The synthesis of aligned silicon nanowires under ambient atmospheric pressure

Highly ordered amorphous silicon nanowires were successfully synthesized from single crystalline silicon wafer at the pyrolysis temperature of 1050 °C under ambient atmospheric pressure. Both poly (phenylcarbyne) and nickel nitrate played important roles in the growth of silicon nanowires. The fabrication of ordered silicon nanowires was controllable and repeatable, confirmed by the experimental results. The morphology and microstructure analysis of the as-obtained samples showed the highly ordered amorphous silicon nanowires were obtained, determined by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and FT-infrared spectroscopy. A solid-liquid-solid growing process was proposed.     

Properties of some perovskites synthesized under pressure

Interstitial solid solutions with the perovskite – like structure were synthesized by solid phase reactions at high pressure and temperature. The synthesis conditions of single phased samples, their stability on heating in air were determined. The magnetic and electrical properties were measured.     

The structure of liquid bismuth under pressure

The structure factor, S(Q), of liquid bismuth near the melting temperature at pressures up to 7.3 GPa has been measured by an energy-dispersive X-ray diffraction method, using synchrotron radiation and a cubic-type high-pressure apparatus. The structure of liquid bismuth at atmospheric pressure shows similarity to high-pressure solid phases rather than to the atmospheric-pressure phase. With increasing pressure below 7 GPa, S(Q) changes continuously, becoming simpler in form and with the first peak becoming higher. In addition, the structure factor obtained at 7.3 GPa is totally different from those at lower pressures. It has extremely sharp peaks located at regular intervals in Q. It is concluded that liquid bismuth changes its structure drastically between 6 and 7 GPa. A model for the structure of liquid bismuth above this structural change is proposed.     

Tammann paradox and amorphization of substances under pressure

It is shown that amorphization of substances under pressure can be related to the fundamental property of the isobaric-isothermal potential surface as a function of temperature and pressure: its convexity. Amorphization is cold melting. The peaks in the pressure dependences of the melting temperature are not anomalies but a manifestation of the general case, whose implementation is limited by polymorphism. A model of the amorphous state near the absolute zero of temperature is proposed: specifically, nanoceramics formed according to the mechanism of polysynthetic twinning.     

Stability of amorphous SETE mixtures under pressure

A sharp semiconductor-metal transition is observed for a-Se₈₀Te₂₀ mixture at about 90 kbar. The resistance for the specimen which experiences the semiconductor-metal transition is recovered after releasing pressure. The sharp semiconductor-metal transition is reproducible in the second compression. X-ray diffraction measurements reveal that the amorphous specimen is transformed into the crystal with layer structure on the transition to metallic state and on releasing pressure the specimen is transformed again into the amorphous state. A brief comment is given for the interesting phenomena that the high-pressure crystalline phase can be recovered to the amorphous phase on releasing pressure.     

A volumetric and dielectric study of m-fluoroaniline under pressure

We have studied the mass density and the dielectric permittivity of the glass forming liquid meta-fluoroaniline under hydrostatic pressure up to 700 MPa. Isothermal, isobaric, and isochoric glass transitions were induced which demonstrate the dependence of the glassy state on the thermodynamic path. The pressure dependence of the secondary relaxation and of the glass transition/melting temperature and the lower density of the pressure-induced glass compared to the liquid support a cluster model with defects for this hydrogen bonded glass.     

Glass transition temperature shift under pressure for some semiconducting glasses

Glass transition temperatures (T_g) were measured as a fucntion of pressure for the semiconducting glasses As₂S₃, Te₁₅Ge₂As₃ and Cd₆Ge₃As₁₁, using changes in the transit time of an ultrasonic pulse to detect the glass transition. The measurements of T_g as a function of pressure were compared to the predictions of the free volume theory of the glass transition. For As₂S₃ and Te₁₅Ge₂As₃, T_g first increased and then levelled off with increasing pressure, as predicted by the free volume theory. The amorphous semiconductor Cd₆Ge₃As₁₁ exhibited anomalous behavior: T_g first increased with pressure, but at the relatively low pressure of 1.5 kbar it began to decrease with increasing pressure.     

