silicate water glass of the nominal composition Na₂O/SiO₂ = 1:3 was carried out from 100 °C up to 800 °C and the advancement of the resulting phases was followed up by powder X-ray diffraction, scanning electron microscopy and thermogravimetry along with differential thermal analysis. The water glass, initially being an amorphous solid, starts to form crystals of β-Na₂Si₂O₅ at about 400 °C and crystallizes the SiO₂ modification cristobalite at about 600 °C that coexists along with β-Na₂Si₂O₅ up to 700 °C. At 750 °C Na₆Si₈O₁₉ appears as a separate phase and beyond 800 °C, the system turns into a liquid.     

Pyrolytic conversion studies of blended polycarbosilane and polyaluminasilazane

Multinuclear solid-state NMR spectroscopy, FTIR and Raman experiments are employed to investigate the pyrolytic conversion of blended polycarbosilane and polyaluminasilazane (denoted CA) up to 800 °C, with the aim of studying structural evolutions and interactions between polycarbosilane and polyaluminasilazane during the pyrolysis process. Vinyl and SiCH₃ units can react with Si–H, SiCH₃ and Si–CH₂–Si groups below 400 °C. These crosslinking reactions can increase the ceramic yield of the blended precursors. At 500 °C aromatic carbon is formed, and N–H and Si–H groups vanish at 600 °C and 700 °C, respectively. At 600 °C, SiCH₃ and Si–H units can further react with SiCN₃, SiC₂N₂, N–H and C–H units. An amount of amorphous carbon and CSi₄ and CSi₃H groups are detectable at 800 °C. Even at this temperature there are still many aromatic protons. In addition, there are also SiC₄, SiC₃N, SiCN₃ and SiN₄ units. Silicon forms SiN₄ more readily than SiC₄. Many AlN₅ groups transform into AlN₆ groups. The D and G bands of graphite are observed in CA pyrolyzed at 1400 °C. According to the XRD patterns, the reflection of crystalline β-Si₃N₄ vanishes at 1700 °C, and the residue pyrolyzed at 1800 °C mainly contains a large number of 2H-SiC/AlN solid solution crystals and a few β-SiC crystals.     

Influence of Bi2O3 content on the crystallization behavior of TeO2–Bi2O3–ZnO glass system

Glass ceramic materials with composition 75TeO₂–xBi₂O₃–(25-x)ZnO (x = 13, 12, 11) possessing transparency in the near- and mid-infrared (MIR) regions were studied in this paper. It was found that as the Bi₂O₃ content increased in the glass composition, the observed crystallization tendency is enhanced, and high crystal concentrations were obtained for the glasses with high Bi₂O₃ content while maintaining transparency in the MIR region. Crystal size in the glass ceramic was reduced by adjusting the heat treatment conditions; the smallest average size obtained in this study is 700 nm. Bi_0.864Te_0.136O_1.568 was identified using X-ray Diffraction (XRD) and found to be the only crystal phase developed in the glass ceramics when the treatment temperature was fixed at 335 °C. The morphology of the crystals was studied using Scanning Electron Microscopy (SEM), and crystals were found to be polyhedral structures with uniform sizes and a narrow size distribution for a fixed heat treatment regime. Infrared absorption spectra of the resulting glass ceramics were studied. The glass ceramic retained transparency in the infrared region when the crystals inside were smaller than 1 μm, with an absorption coefficient less than 0.5/cm in the infrared region from 1.25 to 2.5 μm. The mechanical properties were also improved after crystallization; the Vickers Hardness value of the glass ceramic increased by 10% relative to the base glass.     

Cation diffusion in soda-lime-silicate glass melts

Cation diffusion was experimentally investigated in soda-lime-silicate glass melts (composition in mol%: 74SiO₂–16Na₂O–10CaO) at temperatures from 1000 to 1200 °C using the diffusion couple technique. One half of each diffusion couple was doped with 11 trace elements (500 ppm by weight of Rb, Cs, Sr, Zn, Cd, Nd, Eu, In, Sn, Ge and 1000 ppm by weight of Fe). Experiments were performed in an internally heated gas pressure vessel at a confining pressure of 100 MPa to avoid convective fluxes in the diffusion samples. The distribution of major elements was analyzed by electron microprobe. IR spectroscopy was used to quantify concentrations of dissolved water in the run products. Trace element diffusion profiles were measured simultaneously employing synchrotron X-ray fluorescence microanalysis. In all analyzed glasses the highest diffusion coefficients were observed for Rb whereas Nd was always the slowest element, e.g. at 1000 °C the diffusivity decreases from (1.51 ± 0.35) × 10⁻¹¹ m²/s for Rb to (1.29 ± 0.34) × 10⁻¹³ m²/s for Nd. The diffusivity of Nd is close to the chemical diffusivity of network former calculated from viscosity data using the Eyring relationship. Surprisingly, the rare earth elements Nd (3+) and Eu (mixed 2+, 3+) diffuse more slowly than the tetravalent Ge. Activation energies for diffusion increase from (132.1 ± 1.5) kJ/mol for Rb to (205 ± 16) kJ/mol for Eu. Based on the diffusion data for Eu, Sr and Nd we estimated that Eu²⁺/Eu_total ratios in soda-lime-silicate glass melts are below 0.04 both at reducing and oxidizing conditions.     

Polyhedral to nearly spherical morphology transformation of silver microcrystals grown from vapor phase

Silver microcrystals of various shapes were grown from co-evaporation of Ag₂O and SiO powders in the nominal evaporating temperature range (heat source temperature) from 800 to 1200 °C under a reductive atmosphere. Pathways for the polyhedral to nearly spherical morphology transformation were established. At evaporating temperatures below 900 °C, the polyhedral crystals are generally bounded by {1 1 1}-facets and {1 0 0}-facets. With increase in evaporation temperature facets of the {1 1 0}-, {2 1 0}- and {3 1 0}-families come forth successively, that make the Ag polyhedron microcrystals transform into nearly spherical microcrystals. A mechanism based on the recession of {1 0 0} or {1 1 0} steps induced by the solvent SiO_x (x～2.0), which modifies the growth kinetics, is proposed to explain the emergence of {2 1 0}- and {3 1 0}-facets at high temperatures. By evaporating at 1200 °C, smooth-looking spherical Ag crystals with 6 nm multi-layered steps on the {1 1 1} facets were obtained. These results are also of significant industry relevance when preparation of noble metal particles with high-index facets is of concern.     

Mechanism of crystallization of fast fired mullite-based glass–ceramic glazes for floor-tiles

The mechanism of crystallization from a B₂O₃-containing glass, with composition based in the CaO–MgO–Al₂O₃–SiO₂ system, to a glass–ceramic glaze was studied by different techniques. Glass powder pellets were fast heated, simulating current industrial tile processing methods, at several temperatures from 700 to 1200 °C with a 5 min hold. Microstructural study by field emission scanning electron microscopy revealed that a phase separation phenomenon occurred in the glass, which promoted the onset of mullite crystallization at 900 °C. The amount of mullite in the glass heated between 1100 and 1200 °C was around 20 wt%, as determined by Rietveld refinement. The microstructure of the glass–ceramic glaze heated at 1160 °C consisted of interlocked, well-shaped, acicular mullite crystals longer than 4 μm, immersed in a residual glassy phase.