High pressure solution growth of GaN

An internally heated pressure vessel was used to study the decomposition reaction of GaN at temperatures above 900°C and the phase equilibria in the system Ga-N₂. As a consequence of these studies we have undertaken the crystal growth of GaN free crystals and epitaxial layers on sapphire by a VLS process. We have succeeded in the synthesis of high quality epitaxial layers showing the terrace structure typical of LPE. As a function of the growth conditions both n and p type gallium nitride were obtained, the latter only in polycrystalline form.     

The phase transformation behaviors of Sn2+-doped Titania gels

The transformation behaviors of Sn²⁺-doped titania gels, such as transformation temperature, conversion rate and the crystal granularity, were studied by using thermo-gravimetric/differential thermal analysis and X-ray diffraction methods. Experimental results show that, anatase, rutile and SnO₂ phases can exist in the sintering products by varying Sn²⁺ content and sintering temperature. Sn²⁺-doping greatly depresses the growth of anatase and rutile crystals and so obtaining nanosized crystals The transformation temperatures of gel to anatase and anatase to rutile first decrease and then increase with an increase of Sn²⁺ content, while both of the transformation rates are associated with Sn²⁺ content, change in its valence and the sintering temperature. It is suggested that by adjusting Ti⁴⁺/Sn⁴⁺ ratio and sintering temperature, the crystal granularity and the ratio of anatase to rutile can be tailored to optimize properties of TiO₂–SnO₂ humidity sensing ceramics.     

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.     

High pressure effects on liquid viscosity and glass transition behaviour,polyamorphic phase transitions and structural properties of glasses and liquids

Since the pioneering work of Bridgman it has been known that pressure affects the glass transition of polymers and liquid state viscosities. Usually the T_g and viscosity both increase as a function of pressure as expected from ‘free volume’ theories. However, H₂O provided a notable exception in that the viscosity passes through a minimum at low temperature. It was thought that this might be linked to the anomalous thermal expansion behavior. However further research on geologically important aluminosilicate liquids revealed that they could show anomalous viscosity decreases with increasing pressure and this behavior is given a structural interpretation as five-fold coordinated Si⁴⁺ and Al³⁺ species are formed. Also the existence of polyamorphism or density-driven liquid–liquid phase transitions in certain systems can lead to anomalies in the T_g or η vs. P relations. This may be the case for H₂O, for example. Current research is focusing on investigating structural changes in liquids and glasses at high pressure as the rich variety of behavior is becoming recognized. Both experimental studies and computer simulations are important as the underlying phenomonology is linked to changes in the glass or liquid structure as a function of densification.     

Time and temperature induced phase transformation in L‐isoleucine hydrochloride monohydrated crystal

Semi organic crystals have been intensively studied aiming applications in nonlinear optical (NLO). Such applicability requires crystals with both high quality and thermal stability, which make the full characterization of this material a topic issue. In this paper, single crystals of L‐isoleucine hydrochloride monohydrated (L‐Ile.H2O.HCl) was synthesized by slow evaporation technique and characterized by thermal analysis and X‐ray diffraction (XRD) measurements. XRD results at 298 K showed that the sample crystallize with the orthorhombic structure, and the lattice parameters obtained by Rietveld refinement were a = 5.873(3) Å, b = 24.814(4) Å, and c = 6.873(5) Å. Thermal analysis measurements shows four decomposition stages between 328 ‐480 K which were associated with loss of water by desolvation, loss crystallization water and decomposition of L‐Ile.HCl. XRD measurements as a function of temperature up to 428 K show an irreversible phase transformation. This transformation was obtained after 32 hours keeping the L‐Ile.HCl.H2O sample at 413 K. The analysis shows that phase transformation occurs due to water and chlorine losses without destroy the amino acid carbon chain and in the end of transformation only the precursor amino acid remains. L‐Ile.HCl.H2O present low thermal stability and the phase transformation is time and temperature dependent.     

Luminescence of CaS and MnS nanocrystallites co-activated sol-gel derived silica xerogel

CaS and MnS nanocrystallites co-activated sol-gel derived silica xerogel has been prepared by sol-gel processing. Their photoluminescence characteristics have been evaluated and compared with those of the undoped silica xerogel. Two emission bands have been observed from the doped sample, one at 440 nm while the other at 580 nm. CaS and MnS nanocrystallites embedded in sol-gel derived silica xerogel show sharp emission band. The novel luminescence phenomenon is attributed to the luminescent centers of CaS and MnS in the silica xerogel.     

Polyhedral to nearly spherical morphology transformation of silver microcrystals grown from vapor phase

Silver microcrystals of various shapes were grown from co-evaporation of Ag₂O and SiO powders in the nominal evaporating temperature range (heat source temperature) from 800 to 1200 °C under a reductive atmosphere. Pathways for the polyhedral to nearly spherical morphology transformation were established. At evaporating temperatures below 900 °C, the polyhedral crystals are generally bounded by {1 1 1}-facets and {1 0 0}-facets. With increase in evaporation temperature facets of the {1 1 0}-, {2 1 0}- and {3 1 0}-families come forth successively, that make the Ag polyhedron microcrystals transform into nearly spherical microcrystals. A mechanism based on the recession of {1 0 0} or {1 1 0} steps induced by the solvent SiO_x (x～2.0), which modifies the growth kinetics, is proposed to explain the emergence of {2 1 0}- and {3 1 0}-facets at high temperatures. By evaporating at 1200 °C, smooth-looking spherical Ag crystals with 6 nm multi-layered steps on the {1 1 1} facets were obtained. These results are also of significant industry relevance when preparation of noble metal particles with high-index facets is of concern.     

