Induced smectic A phase by homeotropic orientation in 4, n-octyloxybenzoic acid (OOBA) at nematic-smectic C phase transition

Using homeotropic and quasi-homeotropic orientation in thin film of 4-n-octyloxybenzoic acid, we indicated intermediate smectic A-like state during the nematic-smectic C phase transition. Pseudoisotropic and focal conic textures are characteristic for homeotropic and quasihomeotropic orientations respectively. The smectic A-like state induced by the boundary conditions exists in a very short (about 0.5 °C) temperature region.     

Discontinuous Freedericksz transition in the smectic C phase

The threshold field of the Freedericksz transition in planar oriented S_C monocrystals possessing positive dielectric anisotropy is calculated for two boundary conditions. It is shown that the threshold field is relatively low when a discontinuous transition is taken into consideration. The effective elastic constant is determined from the experimental data.     

The character of FeMn-1# powder catalyst and its influence on the synthesis of diamond

In this paper industrial diamond crystal was synthesized using FeMn-1# powder catalyst in China-type cubic high-pressure apparatus at 5.7 GPa and 1400–1600 °C. The growth feature of diamond in the graphite–FeMn-1# system was researched. Optical microscope observation showed that all the diamond crystals were light yellow octahedral with grain size of 0.3–0.5 mm. There are also plenty of bubbles in the crystals. By SEM, we can see that the surface of diamond is smooth and the crystal is intact. Mössbauer spectrum was used to detect the impurity in the diamonds.     

Peculiarities of the phase behavior of liquid-crystal mixtures at the Cholesteric → Smectic (Ch → Sm) transition boundary

It is found that an interphase state, characterized by a large Bragg reflection coefficient, is formed in smectogenic cholesteric liquid-crystal mixtures at the Ch-Sm transition boundary. The interrelation between the mixture composition and the optical and spectral characteristics is studied. A possible structure of the interphase state is proposed.     

The nature of the glass transition in a columnar hexagonal ordered phase

The glass transition taking place in a columnar discotic phase is connected with the freezing-in of the correlated rotation of the disc-like molecules in the columns about the columnar axis. X-ray investigations on the temperature dependence of the linear expansion coefficients along the columnar axis and perpendicular to this axis reveal that the glass transition leads to a stepwise change of the expansion coefficient along the columnar axis and to a continuous variation for the corresponding coefficient perpendicular to this direction. The glass transition in such ordered systems is highly anisotropic both with respect to dynamic and to thermodynamic aspects.     

Effect of preparation conditions on the phase transformation of mesoporous alumina

A post-hydrolysis method is proposed for the easy-and-fast preparation of mesoporous γ-alumina (MA) at room temperature using an alkyl carboxylate as a chemical template. Water was used as an initiating solvent in the hydrolysis of aluminum alkoxide in the terminal reaction procedure. Hydrolysis and condensation reactions were simultaneously induced. The initial phase of as-made MA was bayerite and/or (pseudo)boehmite, and was transformed into active alumina at 250 °C, which remained thermally stable at temperatures up to 600 °C. The phase transformation of alumina was analyzed with DTA, XRD and ²⁷Al MAS NMR. Based on an ²⁷Al MAS NMR analysis, calcined alumina prepared via a hydrothermal method contained a high ratio of Al^VI to Al^IV (∼4.8), which is not fully transformed into active alumina, and aluminum hydroxide (AlOOH or Al(OH)₃) was detected. These results suggest that high temperature is required to obtain an active form of alumina, when the hydrothermal method is used. Another factor is the ratio of Al^VI to Al^IV in the MA, which is also dependent on the pH of the reactant. Pore size, surface area, and pore structure could be controlled by adjusting the molar ratio of water to aluminum precursor, and the preparation temperature. The pore size of MA was adjustable from 2 to 7 nm, and the MA indicated a large surface area of 300 ∼ 500 m²/g.     

On the shape of the phase boundary of melt-grown ribbon-shaped crystals

Approximated analytical solution of the stationary Stefan problem for the ribbon crystals pulled from the melt are obtained. The results of calculation of the crystallization front are compared with the numerical solution obtained by finite elements method. The good coincidance is demonstrated.     

The effect of chemical distribution on the magnetocaloric effect: A case study in second-order phase transition manganites

Ferromagnetic manganites of general formula ${\text{ABMnO}}_3$ (where A is a trivalent rare-earth ion and B is a divalent dopant) are candidates for magnetic cooling applications, since they can present either second- or first-order magnetic phase transitions, and the perovskite structure allows for substantial chemical substitution and tuning of the magnetocaloric properties. The consequent chemical distribution from substitution affects the magnetic and magnetocaloric properties of the compound. The change of relative cooling power with chemical disorder is discussed by the use of the molecular mean-field model. We present experimental results of the ferromagnetic, second-order phase transition ${\text{La}}_{0.70-x}(\text{Er,}\text{Eu}{)}_x{\text{Sr}}_{0.30}{\text{MnO}}_3$ system.     

Specific character of melt growth of low-temperature phase of aluminum beryllate

It is shown that the melt growth of low-temperature phase aluminum beryllate crystals, Al₂BeO₄ (ABO), requires specific metastable conditions. These are, first of all, the melt pretreatment and insertion of a “cold” seed preserving the stability of its structure and providing a local decrease in melt temperature in the seeding zone. Contact of thermally treated melt with the high-temperature phase must be avoided as the melt loses its metastable state. The role of d-element dope in polymorphous transformations of ABO and narrowing of crystallization field of its high-temperature phase from melt is demonstrated. Some features of polymorphism and crystallization of ABO melt are interpreted in terms of associative complexing as the basis of mechanism of structure formation in condensed media.     

