Amorphous phase separation and crystallization in a lithium silicate glass prepared by the sol-gel method

A 10Li₂O---90SiO₂ (mol%) gel-glass has been prepared by using tetramethyl orthosilicate and lithium iso-propoxide as starting materials. The phase separation and crystallization behaviour was compared with the corresponding conventionally melted glass using DTA, X-ray diffraction and TEM. The same crystallization phase was found in both the gel glass and melted glass upon heating above 650°C. However, the rate of crystallization in the gel-glass was higher than in the melted glass. TEM revealed amorphous phase separation in the gel glass and melted glass. However, the morphologies were different, an interconnected microstructure being observed in the gel glass and a droplet structure in the melted glass. These differences can be partly attributed to differences in OH content. Other potential influencing factors are also considered. After 650°C for 2 h lithium disilicate crystals were observed in the volume of the gel glass by TEM. As the crystals grew they absorbed Li₂O from the surrounding lithia-rich amorphous phase so that silica-rich (lithia depleted) diffusion zones formed around them.     

Bridgman growth and properties of potassium lithium niobate single crystals

Transparent KLN crystals 10mm in diameter and 25 to 45mm in length have been grown by the modified vertical Bridgman technique from different melts in the range of 3035mol% K₂O, 1723mol% Li₂O and 4350mol% Nb₂O₅. The growth conditions are a growth rate of less than 0.25 mm/hr, temperature gradient in solid-liquid interface of 23 °C/mm and growth direction of <110>. As-grown KLN crystals have tetragonal tungsten bronze structure. Most of the as-grown crystals do not crack when cooling through the paraelectric/ferroelectric phase transition. 180° domain structures are observed after the KLN crystal was etched in boiling 2HNO₃:Hf. Dielectric properties and transmission spectrum of the as-grown KLN crystals are measured.     

Growth of Na modified potassium lithium niobate crystal by the Czochralski method

Transparent Na modified potassium lithium niobate (Na_0.23K_2.60Li_1.82Nb_5.35O_15.70; NKLN) crystal was successively grown by the Czochralski method using RF induction heating from melt composition Na₂O:K₂O : Li₂O:Nb₂O₅=2:30:25:43 mol%. NKLN crystal showed a tetragonal tungsten bronze structure with lattice constants a=12.5446±0.0010 Å and c=4.0129±0.0005 Å at room temperature. The dielectric constant along the c-axis ε₃₃ showed a sharp maximum around 480 °C. Optical transmission edge was 370 nm and optical transmission spectra showed no absorption at wavelengths ranging from 380 to 800 nm. The structural and optical properties of NKLN were similar to those of the near stoichiometric KLN crystals. We believe that the growth of NKLN by the Czochralski method has an advantage for a large size and high-quality crystal.     

Growth and morphology of ruby crystals with unusual chromium concentration

Single crystals of ruby have been obtained from fluxed melts based on the systems Li₂O–MoO₃, Li₂O–WO₃, Na₂O–WO₃, 2PbO–3V₂O₅, PbO–V₂O₅–WO₃, PbF₂–Bi₂O₃ and Na₃AlF₆ by both the TSSG method and spontaneous crystallization at the temperatures 1330–900 °C. Al₂O₃ solubility has been measured for the flux composition of 2Bi₂O₃–5PbF₂ in the temperature range 1200–1000 °C and dissolution enthalpy has been defined as 29.4 KJ/Mol. The composition of grown crystals was studied by electron microprobe analysis. The synthetic ruby contains from 0.51 to 6.38 at% of chromium admixture depending on the crystal growth conditions. Experimental results on growth conditions, composition and morphology of grown crystals are presented for each flux and temperature interval.     

Chemical strengthening of Na2O---Al2O3---SiO2 system glasses by surface controlled crystallization

Appropriate glass compositions of the Na₂O---Al₂O₃---SiO₂ system and glass melting technology were investigated. The dependence of some factors, such as composition of molten salts, time and temperature of ion exchange treatment on the modulus of rupture was studied. As expected, the coefficient of interdiffusion is a function of concentration and has the mixed-alkali effect. The order of the interdiffusion coefficient is 10⁻⁶ cm²/s. The effect of binary and ternary salt baths on the surface controlled crystallization was reported. The experimental data showed that there is a series of technological advantages from applying molten salts of the ternary system Li₂SO₄---Na₂SO₄---K₂SO₄ in comparison with the binary system Li₂SO₄---Na₂SO₄.     

Ni-containing spinel aluminates glass-ceramic materials obtained from cordieritic bulk glasses

Monolithic glasses with compositions 2MO · 2Al₂O₃ · 5SiO₂, being M=Ni and equimolar mixtures of Ni and Mg, were prepared at 1650 °C by melting mixtures of raw materials. The crystallization of monoliths was produced by heat-treatment at several temperatures up to 1200 °C. The crystallization sequence was followed by differential thermal analysis (DTA), X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) and diffuse reflectance spectroscopies. Surprisingly, the only crystalline phase formed after heating up to 1100 °C was a nickel-containing aluminate spinel for both compositions. The microstructural characterization indicated the volume crystallization of well formed octahedral crystals of spinel with smaller size than 500 nm. Finally, it has been proved that nickel-containing aluminosilicate glasses could be converted into μ- or -cordierite- and spinel-based glass-ceramics by thermal treatment of powdered glasses and monolithic bodies, respectively.     

