Oriented lithium disilicate glass–ceramics prepared by electrochemically induced nucleation

Glasses with the compositions Li₂Si₂O₅ and 9Li₂Si₂O₅ · BaSiO₃ were crystallized using electrochemically induced nucleation. A platinum wire was inserted into the melt. A dc-current was supplied between the platinum wire (cathode) and the crucible (anode) at various temperatures below the liquidus temperature. This led to nucleation at the cathode and subsequently to radial growth of lithium disilicate crystals. The crystals exhibit dendritic growth. The main crystal growth direction is the crystallographic c-axis of lithium disilicate. This axis is oriented perpendicular to the electrode surface. A crystal growth selection near the cathode results in a highly textured glass–ceramics. The electrochemically induced nucleation is based on the reduction of the silicate glass to elementary silicon and an alloy formation between Si and Pt with a depletion of Si in the melt near the cathode. The crystalline products were characterized and the effects of experimental parameters were studied.     

Novel monolithic mesoporous foamed carbons prepared using micro-colloidal particles as templates

Novel mesoporous foamed carbons were synthesized from carbonization of organic gels templated by polymer micro-colloidal particles. Resorcinol and formaldehyde were allowed to gel in dilute polymethylmethacrylate (PMMA) microemulsion latex, subsequently the water in the gel was solvent exchanged with methanol and the wet gel was dried under ambient pressure. Pyrolysis was carried out at 800 °C to afford carbon xerogels with porous structures similar to those of resorcinol–formaldehyde (RF) carbon aerogels, but of higher density (>1.2 g/cm³), which provide the carbon materials with relatively higher volumetric surface area (up to 918 m²/cm³). Brunauer–Emmett–Teller (BET) adsorption results indicate that PMMA micro-colloid particles with mean diameter 25 nm contributed to the formation of mesopores of mean diameter at 5 nm.     

Double heterostructure lasers prepared by LPE on medium-quality GaAs substrates under controlled as vapour conditions

The fact that defect formation in (Ga, Al)As LPE layers can be suppressed by the presence of arsenic vapour is employed to improve properties of double heterostructure (DH) lasers. The controlled vapour pressure (CVP) method is implemented using a modified LPE horizontal carbon boat. Growth kinetics study under near-equilibrium conditions shows that the presence of arsenic vapour diminishes the growth rate of aluminium-containing layers; no such influence has been observed with the GaAs layers. The CVP method, compared with the customary LPE, has a beneficial effect on the DH surface and cross sections perfection, as well as on the lasing characteristics (differential quantum efficiency, threshold current). Observed values of the parameter T₀, characterizing temperature dependence of the laser threshold current, fall into the vicinty of 200 K. The results have been obtained on medium-quality GaAs substrates.     

Transparent SiO2 aerogels prepared by ambient pressure drying with ternary azeotropes as components of pore fluid

Transparent SiO₂ aerogels were prepared by two-step sol–gel processing followed by ambient pressure drying at temperatures from 70 to 250 °C. The wet gels were synthesized via acid–base catalysis using tetraethyl orthosilicate as a silica precursor and isopropanol as a solvent. Isopropanol was exchanged with n-butanol, and the gel surface was modified using a trimethylchlorosilane solution in n-butanol. Next, the solvent was exchanged in several steps with saturated hydrocarbon in order to obtain pore fluids containing azeotropic mixtures of water, n-butanol and a corresponding hydrocarbon (hexane, heptane, octane, nonane). Ambient pressure drying was performed in two steps, at the boiling points of the ternary azeotropes and hydrocarbons, respectively. In this way, transparent, crack-free aerogels of different shapes, with a specific surface area of 1000 m²/g, average pore diameter of ～40–55 Å and density in the range 0.4–0.57 g/cm³ were obtained.     

Optical properties of flower‐like CdS prepared by a hydrothermal method with SDBS as surfactant

In this article, flower‐like CdS structures have been prepared by a hydrothermal method with SDBS as surfactant. The influences of different experimental conditions on the morphologies, UV‐Vis and fluorescence properties of CdS have been investigated. The performances of CdS have been analyzed by X‐ray diffraction (XRD), field emission scanning electron microscopy (FESEM), ultraviolet‐visible (UV–Vis) and room‐temperature photoluminescence (PL). The XRD result indicates that the flower‐like CdS structures are of hexagonal phase. The FESEM results indicate that the main role of SDBS is to make the CdS crystals assemble together to form the flower‐like structures. The UV–Vis results show CdS has a strong absorption in the ultraviolet region and visible‐light region. The PL results show CdS has two emission peaks, respectively at 461 nm and 553 nm. The growth mechanism for the formation of flower‐like CdS structures is also described.     