Fabrication of highly homogeneous As2Se3 glass under argon flow

The present study relates to a new method for the synthesis of As₂Se₃ glass in a controlled atmosphere. The advantage of this technique is that it does not require sealing of the silica reaction container and therefore makes it likely to substitute the current industrial batch by batch synthesis which actually needs very expensive single-use sealed silica vessels.An experimental device has been developed for these purposes. It is equipped with a stirring mechanism to homogenize the molten bath. In order to avoid contamination by oxygen and moisture, the synthesis is carried out under argon flow (pressure of 1 bar). Material losses during synthesis can be reduced to less than 2% when temperature is progressively increased up to 430 °C. Bulk glass ingots are finally obtained according to a two-step annealing process. Their chemical composition is analyzed by EDS and shows a variation range of less than 0.2%. The excellent reproducibility of the given method is also confirmed by the refractive indexes, that do not differ for more than 1 · 10⁻³ from one another.Adverse absorption bands due to oxygen do not occur in the 8-12 μm spectral region when 1000 ppm of Mg is added. As no distilling operation has been carried out until now, the magnesium oxide partially keeps staying in the glass and leads to scattering losses at short wavelengths.     

Mechanical properties of polymer-modified silica aerogels dried under ambient pressure

Silica gels prepared by copolymerizing tetraethylorthosilicate with 3-aminopropyltriethoxy-silane were modified using polymer derived from toluene diisocyanate and dried under ambient pressure. The successful preparation of silica aerogels depended on the effective control of shrinkage during drying. The resulting material, polymer-modified silica aerogel, was then characterized by thermogravimetric analysis and uniaxial compression tests. Results indicated that the apparent elastic modulus and compressive strength of the polymer-modified silica aerogels decreased with increasing amounts of incorporated polymer because of decreasing shrinkage and density, while the strains at the surface cracking point and the final failure point increased significantly during compression tests. The strength and modulus of the silica skeleton could be calculated from the apparent strength and modulus of the silica aerogels respectively. It was interestingly shown that the elastic modulus of the silica skeleton of the silica aerogels increased because of the incorporated polymers, while the polymers had no effects on the compressive strength of the silica skeleton. In addition, the relationships between the apparent elastic modulus or the apparent compressive strength of the polymer-modified silica aerogels and their shrinkage were quantitatively expressed.     

Silica under hydrostatic pressure: A non continuous medium behavior

The homogeneous/inhomogeneous structure of glasses is still a debated question. Hydrostatic high pressure experiments allow us to determine if a glass behaves as an elastic continuous random network or if a nanometer scale heterogeneity has to be taken into account. In order to get information on the homogeneous/inhomogeneous structure of glasses, in situ high pressure Raman experiments are performed on silica in the elastic domain up to 4.7 GPa. A strong decrease of the Boson peak intensity is observed between 1 bar and 3 GPa. We show that this decrease does not correspond quantitatively to the effect of pressure on a homogeneous elastic medium. From the interpretation of the narrowing of the main Raman band width under pressure as a narrowing of the θ inter-tetrahedral Si–O–Si angle distribution it is shown that the decrease of the Boson peak intensity is correlated to the decrease of the intrinsic inhomogeneity of the silica glass. These results confirm the occurrence of an intrinsic inhomogeneity at a nanometer scale even in a single component glass like SiO₂ which is very important for the interpretation of the optical or mechanical properties of the glasses.     

Cost-effective synthesis of silica aerogels from fly ash via ambient pressure drying

Silica aerogels were synthesized from the industrial fly ash by ambient pressure drying method. The process consists of two stages, preparation of sodium silicate solution from fly ash by hydrothermal reaction with sodium hydroxide, and synthesis of porous silica aerogels from the obtained sodium silicate solution. Silica wet gels were formed by vitriol-catalysis or resin-exchange-alkali-catalysis of the obtained sodium silicate solution. The trimethylchlorosilane(TMCS)/ethanol(EtOH)/hexane mixed solution was used for solvent exchange/surface modification of the wet gel so as to obtain porous silica aerogels via ambient pressure drying. The results indicated that the synthesized silica aerogels were lightweight and hydrophobic. The BET specific surface area, pore volume and average pore diameter were 362.2-907.9 m² g^− 1, 0.738-4.875 cm³ g^− 1, and 7.69-24.09 nm respectively. Particularly, the synthesized silica aerogels by resin-exchange-alkali-catalysis method showed uniform mesoporous structure, and had much higher specific surface area (907.9 m² g^− 1) and pore volume (4.875 cm³ g^− 1) than that of by vitriol-catalysis process.     

Refractometry of TGS crystals doped with L-threonine impurity under uniaxial pressure

The temperature and spectral dependences of the refractive indices of triglycine sulphate (TGS) crystals doped with L-threonine impurity have been investigated. It is established that the introduction of an impurity weakens the temperature dependence of refractive indices. The electronic polarizability, refractions, and parameters of UV oscillators of mechanically deformed impurity crystals are calculated. The temperature coefficients of the phase transition shift are determined.     

