Preparation and characterisation of alkoxide-derived celsian glass-ceramic

An all-alkoxide route to stoichiometric celsian glass has been developed. Sintering to 97% of full glass density, prior to the onset of appreciable crystallisation, was achieved after 30 min at 900 °C. However, crystallisation of metastable hexacelsian, rather than the preferred monoclinic celsian, from stoichiometric glass was difficult to avoid.     

A new graphite boat construction for the LPE growth of thin GaAs layers with a new technique

The characteristics of planar devices made of thin epitaxial GaAs layers on semiinsulating substrates, such as Gunn efect devices and MeSFETs, sensitively depend on the quality of the interface between the epitaxial layer and the substrate. The origin of an n-type, low resistivity interface layer between the epitaxial layer and the semiinsulating substrate is the semiinsulating substrate itself. The measured changes in the substrate-surface layer characteristics occur during the pregrowth heat treatment of the substrate in purified hydrogen. One solution of the problem could be shortening of the pregrowth heat treatment, but a low background doping concentration and a layer surface without any morphological defects only can be obtained after a relatively long pregrowth heat treatment of the melt. In this paper the technique of pregrowth melt preparation by heat treatment of the melt and the melt-back of the substrate prior to the growth is described. A new turnable sliding boat is presented which allows the use of as many melts as necessary for the required device (multilayer structures). Results (change of the electron concentration at the interface epitaxial layer-semiinsulating substrate, trap concentration, Hall mobility) and device characteristics (Gunn device under pulse and dc conditions and MeSFET) obtained with the improved technology are presented.     

低温热处理温度对SiC衬底上CuAlO2薄膜特性的影响

为了解决双极型碳化硅(SiC)功率器件中由于p型SiC在室温下难以完全电离所导致的p+n发射结注入效率低的问题,提出将p型CuAlO₂与n型SiC形成的异质结作为发射结以提高该结的注入效率。本文利用溶胶凝胶(sol-gel)方法,在4H-SiC衬底上制备了CuAlO₂薄膜,研究了低温热处理温度对CuAlO₂薄膜晶体结构、表面形貌、光学特性的影响。结果表明:较高的热处理温度可以促进中间产物CuO的生成,进而在固相反应阶段促进CuAlO₂相的产生,最终制备的CuAlO₂薄膜主要以CuAlO₂相的(012)晶向择优取向。随着低温热处理温度的升高,薄膜的表面均匀致密,空位缺陷含量降低,结晶质量提高。当低温热处理温度为300 ℃时,CuAlO₂薄膜晶粒尺寸约为35 nm。此外,CuAlO₂薄膜在可见光范围内的透过率超过70%,且随着预处理温度升高,薄膜光学带隙略有增加。     

Nonlinear optical studies of lead lanthanum borate glass doped with Au nanoparticles

Synthesis, characterization and optical nonlinearity of lead lanthanum borate glass embedded with gold nanoparticles have been investigated. DSC thermogram shows characteristics glass transition temperature at T_g = 775 K. Glasses doped with Au were subjected to heat treatment at 823 K with different annealing time and then, slowly cooled to room temperature show striking ruby color. SAED and TEM analyses have confirmed that f.c.c. Au nanoparticles of ~ 40 nm size are present in these glasses. An absorption peak centered on 563 nm has been observed in heat treated samples, which is attributed to surface plasmon resonance of gold nanoparticles. Nonlinear optical studies with open aperture Z-Scan technique show saturable absorption for heat treated samples at low intensity and reverse saturable absorption in samples without heat treatment at high intensity.     

Temperature, H2 loading and ultra violet irradiation effects in germanosilicate optical fibers: laser spectroscopy measurements

The effects of heat-treatment and ultraviolet (UV) light at 325 nm on virgin and hydrogen loaded germanosilicate optical fibers have been investigated by using laser spectroscopy technique. The luminescence band around 650 nm is sensitive to heat treatment. The hydrogen reloading of the hydrogen loaded fibers subsequent to heat treatment showed a new emission band around 570 nm. The irradiated fibers have an emission band around 383 nm which was not detected in hydrogen loaded fibers.     