InGaAs---GaAs pseudomorphic heterostructure transistors prepared by MOVPE

In this paper, we will demonstrate two new InGaAs-GaAs pseudomorphic heterostructure transistors prepared by MOVPE technology, i.e. InGaAs-GaAs graded-concentration doping-channel MIS-like field effect transistors (FET) and heterostructure-emitter and heterostructure-base (InGaAs-GaAs) transistors (HEHBT). For the doping-channel MIS-like FET, the graded In_0.15Ga_0.85As doping-channel structure is employed as the active channel. For a 0.8 × 100 μm² gate device, a breakdown voltage of 15 V, a maximum transconductance of 200 mS/mm, and a maximum drain saturation current of 735 mA/mm are obtained. For the HEHBT, the confinement effect for holes is enhanced owing to the presence of GaAs/InGaAs/GaAs quantum wells. Thus, the emitter injection efficiency is increased and a high current gain can be obtained. Also, due to the lower surface recombination velocity of InGaAs base layers, the potential spike of the emitter-base (E-B) junction can be reduced significantly. This can provide a lower collector-emitter offset voltage. For an emitter area of 4.9 × 10⁻⁵ cm², the common emitter current gain of 120 and the collector-emitter offset voltage of 100 mV are obtained.     

Activation parameters of some differently prepared thermoluminophors

For some thermoluminophors (LiF, Li₂B₄O₇, CaF₂, and CaSO₄) a thermoluminescence peak analysis was made in order to characterize the observed processes by their activation parameters. The resulting values for E₀ and K as well as the kinetic order are given. For different sample geometry the glow curve parameters can slightly be altered but the activation parameters remain almost invariant. For LiF the whole TL curve was described by a spectrum of resolved, independent levels and corresponding peaks.     

Amorphous iron-chromium oxide nanoparticles prepared by sonochemistry

Amorphous Fe₂O₃ and Fe_1.9Cr_0.1O₃ materials have been prepared by sonochemical method. X-ray diffraction patterns, transmission electron microscopy, Raman and infrared spectra, differential scanning calorimetry, Mössbauer and magnetic measurements revealed many interesting behaviors of the samples. Reaction to form the materials only occurred at the preparation temperatures of 70 °C or above. Upon heating, the sample prepared at 70°C presented a strong ferromagnetic behavior due to the presence of the magnetite phase coexisting with the hematite phase whereas the samples prepared at higher temperatures presented only the existence of the hematite phase. Thermal analyses of the sample prepared at 80°C revealed three exothermic peaks which were corresponding to the phase changes of dehydroxylation, crystallization of the maghemite phase and maghemite–hematite transition, respectively. The activation energies of the phase changes deduced from the thermal analyses showed that the presence of Cr enhanced the activation energy which can slow down the ageing effect of the amorphous state when being used in practice.     

Interface engineering of photoelectrochemically prepared Si surfaces

The oxide of Si(1 1 1) formed by electropolishing in dilute ammonium fluoride solution is analysed by photoelectron spectroscopy using synchrotron radiation. The oxidic layer is about 3.1 nm thick and contains Si-F_x species as well as oxyfluorides. The oxyfluorides are found preferentially at the electropolishing layer surface. SiOH species are concentrated at the oxidic film/substrate interface. The full width half maximum of the Si 2p line indicates that the Si/electropolishing oxide interface is smoother than the Si/natural oxide interface.     

Ultralow density silica aerogels prepared with PEDS

This paper deals with the synthesis of ultralow density silica aerogels using polyethoxydisiloxanes (PEDS) as the precursor via sol-gel process followed by supercritical drying using ethanol solvent extraction. Ultralow density silica aerogels with 5 mg/cc of density were made for the molar ratio by this method. A remarkable reduction in the gelation time was observed by the effect of the catalyst NH₄OH at room temperature. The microstructure and morphology of the ultralow density silica aerogels were characterized by the specific surface area, S_BET, SEM, TEM and the pore size distribution techniques. The results show that the diameter of the silica particles is about 13 nm and the pore size of the silica aerogels is about several nm. The specific surface area of the silica aerogel is 339 m²/g and the specific surface area, pore volume and average pore diameter decrease with increasing density of the silica aerogel.     

Amorphous metal-hydrogen alloys prepared by mechano-chemical reaction

The mechano-chemical formation of crystalline and amorphous hydrides is described. Niobium transformed into β-Nb hydride by milling with liquid normal-heptane. Chromium and manganese were transformed into amorphous hydrides. The hydrocarbon served as the source of hydrogen atoms through a dehydrogenation reaction.     