Isolation,characterization and phase transformation of new ginsenoside compound k hydrate and methanol solvates

Two new solvates of ginsenoside compound K (nonstoichiometric hydrate/CKH and methanol solvate/CKM) have been discovered and characterized in this paper. They were obtained through cooling crystallization in different solvents, and CKM could be prepared by transformation from CKH as well. The solvates were analysed by Power X‐ray diffraction (PXRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and fourier transform infrared (FTIR) spectroscopy. From the thermal studies, it is shown that the two new products are both solvates with different onset melting points. The PXRD and FTIR data support different crystal structures of them. It also describes the solution‐mediated phase transformation from CKH to CKM with a combination usage of process analytical technology tools. It is shown that the transformation process can be divided into three stages. The results reveal that seeding and low temperature help to accelerate the transformation, but initial solution concentration do little to the transformation kinetics. The kinetics and the rate‐controlling step for the transformation depend on the nucleation of the CKM. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of heat treatment on phase transformation of aluminum nitride ultrafine powder prepared by chemical vapor deposition

Ultrafine aluminum nitride (AlN) powders were obtained by chemical vapor deposition via AlCl₃–NH₃–N₂ system operated at various temperatures and at a same 200 cm³/min flow rate of NH₃ and N₂, respectively. It has been shown that when the reaction temperature of AlCl₃ and NH₃ was above 600°C, then crystalline AlN powder can be formed; whereas, amorphous AlN was obtained with NH₄Cl if reacted in a lower-temperature zone of the reaction chamber. The amorphous AlN powder was heat treated at 1400°C under NH₃ and N₂ atmosphere for 2 h, then the crystalline phases of the obtained powder belong to a single phase of AlN; a mixture of AlN and Al₂O₃ and only AlON, respectively. On the other hand, if crystalline AlN powder is heat treated at 1400°C under N₂ atmosphere for 2 h, the crystalline phases were composed of the major phase of AlN and a minor phase of Al₂O₃. The morphology, particle size and agglomerate size of the AlN powder were strongly dependent on the heat-treatment temperature. The particle size of AlN powder increases from 28.1 to 45.0 nm, as the heat treatment temperature increases from 800 to 1400°C.     

Preparation of anhydrous dicalcium phosphate, DCPA, through sol-gel process, identification and phase transformation evaluation

Anhydrous dicalcium phosphate, CaHPO₄, DCPA, as a member of the series of calcium phosphates, has considerable biological importance to the biomineralization processes in bones and teeth, and has found practical applications in dental cements and restorative materials. This compound, can reconstruct hard tissues because of high solubility in vivo in comparison with many of the other calcium phosphate compounds. In this work, Ca-deficient DCPA was synthesized through sol-gel process as a new method. Ca(NO₃)₂ · 4H₂O and H₃PO₄ were employed as precursors and identified by XRD, FTIR and SEM spectroscopic techniques. The later technique makes the elemental analysis possible. The measured Ca/P molar ratio suggests, Ca_1−x(H₂PO₄)_2x(H₂PO₄)_1−2x (x = 0.02, 0.04 and 0.07) formula for the synthesized compounds. The effect of aging time and heat treatment (sintering stage) on phase purity and phase transformations of products were systematically studied. The minimum necessary aging time for Ca-deficient DCPA preparation was 24 h. From X-ray patterns of powders sintered at different temperatures, the sintering temperature of Ca-deficient DCPA was confirmed to be 300 °C. According to the experimental results, varying the temperature from 300 to 1000 °C gave different stable phases as products: Ca-deficient DCPA at 300 °C, hydorxyapatite (Ca₅(PO₄)₃OH, HA) at 600 °C and β-tri calcium phosphate (β-Ca₃(PO₄)₂, β-TCP) at 1000 °C.     

Microstructure and phase transformation of Ti3AC2 (A = Al,Si) in hydrofluoric acid solution

In this paper, Ti₃AC₂ (A = Al, Si) were prepared by pressureless argon shielding synthesis technique. The microstructure and phase transformation of as‐prepared Ti₃AC₂ (A = Al, Si) in hydrothermal hydrofluoric acid (HF) solution were investigated systematically. Results showed that the obtained Ti₃AlC₂ and Ti₃SiC₂ were closely aligned layered structure. In hydrothermal HF solution, Al or Si element was preferentially etched from the layered structure, inducing obvious transformation of microstructure and phase composition. For Ti₃AlC₂, Al atoms diffused out of the structure and reacted with HF to form AlF₃•H₂O, which induced the rearrangement of the Ti and C atoms, and finally resulted in the formation of TiC_x cubic phase. With the hydrothermal temperature and reaction time increasing, the TiC_x phase gradually disappeared and the grain size of AlF₃•H₂O gradually increased. When Ti₃SiC₂ was immersed in hydrothermal HF solution, the main products were TiC and SiC. Interestingly, with the hydrothermal treatment temperature and reaction time increasing, TiC gradually disappeared, while SiC nearly kept unchanged. This can be explained that SiC was covalently bonded carbide, while TiC was metallically bonded, having relatively weak bond energy and consequently being unstable in hydrothermal HF solution.     

