Theoretical investigation of the electronic and structural properties of AlN thin films

Studying the electronic and structural properties of AlN thin films is an important problem because such films are widely used as a buffer layer when growing GaN-based semiconductor heterostructures on Si substrates. In this paper, we carry out a theoretical investigation of the properties of an Al-terminated AlN(0001) surface in the framework of the density functional theory. Ab initio calculations allow us to analyze the effect of the in-plane lattice strain on the energy of this surface. It is shown that compressive strain causes a decrease in the AlN(0001) surface energy, while tensile strain leads to its increase. Knowing the surface energy values allows us to evaluate the stress of the surface under investigation. In addition, the curvature of the AlN surface is calculated for various AlN film thicknesses in the case of free growth. The obtained values of the surface curvature are in close agreement with the known experimental results.     

Physical Properties of Polyurethanes Produced from Polyols from Seed Oils: I. Elastomers

Polyols synthesized by ozonolysis and hydrogenation from canola oil were reacted with aliphatic 1,6-hexamethylene diisocyanates (HDI) to produce polyurethane (PU) elastomers. The properties of the materials were examined by dynamic mechanical analysis (DMA), thermomechanical analysis (TMA), modulated differential scanning calorimetry (MDSC), and thermogravimetric analysis (TGA), and measurements were taken of tensile properties. The effect of dangling chains on network properties was assessed. The formation of hydrogen bonds was observed by FTIR. The measured properties were found to be strongly dependent on processing-dependent factors such as the crosslinking density and the molar ratio of polyols to HDI component. The glass transition temperatures (T _g) of the elastomers were found to increase as the OH/NCO molar ratio decreased. With the same OH/NCO molar ratio, T _g of canola-oil-based PU was higher than that of soybean-oil-based PU. The TGA thermographs showed two well-defined steps of degradation for all the elastomers. In the first step, up to 30% weight loss, the fastest rate of loss was found at 345 °C for canola-oil-based PU while soybean-oil-based PU lost most of the weight in the second step. With the same OH/NCO molar ratio, the elastomers made from canola-oil-based polyol showed slightly higher Young’s modulus and tensile strength.     

The difference between cocoa butter and salatrim® lies in the microstructure of the fat crystal network

The solid fat content of Salatrim® at 22°C is 9% higher than that of cocoa butter; however, its storage modulus, G′ (1 Hz), or solid-like character, is one order of magnitude lower (2.5 MPa vs. 52 MPa). This difference may be explained by structural differences in the microstructure of the fat crystal network of the respective fat systems. Polarized light micrographs of these two confectionery fats show that cocoa butter forms a fat crystal network characterized by discrete crystalline particles which aggregate to large microstructures which then form a three-dimensional network, while Salatrim®’s network is composed of randomly arranged noncrystalline, translucent platelets. Rheological measurements on both fat networks yielded fractal dimensions of 2.37 for cocoa butter and 2.90 for Salatrim®. Image analysis of the microstructure of cocoa butter yielded a fractal dimension of 2.31; however, the microstructure of Salatrim® does not lend itself to fractal analysis via image analysis. It was observed that the microstructure of Salatrim® is random instead of fractal. The proposition is made that the macroscopic mechanical properties of Salatrim® are related to the mechanical properties of the platelets that make up the network and the nature of the links between the platelets. Furthermore, the random spatial distribution of the platelets does not provide an indication of the strength of the network. For cocoa butter, the macroscopic mechanical properties are integrally related to the fractal spatial distribution of the solid mass in the network, the nature of the links between the microstructures, and the strength of the microstructures.     

Thermoelastic Fields in Sapphire

The temperature and thermoelastic fields in growth systems are considered theoretically in order to assess their effect on the optical symmetry of the growing crystal. The process is modeled using three-dimensional curvilinear coordinates to describe a closed, low-strain thermoelastic system, with allowance made for the temperature variations of the thermal properties of the multilayer growth system and nonlinear and unsteady-state processes with arbitrary boundary conditions. The results, presented as plots of the strain, stress, displacement, and temperature fields, demonstrate the potential of the method for designing new growth units and improving the existing ones and suggest that crystals without frustration of optical symmetry can, in principle, be grown.     

Synthesis and Characterization of Novel Diol,Diacid and Di-isocyanate from Oleic Acid

Novel linear diol 2, diacid 3, and diisocyanate 4 with functional groups located at each end of the chain have been produced from oleic acid via a diester precursor. All the compounds were fully characterized by IR, ¹H NMR, ¹³C NMR and Mass Spectrometry. These unbranched terminally-functionalized compounds are important monomers for the manufacture of biodegradable thermoplastic polyurethanes and polyesters. The general synthetic scheme should also prove useful for the syntheses of diols, diacids and diisocyanates from any unsaturated fatty acid.