Generation of random network models with periodic boundary conditions

We have investigated a novel procedure for the generation of tetrahedral random network models compatible with periodic boundary conditions. Data for a sequence of such models is presented and critically examined.     

Study on the ferroelectric phase transition in potassium lithium niobate (KLN)

The ferroelectric phase of potassium lithium niobate K₃Li_2–xNb_5+xO_15+2x (KLN) in the range of 0.15 < x < 0.5 is a very promising material for the second harmonic generation (SHG) in the blue visible region (∼410 nm). The ferroelectric phase transition was shown to occur between 400 and 500°C depending on the composition of the KLN phase. In this study several analysis techniques were used to investigate the phase transition on ferroelectric (x = 0.3) KLN samples. The temperature‐dependent measurements of the relative dielectric constant ε₃₃ provided a phase transition temperature of about 470°C. In our DTA experiments, a small but reproducible thermal effect at the phase transition in KLN was indicated. The temperature‐dependent birefringence measurement technique, applied the first time on KLN, shows a second order behaviour at a temperature of 467 °C. However, this phase transition is accompanied by a small thermal effect. The DSC analysis for the other KLN composition (x = 0.5) provided a phase transition temperature of 514 °C. The appearance of a phase transition in the paraelectric KLN phase (Nb content higher than 55 mol%) was also studied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental observation of the surface roughening transition in vapor phase growth

The surface roughening transition has been observed in growth from the vapor phase in both C₂Cl₆ and NH₄Cl. Both these materials are plastic crystals which have cubic structures and sublime at atmospheric pressure rather than melting. The surface roughening transition was detected by observing changes in the growth morphology in these two materials. In C₂Cl₆ the roughening transition is approximately at 100°C where the equilibrium vapor pressure is 40 mm, and in NH₄Cl it is at approximately 365°C and 3 atm pressure.     

The influence of etching conditions on surface contamination during silicon etching in CF4 plasma

The dependence of blanked-etched silicon surface quality on the etch parameters and mask material was studied. First it was tried to find such etch conditions, where the silicon surface keeps clean. It was found that etching in RIE-mode yields better surfaces than in PE-mode. There was no noticeable influence by variation the oxygen content in the CF₄/O₂-etch gas mixture and the total gas flow. If the generator current was raised, the edge zone of the wafer, where the surface is contaminated, becomes greater. Second it is shown that the presence of aluminium as mask material makes worse the surface quality. This effect is stronger during etching in the RIE-mode than in the PE-mode.     

The influence of water steam on the direct phase transformation of zeolite NaA to nepheline by thermal treatment

Heating of zeolite NaA leads to the consecutive formation of amorphous phase, carnegieite and nepheline in the high temperature region. These phase transformations are influenced not only by temperature but also by the presence of water (inside of the crystals). Water causes destruction of zeolite framework by hydrolytic splitting of Si-O-Al-bonds in addition to thermal effects. Thus, the amorphous phase as the first product of phase transformation occurs in the presence of water steam at a lower temperature. The formation of the crystalline phases carnegieite and nepheline was observed in a temperature range, 300 K lower than usually, and without occurrence of the amorphous phase.     

On the piezoelectric properties of multiferroic nanofilms with allowance for the flexoelectric effect on morphotropic phase boundary

The minimum size (nanoscale) of various phases occurring on the morphotropic phase boundary is determined based on the dislocation theory. It is shown that the flexoelectric effect is strong on this boundary, which explains the anomalous piezoelectric properties of nanoscale epitaxial structures, regardless of the type of crystal material. The debated question of the value of piezoelectric coefficients in favor of classical estimate is considered on the basis of dimensional relations and experimental data.     

A Study on the phase transition in decamethylosmocene crystal over a wide temperature range

The reversible structural phase transition accompanied by the doubling of the unit cell parameter c and a change in the space group is found in the decamethylosmocene crystal Cp ₂ ^* at a temperature of 243 ± 5 K. In the high-temperature (HT) phase (space group P2₁/m, Z = 2), the molecule adopts an eclipsed conformation and lies in the mirror symmetry plane. In the low-temperature (LT) phase (space group P2₁/c, Z = 4), the molecule occupies the general position with a small rotation of the ligands. The structural transformations from the high-temperature phase into the low-temperature phase at 120 K are described by the deviation of the molecular axis of fivefold symmetry by 3.2(2)° and the displacement of the center of mass of the molecule by 0.173(2) Å with respect to their positions in the high-temperature phase, and also by the rotation of the Cp* ligands in the same direction by 3.0(2)° and 5.4(2)°, respectively.     

Molecular aspects of the main phase transition in lipid systems as a weak first-order phase transition: 1. Model of thermodynamic behavior of lipid membranes

The possibility to describe the principal phase transition in lipid systems as a weak orientational first-order phase transition with characteristics close to those of a second-order phase transition characterized by pretransition phenomena and rather strong fluctuations has been considered. A first order transition is explained by interactions between fluctuations of molecule orientations and density fluctuations at a low shear modulus. The jumpwise behavior of enthalpy, volume, heat capacity, compressibility, sound velocity and absorption, and the order parameter are analyzed. It is shown that because of molecular van der Waals interactions the shear modulus should give rise to more pronounced jumps of the above quantities with an increase in molecule lengths.