Dependence of RF power on the phase transformation for boron nitride films deposited on graphite at room temperature

Cubic boron nitride (cBN) thick films deposited on mainly c-axis-oriented graphite substrate at room temperature and zero bias by radio frequency (RF) magnetron sputtering were studied. In the growth process, RF power plays a key role in determining the content of cubic phase in films, while the conventional substrate heating and biasing have been neglected. With increase in RF power, the dominated content of films converts from explosion boron nitride (eBN) to cBN. The transformation mechanism has been discussed. The unique structural properties of the “soft” graphite are favorable to propose simple conditions for growing “hard” cBN films. Furthermore, the optical band gap of BN films having ∼90% cubic phase is of ∼5.8 eV obtained from ultraviolet–visible optical transmission measurement. The electron field emission examination shows that cBN film on graphite has a high emission current density of 2.8×10⁻⁵ A/cm² at an applied field of ∼30 V/μm.     

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.     

Optical microscopy at high pressure : P-T phase behaviour of 80cb and CBOOA and their mixtures

By direct texture observations under pressure in a diamond anvil cell, the phase behaviour of 80CB and CB00A and their mixtures have been investigated. The maximum pressure (p_M) of ocurrence of the smectic A phase (smA) runs through a maximum value as a function of mole fraction, the highest p_M being observed at about 2.2 kbar for x(CBOOA)-0.284.     

The effect of dynamic recrystallization on the growth velocity of new phase crystals at polymorphic transformation of tin and thallium

The analysis of dynamic recrystallization influencing the crystal growth of the new phase in high purity tin and thallium (99.9995%) is carried out. It is shown that in tin, in which the β → α transition proceeds on the dislocation or the normal mechanisms (depending upon a supercooling degree, ΔT) a superposition of dynamic recrystallization results in an increase of the growth velocity of α-Sn crystals. The most distinct of acceleration is observed at −21 ÷ − 23 °C, when the growth velocity of recrystallization centers and the maximal growth velocity, V_max, of the crystals are comparable values. The dynamic recrystallization superposition in thallium offers to transform the transformation mechanism. Thereby an erase of the features of the martensite α ⇄ β transition mechanism and an appearance of the features of the normal crystal growth mechanism of the new phase are observed.     

High pressure electrical resistivity study on nonlinear single crystal zinc thiourea sulphate (ZTS)

The Tris Zinc Thiourea Sulphate (ZTS) crystals have been crystallized by slow evaporation technique. The lattice parameters of the grown crystals have been determined by the Energy dispersive x‐ray diffraction technique (EDXRD) and the structure has been confirmed. And, the high pressure electrical resistivity study have been carried out on this crystal and the results have been reported here. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of La/Ce-doping on phase transformation and crystal growth in TiO2-15 wt% ZnO gels

Titania based ceramic is a good candidate for environmental sensing materials. Ion doping is an effective method to improve the properties by modifying their microstructure and phase composition. By using differential thermal analysis and X-ray diffraction methods, the transformation behaviors of La³⁺/Ce³⁺-doped TiO₂-15  wt% ZnO gels were studied so as to modify the phase transformation and decrease the granularity of crystals. Experimental results show that, anatase, rutile and ZnTiO₃ nanocrystals can be tailored by varying La³⁺/Ce³⁺ contents and sintering temperatures. La³⁺/Ce³⁺ doping decreases the transformation temperature of gel to anatase and the forming temperature of ZnTiO₃ phase, enhances the transformation temperature of anatase to rutile and results in appearance of Zn₂Ti₃O₈ interphase. With the increases of La³⁺/Ce³⁺ contents, the transformation rates of ZnTiO₃, gel to anatase and anatase to rutile, as well as the granularity of the crystals are reduced. Ultimately, the action mechanism of La³⁺/Ce³⁺ doping was discussed.     

Processes in phase boundary region during liquid phase growth

The behavior of components within the phase boundary region during liquid phase growth is the subject of the present article. Based on the supposition that the phase boundary is a structured region, repeating the periodicity of the substrate, the distribution of different components within the phase boundary is considered. Assuming that the value of the supersaturation at the upper end of the phase boundary defines the growth rate of the compound, a relation has been derived, concerning the extension of the phase boundary. The case of liquid phase epitaxial growth of GaAs is considered and an effort for evaluation of the phase boundary extension is undertaken. The linear dependence of the stationary growth rate on the cooling rate, predicted previously, is proved experimentally.