The effect of initial oxidation state on crystallization of basaltic glass

Iron-rich silicates mined as basalt may be processed as glass fibers and, woven to textile mats, it may be used as heat insulation. High-temperature stability is however a limiting factor because at least three process of alteration occur upon heat treatment that affect mechanical and chemical properties, micron to nano-crystallization, oxidation and cation-enrichment at the glass surface with a few microns depth. We here evaluate the crystallization behavior of synthetic Fe³⁺-rich basalt (SB) heat treated in air (negligible oxygen potential) in comparison with previously studied Fe²⁺-rich natural basalt (NB) heat treated in air and in Ar (i.e., high and low oxygen potential, respectively). Initial crystals growing in Fe²⁺-rich basalt are micron-sized dendrites, and with temperature of heat treatment they become increasingly granular and are identified as pyroxene. Pyroxene in SB form smaller dendrites; crystallization textures and nucleation and growth rates, derived from crystal size distribution, are independent of the temperature of heat treatment in the range between 850 and 1030 °C. Here, the activation energy of pyroxene vanishes, though crystallization rates are smaller than those of NB. Whereas in NB pyroxene growth occurs diffusion limited, in SB it is likely to be limited by the attachment of ions to new crystal surface.     

Electrical properties of amorphous and crystalline InSb and InAs thin films

Amorphous films of indium antimonide (0.02–0.26 μm) and indium arsenide (0.03–0.3 μm) were formed on goldseal glass, freshly cleaved mica and NaCl substrates by using a “flash evaporation” technique. The post-deposition heat treatment was carried out on these films when the amorphous → crystalline transformation was observed. The transformation was characterized by a sudden and large fall in the resistance of the film at a particular temperature depending on the thickness. This transformation was confirmed by transmission electron micrographs and diffraction patterns obtained on the films before and after heat treatment. The transformation temperatures lie between 495–525 K for indium antimonide and 550–575 K for indium aresenide, for the thickness range involved in our investigations. The electrical conductivity measurements showed a temperature dependent activation in the high temperature region and hopping conduction in the low temperature region (Mott's theory). The activation energies, at different temperatures for various thicknesses were calculated and presented. While no Hall mobility could be observed in as-deposited films, very low mobilities were observed in annealed thick films (t > 2000 Å). Thermoelectric power for InSb films was found to vary from 0.075–0.17 mV/K for films of thickness ranging from 1000–2300 Å, whereas for InAs films, its value varied from 0.09–0.27 mV/K for the thickness range, 1250–2500 Å. These measurements indicated the conductivity to be n-type and supported the hopping conduction mechanism observed in low temperature conductivity measurements.     

高炉熔渣微晶玻璃的结构与性能研究

高温熔渣具有大量显热与渣体.采用熔融法制备微晶玻璃可以更好地利用其热和渣,达到高效利用的目的.通过高温条件下混熔的方式制备性能稳定的基础玻璃.利用差示扫描量热仪(DSC)确定基础玻璃的热处理工艺制度.结合高分辨透射电子显微镜(TEM)、X射线衍射分析(XRD)、场发射扫描式电子显微镜(FE-SEM)对微晶玻璃的结构进行了研究.研究表明:基础玻璃中氟含量的增加,有利于促进微晶玻璃成核和晶体长大,降低微晶玻璃的形核结晶温度.在低温热处理得到微晶玻璃的主晶相为萤石,高温热处理得到的微晶玻璃析出了霞石和萤石两个微晶相.高炉渣微晶玻璃中,微晶相的出现可有效提高微晶玻璃的力学性能,试样的显微硬度最高可达585.68 MPa,抗折强度最高可达126.21 MPa.     

