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.     

Phase transition of nematic to smectic C through an intermediate boundary induced smectic A structure

An intermediate phase has been observed at the transition nematic to smectic C where a smectic A structure induced by the boundaries and a nematic phase exist simultaneously. The temperature interval of existence of the intermediate phase is of the order of one tenth of degree. Several conditions are necessary for its appearance: the latent heat L of the phase transition nematic-smectic C of the liquid crystal should be sufficiently small (L < k_BT_c); the nematic phase should undergo a bend deformation by the boundaries (possibly a twist one as well) and the transition nematic-smectic C should be carried out as slower as possible.     

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.     

The influence of crystallization conditions on the onset of dendritic growth of calcium carbonate

Understanding the crystallization of calcium carbonate is relevant in numerous fields like biomineralization, geology and industrial applications where calcium carbonate forms. In order to enhance the knowledge about the formation of calcium carbonate with focus on the vaterite polymorph, in this work calcium carbonate has been crystallized in aqueous solutions at temperatures from 5 °C to 90 °C. Special attention has been directed to higher temperatures for which the effect of the initial supersaturation on the resulting crystal morphologies and the onset of dendritic growth have been studied. In analogy to snow crystal formation, it has been found that in a certain temperature range hexagonal plate‐like crystals form at low supersaturation whereas dendritic patterns start to appear at higher supersaturation. The symmetrical branches characteristic for dendritic growth get less pronounced as the temperature is decreased. The results reported here related to the interdependence between supersaturation, crystal morphology and growth mechanisms, can be used in future works to predict particle formation and to design crystal architectures. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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)     

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.     

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 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.     

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.     

Recent research on pseudobiological hydroxyapatite crystal growth and phase transition mechanisms

We have studied the growth and phase transition mechanisms of hydroxyapatite (HAP) and its interaction with a growth factor protein in a simulated physiological environment. Using atomic force microscopy (AFM) and real-time phase shift interferometry, we performed in situ observations of growth in simulated human body fluid solutions seeded with millimeter-sized HAP single crystals produced by hydrothermal synthesis, and we measured the normal growth rate. The step kinetic coefficient (derived from the velocity of growth steps) and the edge free energy (calculated from the variation in the normal growth rate with the degree of supersaturation) both deviated greatly from the standard values for typical inorganic salt crystals and were found to be close to those of protein crystals. This suggests that the growth units of HAP crystals are clusters rather than simple ions and that growth proceeds through the accumulation of these clusters. Observations using dynamic light scattering confirmed the presence of clusters with a diameter of about 1 nm in simulated body fluids. Ab initio analysis of the cluster energy stability indicated that calcium phosphate clusters based on Ca₃(PO₄)₂ units achieve an energy minimum for clusters of the form [Ca₃(PO₄)₂]₃. These clusters have S₆ symmetry, and, when they are used to build a HAP crystal structure, their structure is likely to become slightly modified, resulting in the formation of C₃ structures.Since these clusters would also be the building blocks of amorphous calcium phosphate (ACP), they provide vital clues to the phase transition from ACP to HAP. Using time-resolved static light scattering, we tracked the ACP–HAP phase transition process and found that the degree of coarseness inside a cluster aggregate changes abruptly within a specific time interval and HAP is formed and deposited in the final stages. This suggests that an ACP aggregate changes into HAP as its internal structure becomes regularized. Using this phase transition process, we produced a complex of HAP and the FGF-2 growth factor protein. This complex releases the FGF-2 into a physiological saline solution over a period of at least a week, meaning that it can be used as a pharmacologically active third-generation biomaterial.