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Thermal carbonization of porous silicon surface by acetylene

Three different thermal carbonization processes of porous silicon (PS) surface by acetylene occurring between 400 and 950 °C were observed by in situ measurements. The peak temperature of oxidation shifts from 300 to 600 °C and the activation energy from 130 to 500 kJ/mole in the samples treated at 900 °C compared to untreated PS. Using Fourier transform infrared spectroscopy (FTIR), the bonding configurations associated with the temperature dependent processes were found. In the samples treated at or below 600 °C, traces of C–Hx bonds were clearly seen, but above 600 °C the FTIR spectra started resemble those of the SiC. The graphitization process was found to take place on the surface of PS samples treated above 800 °C. By changing the time between the C2H2 flush and thermal treatment, the graphitization can be controlled and even avoided. The weight increase was observed to be nearly proportional to the reduction of the surface area while the decrease in the surface area due to the thermal carbonizatio...

Effects of grinding and compression on crystal structure of anhydrous caffeine

Abstract The extent of the polymorphic transformation of anhydrous caffeine has been studied as a function of grinding time and compression pressure by using quantitative X-ray diffraction analysis. The measurements show that both grinding and compression induce the transformation from the metastable form I into the stable form II. The transformation can be observed even after 1 min grinding or by using a compression pressure of about 50 MPa in tableting. The degree of transformation is greater near the surface than in the middle part of the tablet.

Thermal oxidation of free-standing porous silicon films

We have studied the thermal oxidation of free-standing porous silicon films from room temperature to 730 °C with a differential scanning calorimeter and a thermogravimeter. We have observed three different thermal oxidation processes for the porous silicon. The change of enthalpy (ΔH) and activation energy (Ea) for the first reaction has been calculated. The oxidation of a fresh sample has been compared with those of aged samples, which were stored in dry relative humidity (RH 0%), humid (RH 100%) and normal (RH 25%–35%) laboratory air atmospheres. We also used Fourier transform infrared spectroscopy to clarify the bonds for each process.

Studies of Thermally‐Carbonized Porous Silicon Surfaces

Thermally-carbonized porous silicon films have been prepared by exploiting the dissociation of acetylene. Thermoanalytical methods have been used to study the oxidation behavior of these films in different oxidizing ambients. The results have been compared to other stabilization methods. Due to enhanced adsorption and only slightly reduced specific surface area, the carbonization of porous silicon was found to be an attractive treatment for sensing applications. The possible post-treatments are also discussed.

Ruthenium-modified MCM-41 mesoporous molecular sieve and Y zeolite catalysts for selective hydrogenation of cinnamaldehyde

Abstract Ru-modified MCM-41 mesoporous molecular sieve and Y zeolite catalysts were synthesised and characterised using XRD, electrochemical voltammetry, ESCA (XPS), nitrogen adsorption and H2-TPD. Selective liquid-phase hydrogenation of cinnamaldehyde to cinnamyl alcohol was investigated over these catalysts and compared to hydrogenation on a commercial Ru/C catalyst. The effect of support (MCM-41, Y and C), solvent (cyclohexane, hexane and 2-propanol) and catalyst pre-treatment (calcination) on the conversion of cinnamaldehyde and selectivity to cinnamyl alcohol was studied. The zeolite structure, pore size, acidity, catalyst pre-treatment as well as the solvent influenced the activity and selectivity. Non-calcined Ru/Y exhibited the highest selectivity to cinnamyl alcohol. The activity of Ru/Y was highest with 2-propanol as a solvent.