Linear and nonlinear optical properties of glasses doped with Bi nanoparticles

Transparent glass samples doped with bismuth nanoparticles are prepared by heat treatment of as-made glass samples. According to the results of X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectra, Bi nanoparticles are well distributed inside glasses after heat treatment. The average size of Bi nanoparticles increases with the increasing of heat treatment temperature. Because of the size effect and multiple scattering of nanoparticles, the fundamental absorption edge shows a red-shift behavior with the increasing of heat treatment temperature. Nonlinear optical properties of Bi nanoparticles doped glasses are investigated by using Z-scan technique. The maximum value of χ⁽³⁾ of the glasses is estimated to be 2.49 × 10^? 7 esu at 800 nm. These results indicate that Bi nanoparticles doped glasses may be promising as material for optical switching.     

Spectroscopic studies on vitreous and polycrystalline heavy metal gallium–bismuthates

Local order changes determined by Fe₂O₃ (0–20 mol%) addition to Bi₂O₃–Ga₂O₃ matrix in glass and vitroceramic samples were investigated by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopies. Glass samples were prepared using the melt-quench technique. The vitroceramic samples were obtained by crystallization, as a result of the heat treatment applied on glass samples. The glass network mainly consists of [BiO₆] octahedral units. After heat treatment induced crystallization, [BiO₃] pyramidal units are predominant in samples. As evidenced by electron spin resonance, the Fe³⁺ ions surrounding is characterized by low crystal fields, excepting the vitreous sample with the lowest Fe₂O₃ content, wherein the Fe³⁺ ions occupy sites of low symmetry, characterized by high crystal fields.     

Ion-exchange-induced formation of glassy electrooptical and nonlinear optical nanomaterial

Subsurface electrooptical glass-ceramic layers are formed in an alkaline-silica-niobate glass using ion exchange technique. Ways of selection of glass compositions and ion exchange conditions to form such layers are discussed. It is shown that there are two ways to form glass-ceramics; the first one is the glass ceramization during the ion exchange treatment at higher temperatures, and the second one is the glass ceramization in heat treatment of the samples ion exchanged at lower temperatures, at which glass crystallization does not take place. The latter allows producing glass-ceramics layers of controllable thickness, because in this case diffusion process strongly obeys Fick equation.     

UV assisted local crystallization in Er3+ doped oxy-fluoride glass

We report on fabrication of Er³⁺-activated LaF₃ nanocrystals in transparent glasses using an original technique, which combines both heat treatment, below glass crystallization temperature, and ultraviolet laser irradiation at 244 nm. The main advantage of this method is to control the spatial localization of the nanoparticles in the glass sample, whereas annealing solely at the crystallization temperature leads to a fully crystallized glass sample. Thermal differential analysis was used to determine the crystallization temperature of the sample. The photoluminescence spectra behaviour of Er³⁺ ions, collected from the UV-irradiated and unirradiated regions, allowed us to follow and to distinguish the structural changes in the glass network under heat treatment and ultraviolet exposure.     

Positron lifetime studies of X-irradiated N-acetyl-D,L-alanine

Polycrystalline samples of N-acetyl-D,L-alanine (AA) which had received an X-ray dose of approximately 6 Mrad have been studied by the positron annihilation technique. Data were obtained as a function of time following the radiation treatment, and also as a function of subsequent annealing temperature. The large decrease of the intensity of the longest lifetime component (I₃) was observed to be reversible when assisted by heat treatment.     

Ammonia-free chemical bath deposition of CdS films: tailoring the nanocrystal sizes

We report on the control of nanocrystal sizes in CdS nanocrystalline films prepared by ammonia-free chemical bath deposition technique. We studied the effect of deposition duration, bath temperature during deposition, and post-preparation heat treatment. Nanocrystals (NCs) with radii from 2.6 nm to more than 10 nm were prepared and characterised by transmission electron microscopy, X-ray diffraction, and optical photoluminescence and absorption spectroscopy. We observed cubic to hexagonal phase transition for large NCs prepared by the heat treatment.     