Chemical bonding and optical properties of germanium–carbon alloy films prepared by magnetron co-sputtering as a function of substrate temperature

Amorphous non-hydrogenated germanium carbide (a-Ge_{1 − x}C_x) films have been prepared by magnetron co-sputtering system designed by ourselves. The chemical bonding and microstructure have been analyzed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. The optical properties of the films have been investigated by means of spectroscopic ellipsometry. The relationship between the chemical bonding and the optical properties has been explored. It has been found that all films with the constant carbon content are amorphous. The sp² CC and sp³ GeC bonds increase with T_s, and some sp² CC bonds gain infrared activity. The fraction of sp³ GeC bonds rises with T_s, but the fraction of sp³ GeGe bonds gradually drops down. In addition, the refractive index and extinction coefficient increase with T_s. The film optical gap is seen to reach 1.15 eV when T_s is 200 °C. However, the optical properties of a-Ge_{1 − x}C_x films almost remain stable with the substrate temperature.     

Sodium borosilicate glasses prepared by the sol-gel process

A sodium borosilicate gel of composition 80SiO₂·15B₂O₃·5Na₂O (wt%) was prepared from tetraethyl orthosilicate, trimethyl borate, sodium methylate, H₂O, and HCl as the catalyst. Variation of specific surface area and porosity as a function of heating temperature indicated that closed pores were opened at temperatures lower than 400°C and collapsed above 450°C. From TG and DTA curves, about 19% Si and B atoms are evaluated to have −OH bonds. X-ray diffraction patterns indicated crystallization of low-cristobalite out of the gel when it was heated at 700°C for 5 h, showing a difference from a melt-quenched glass of the same composition.     

Thermosoftening phenyl siloxane glasses prepared via sol concentration

Thermosoftening phenyl polysiloxane glasses have been prepared by concentration of the corresponding sol above the melting temperature of the resultant materials, which we call the ‘sol concentration method’ (SCM). Because the glass melt is directly obtained without a gelation process, a remarkable shortening of the process time and suppression of crack formation in the sol–gel transition region have been realized. That is the whole process time of the SCM (about 24 h) is much shorter than that of the conventional sol–gel process (about 10 days). Thermosoftening phenyl polysiloxane glasses with high transparency in the UV region were obtained by the SCM.     

Microcrystalline silicon prepared at magnetic field modified nucleation

We present a complex characterization of thin silicon films, the growth of which has been influenced by permanent magnet, placed under the substrate. The pattern of microcrystalline regions in otherwise amorphous film varied with the orientation of the magnetic field. Study by atomic force microscopy and by micro-Raman spectroscopy revealed that the microcrystalline regions resulted from increased nucleation density of crystalline grains at the locations where the magnetron effect could be expected. This phenomenon allowed us to study in detail the transition between amorphous and microcrystalline growth. Moreover, it can be used as a kind of ‘magnetic lithography’ for preparation of predefined microcrystalline patterns in otherwise amorphous silicon films.     

Structural state of expanded graphite prepared from intercalation compounds

The structural state of nanocrystalline samples of expanded graphite is investigated using X-ray diffraction and neutron diffraction analyses. The expanded graphite samples are prepared by a rapid thermal decomposition of intercalation compounds of oxidized graphite based on fluorinated graphite, graphite oxide, and graphite aminofluoride. It is established that the main phase of expanded graphite belongs to the hexagonal crystal system (space group P6₃/mmc) and that carbon atoms in the structure occupy the 2b and 2c positions. The unit cell parameters and the unit cell volume in the structure of expanded graphite samples are larger than those in the structure of massive graphite.     

Spectroscopic investigation of porous GaAs prepared by laser-induced etching

We have investigated the etching mechanism in Cr–O doped GaAs wafer under super- and sub-bandgap photon illumination. A comparison of the etching rate and properties of nanostructures from two samples which are etched with argon-ion laser (2.41 eV) and Nd:YAG laser (1.16 eV), are presented here. The etching mechanism is found different for these different illuminations, which play the key role in the formation of defects. It is observed that the etching process starts vigorously under sub-bandgap photon illumination through the mediation of intermediate defect states. SEM micrograph shows the formation of distinct GaAs nanostructures in sample etched by Nd:YAG laser. Porous structure produced by Nd:YAG laser shows strong room temperature luminescence in the red region. The size and size distribution of the nanocrystals are investigated by non-destructive Raman and photoluminescence spectroscopies. The data are analyzed within the framework of quantum confinement models.     

Amorphous Si prepared in a UHV plasma deposition system

Although glow discharge decomposition of SiH₄ has been demonstrated to yield a-Si:H films with potential for large area device applications, the a-Si:H films often have high impurity level (～ 10¹⁹t- 10²⁰/cc). Moreover, cross contamination of dopants at interfaces between differently doped layers is a problem. Since both issues may be the key to improving material quality, a UHV plasma deposition systems has been built to address them. By controlling all sources of contaminants such as chamber walls, gas lines and gas bottles, the impurity levels have been significantly lowered to ～10¹⁷?10¹⁸/cc. In addition, sharp interfaces between differently doped layers are obtained, which allow the preparation of multilayer structures.