Infiltration under isostatic pressure of porous silica glasses with silica sols

In this work a SiO₂ matrix with more than 50% porosity was developed and infiltrated with a pure silica sol under isostatic pressure, as a prior step to the immobilization of radioactive waste using this technique. The silica glass was prepared through the acidic leaching of phase-separated and partially-sintered sodium–borosilicate glass powder compacts. Phase separation was promoted at different stages of the sintering process to obtain different total porosity or pore size distributions, which in all cases showed macro, meso and micropores. Infiltration leads to a significant increase in weight, reflecting the initial porosity of the substrates. Porosimetry techniques (Hg porosimetry and N₂ adsorption isotherms) show that the silica sol fills practically all the pores with diameters over 3 nm. Preliminary sintering tests show that the infiltration technique lowers the sintering temperature by more than 150 °C.     

A survey of the elastic properties of some semiconducting glasses under pressure

We have measured both shear and longitudinal velocities for 10 MHz sound in amorphous As₂S₃, As₂Se₃, As₂Ge₂SeTe, As₂Ge₃Te₁₅ and Cd₆GeTe and Cd₆Ge₃As₁₁ over the pressure range 0–8 kbar and the temperature range 25–100°C. A variety of elastic constants has been computed. Those glasses with larger values of the connectedness were stiffer and showed less pressure effect.     

Simulation studies of structural changes and relaxation processes in lysozyme under pressure

The paper describes preliminary results of a molecular dynamics simulation study on the influence of non-denaturing hydrostatic pressure on the structure and the relaxation dynamics of lysozyme. The overall compression and the structural changes are in agreement with results from recent nuclear magnetic resonance experiments. We find that moderate hydrostatic pressure reduces essentially the amplitudes of the atomic motions, but does not change the characteristics of the slow internal dynamics. The latter is well described by a fractional Ornstein–Uhlenbeck process, concerning both single particle and collective motions.     

Photoluminescence of annealed LLGG:Cr3+ crystals under high pressure

High pressure photoluminescence has been used for analysis of lattice disorder in La_2.7Lu_2.29Cr_0.01Ga₃O₁₂ and La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂ crystals. Photoluminescence of samples before annealing and those annealed for 5 h in oxygen and hydrogen atmosphere at 923 K has been measured. The pressure dependence of Cr³⁺ luminescence has been used to obtain the crystal field distribution (lattice disorder) and its dependence on the thermal treatment. The distribution energy of the ⁴T₂ state has been considered as a measure of disorder. Annealing in oxygen has been found to reduce the energies of ⁴T₂ states for all sites, and annealing in hydrogen – to increase these energies.     

Precise sound velocity measurement for liquid Se50Te50 under high pressure

We have measured the sound velocity in liquid Se₅₀Te₅₀ at 19.5, 32.6, 45.6, and 58.2 MHz simultaneously by means of an ultrasonic pulse transmission/echo method. By using a phase-sensitive-detection technique the relative error was reduced to less than 0.1%. The temperature dependence of sound velocity at 100 MPa exhibits a minimum at 600 ^oC and a maximum at 1035 ^oC, which are related to the large structural change accompanied by the semiconductor–metal (S–M) transition. In addition, sound dispersion has been observed at temperatures from 500 °C to 900 °C for the first time, where anomalous sound attenuation was previously reported. This result implies that a structural relaxation on nanosecond timescale takes place in the S–M transition region.     

Melt-casting of Si-Al-Y-O glasses and glass-ceramics by combustion synthesis under high gravity

Si-Al-Y-O glasses and glass-ceramics were prepared via melt-casting by combustion synthesis under high gravity. The phase assemblage and microstructure of the cast products strongly depended on the content of SiO₂ and the additive ZrO₂ or La₂O₃ in the starting compositions. With increasing content of SiO₂, the glass-forming ability of the melt was enhanced. The additive ZrO₂ was not dissolved into the glass and inclined to crystallize, but La₂O₃ was inclined to remain in the glass matrix instead of precipitation. With a short processing period and lower energy consumption, combustion synthesis under high gravity can offer a new approach to fast fabrication of glass and glass-ceramic materials.     

Refractive indices of (NH4)2SO4 crystals under uniaxial pressure

Crystallography Reports - The effect of uniaxial pressure σ m ≤ 200 bar on the spectral (300–800 nm) and temperature (300–77 K) dependences of the refractive indices...