Specific heat of ethanol at low temperatures

We present here new specific heat measurements at low temperatures (2–20 K) of the different phases of ethanol, characterized by the same calorimetric set-up at higher temperatures. We have extended and improved earlier measurements by implementing higher-accuracy calorimetric methods at low temperatures (using two complementary versions of the thermal relaxation method), as well as at higher temperatures (using a quasi-adiabatic, continuous method). The quantitatively very similar low temperature properties and glass-transition features of both structural glass and orientationally-disordered crystal of ethanol provide clear evidence that the lack of long-range crystalline order typical of amorphous solids is an unimportant factor regarding the universal properties of glasses. We have also employed these new measuring methods to study the possible effect of water impurities on the specific heat of the different solid phases of ethanol, and to study possible variations in the specific heat between different found phases of the monoclinic crystal of ethanol.     

GeO2/SiO2-glasses from gels to increase the oxidation resistance of porous silicon containing ceramics

GeO₂/SiO₂-glasses with relatively high expansion coefficients (between 5 and 7.3 × 10^?6 °C^?1) were prepared by hydrolysis of metal alkoxides (tetramethoxy silane+tetraethoxy germane as well as mixtures of the silane with GeO₂-powder) with subsequent heat treatment up to 1000°C. It is shown that the preparation of thin films on glasses and ceramics is possible but also that this technique can be used for liquid infiltration of porous SiC- and Si₃N₄-materials to increase the oxidation behaviour in the temperature range 1000 to 1400°C. In the case of molybdenumdisilicide as a ceramic material it is shown that the catastrophic inner oxidation in the low temperature range (600 to 800°C) of porous specimens (tested with plasma-sprayed MoSi₂-layers on refractory metals) can be hindered if these materials are infiltrated and heat treated.     

Thermal properties of vitreous silica with He impurities

The specific heat and thermal conductivity of vitreous silica (Suprasil W) doped with He at 420°C and 200 bar has been measured at low temperatures (0.5 K to 8 K). While with respect to an undoped sample the specific heat shows an excess contribution which varies as T^1.7, no effect of doping on the thermal conductivity is observed. This shows that the coupling to phonons of the excitations introduced by He doping is weak compared to that of intrinsic low energy excitations common to glasses. The excess specific heat can be attributed to excitations of He atoms in cavities.     

Solvus of KCI:Pb2+ Solid Solution from Flotation Measurements of Single Crystal Density

The solvus of KCl:Pb²⁺ is found in the concentration range from 1.6 × 10⁻² to 8 × 10⁻² mol.% at temperatures ranging from 450 to 250 °C. The decomposition of the solid solution after various regimes of heat treatment is judged by a change in the crystal density determined by the flotation technique.     

Influence of sol-gel coatings on crack initiation by vickers indentation in soda-lime glass

In this work an indentation technique was used to study the effect of the sol-gel coatings of soda-lime glasses on crack formation. The tendency of samples to develop radial cracks has been evaluated by measuring their number in correspondence with the Vickers impressions as a function of the applied load. Silica coatings of different thickness were obtained from alcoholic solutions of TEOS by the dipping technique on glass sheets with different degrees of surface hydration.

All the coated samples show a lower crackability compared with the uncoated ones. It was found that this parameter is influenced by layer thickness, heat treatment temperature and by the degree of galss surface hydration.     

Magnetite nanoparticles prepared by the crystallization of Na2O–Fe2O3–B2O3–SiO2 glasses

A glass with the composition of 35Na₂O–24Fe₂O₃–20B₂O₃–20SiO₂–1ZnO (mol%) was melted, quenched, using a twin roller technique, and subsequently heat treated in the range 485–750 °C for 1–2 h. This led to the crystallization of magnetite as the sole or the major crystalline phase.Heat treatment at lower temperatures resulted in the crystallization of magnetite crystals 7–20 nm in diameter, whereas heat treatment at higher temperatures produced higher quantities of magnetite and much larger crystals. The room temperature magnetization and coercive force values were in the range of 6–57 emu g^? 1 and 0–120 Oe, respectively for the heat treated glasses.