The room temperature oxidation of porous silicon

The room temperature oxidation of porous silicon was studied using isothermal methods. The oxidation was found to depend on the type of the porous silicon. The microcalorimetric signals from the oxidation of the p+- and n-type porous silicon in dry air were different. In humid air the signals from the oxidation could not be distinguished from the strong signal due to adsorption of water vapour, but when the samples were placed in water similar differences were observed. The reason for differences in reactions is discussed. The oxidation in different liquids was also studied. The signal from reactions in methanol and ethanol were found to be 100 times higher than in water. In FTIR studies the reaction gas produced by reactions between alcohols and the porous silicon, silane (SiH4) was found in the gas. Traces of SiOCH3 and SiOC2H5 groups were also found in FTIR spectra indicating SiOCxHy passivation of the surface.

Formation of dihydrate from carbamazepine anhydrate in aqueous conditions

Abstract According to our observations, carbamazepine dihydrate crystals grow by the whisker mechanism (or mechanisms). The habit, the dimensions of the crystals and their rapid growth indicate this. Also the strong orientation in the X-ray diffractogram is in harmony with the whisker theory. The whisker growth explains also the rapid transition from the anhydrous form to the dihydrate one in water. Dihydrate whiskers which are loosely in the solution as well as ones fastened to the surface of the anhydrous mother crystal has been observed. The oriented crystal structure of carbamazepine dihydrate may be at least one important factor in the growth mechanism of carbamazepine dihydrate whiskers.

A kinetic study of polymorphic transition of anhydrous caffeine with microcalorimeter

Abstract The phase transition of anhydrous caffeine form I → form II was investigated using isothermal microcalorimetry (IMC) and the kinetic results were compared with X-ray diffraction (XRD) studies. As the Arrhenius relationship was assumed to be valid in this study, the comparison was made by defining typical concentration vs. time curves and by generating Arrhenius plots based on times for different degrees of the transformation. The reverse transition was investigated with a differential scanning calorimeter (DSC) to estimate the value of the transition enthalpy Δ H trs of the polymorphic transformation in question. The selected kinetic equations fitted in IMC data gave great variation in the values of Δ H trs though the curves correlated usually quite well with the measured data points. Changes in the transition mechanism as the transition progresses were verified from IMC data via the technique, where the calorimetric heat flow signal d q /d t is plotted as a function of released energy Q for different measuring temperatures. The activation energies at specific degrees of the transformation were calculated from the plots of ln(d q /d t ) vs. 1/ T at corresponding energies. The values for activation energies increased as the transition progressed indicating changes in the transition mechanism. The tendency was obvious according to the XRD measurements, too. However, the kinetics obtained with IMC and XRD differed from each other remarkably, which is due to the differences in the detection principle and provides further information on the transition mechanism itself. The nonlinear correlation between IMC and XRD quantitative results showed the transformation to begin at regions with high stored energy.

Electrostatic measurements on a miniaturized fluidized bed

In the pharmaceutical industry fluidization is often used in drying, coating and granulation processes. During fluidization powder particles contact other particles as well as the walls of the vessel and this leads to generation of electric charges. These charges may result in sparks, dust explosions, fires, reduced process efficiency and particle accumulation on the walls. However, the mechanism of the charging is not well understood. In this paper, an inductive method for charge generation measurement is presented. The system consists of an electrostatic ring probe, which allows charge scanning across the miniaturized fluidized bed without disturbing the fluidization process. The charge coupling from the system to the probe has been modelled and the experimental data have been simulated using an advanced field solving software. Experiments on lactose monohydrate, microcrystalline cellulose and glass beads have been performed and these results are presented.

Electrostatic measurements on lactose–glucose mixtures

Abstract This paper presents the results of an experimental study on electrostatic behavior of lactose and glucose which are commonly used in the pharmaceutical industry as additives. Resistivities and frictional charging of samples with different proportions of lactose and glucose were measured. Powder samples were charged by sliding them down into the Faradays cup via a pipe. Resistivity was found to follow a linear relationship with the concentration but the nature of frictional charging changed considerably when mixtures were charged instead of pure additives. Both pure additives charged positively when slid into the Faradays cup via a glass pipe but some mixtures became negatively charged due to powder–powder contacts within the flow and near the pipe surface.