The temperature gradient technique (TGT) growth and optical properties of Yb-doped YAlO3 single crystal

The dark-brown colored 5 at% Yb-doped YAlO₃ (Yb:YAP) single crystal was grown successfully by temperature gradient technique (TGT) for the first time. The TGT-grown Yb:YAP crystal with the perovskite structure and excellent crystallization perfection were confirmed by the X-ray diffractions techniques. The dark-brown color of TGT-Yb:YAP crystal turned into the colorless after annealing in the air at 1200 °C for 10 h. The absorption spectra, LD-excited infrared emission and X-ray excited luminescence spectra of the air-annealed Yb:YAP single crystal were investigated at the room temperature. The results indicate that the TGT-Yb:YAP single crystals can be used for the laser and scintillation applications.     

Correlation between interfacial structure and c-axis-orientation of LiNbO3 films grown on Si and SiO2 by electron cyclotron resonance plasma sputtering

Thin films of crystalline lithium niobate (LN) grown on Si(1 0 0) and SiO₂ substrates by electron cyclotron resonance plasma sputtering exhibit distinct interfacial structures that strongly affect the orientation of respective films. Growth at 460–600 °C on the Si(1 0 0) surface produced columnar domains of LiNbO₃ with well-oriented c-axes, i.e., normal to the surface. When the SiO₂ substrate was similarly exposed to plasma at temperatures above 500 °C, however, increased diffusion of Li and Nb atoms into the SiO₂ film was seen and this led to an LN–SiO₂ alloy interface in which crystal-axis orientations were randomized. This problem was solved by solid-phase crystallization of the deposited film of amorphous LN; the degree of c-axis orientation was then immune to the choice of substrate material.     

Solidification characteristics of textured 123 phase in YBCO by laser floating zone leveling (LFZL) method

Because of incongruent melting in YBa₂Cu₃O_7-x, it is difficult to grow the textured 123 phase directly from the laser-heated molten zone in the laser floating zone (LFZ) method. To overcome this problem, the laser floating zone leveling (LFZL) method was utilized following a pseudo-binary diagram along the Y₂BaCuO₅−YBa₂Cu₃O_7−x tie line and a crystal rod of textured 123 phases was directly grown from the molten zone. Experimental results indicate that the orientation of the textured tetragonal 123 phases is perpendicular to the c-axis and that the T_c of the grown crystal rod is 91.2 K after annealing at 550°C for 40 h in flowing oxygen. Also, the solidified morphologies of some crystal rods grown at different growth rates were observed.     

Viscosity and IR investigations in the Li2O---B2O3 system

The high viscosity in melts of the Li₂O---B₂O₃ system makes it very difficult to grow large crystals of lithium triborate. The viscosity and IR characteristics of molten li₂O---B₂O₃ system are reported in this paper. When the temperature increases the viscosity of li₂O---B₂O₃ system decreases and follows an Arrhenius-type relationship. With an increasing 13203 ratio in Li₂O---B₂O₃ melts, the viscosity rises gradually to a maximum with a composition Li₂O: 3.513203 then it falls rapidly. In order to find active agents to reduce the viscosity, Na₂O, NaCl, LiF, P205, M003, W03 etc oxides were added to Li₂O---B₂O₃ samples respectively and investigated using the orthogonal method. The experimental results show that the addition of acidic oxides can significantly decrease the viscosity in the Li₂O---B₂O₃ system. For Li₂O: 4.513203, an ideal additive agent is 20wt% Li₂O:: 2MoO₃. Near the composition for crystal growth, the percentage reduction of viscosity is 62.2%. The IR spectra of Li₂O---B₂O₃ system revealed that the BO₄⁻/NO₃⁻ ratio is reduced in the melt using Li₂O: 2MoO₃ as an additive. It is proposed that the M003 reduced the concentration of bridging oxygen atoms of BO₄⁻. The change of structure explains the decline in the viscosity. In the crystal structures of lithium triborate, the matrix spaces are so small that larger other cations than Li+ are very difficult to enter the crystal matrix. So the use of additive agents to reduce the viscosity is a possible method if no new phase appears.     

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.     

Precipitation of In2O3 nano-crystallites from glasses in the system Na2O/B2O3/Al2O3/In2O3

Glasses in the system Na₂O/B₂O₃/Al₂O₃/In₂O₃ were melted and subsequently tempered in the range from 500 to 700 °C. Depending on the chemical composition, various crystalline phases were observed. From samples without Al₂O₃, In₂O₃ could not be crystallized from homogeneous glasses, because either spontaneous In₂O₃ crystallization occurred during cooling, or other phases such as NaInO₂ were formed during tempering. The addition of alumina, however, controlled the crystallization of In₂O₃. Depending on the crystallization temperature applied, the crystallite sizes were in the range from 13 to 53 nm. The glass matrix can be dissolved by soaking the powdered glass in water. This procedure can be used to prepare nano-crystalline In₂O₃-powders.     

Structural transformation of the TiO2---SiO2 system gel during heat-treatment

The gel formation of the (100-x)TiO₂·xSiO₂ (x = 0–10 mol%) system has been studied. The progressive elimination of residues was followed by DTA and TGA curves. DTA curves showed that the formation of anatase during heat treatment could be sensibly slowed down with the increase of SiO₂. The relationship between the gel composition and crystallization temperature of anatase has been systematically investigated. The X-ray diffraction spectra demonstrated that the crystallization temperature of anatase is 400°C for TiO₂ gel and 430°C for 90TiO₂ - 10SiO₂ gel. The infrared absorption spectra were used to follow the structural transformation of gels heat-treated at different temperatures. With the help of EPR it is evident that titanium ions exist only in tetravalence.     

A study of the chemical reactions involved in Li–Ca–N system

The chemical reactions and phases involved in the potential flux system of Li–Ca–N for the growth of bulk GaN crystals have been investigated under varying conditions. It is found that no preferential nitrification of Li or Ca by N₂ in Li–Ca melts at 500 °C. Only the ternary compound LiCaN is identified in the Li–Ca–N system under the present experimental conditions. Static N₂ pressures are found to enhance the formation of LiCaN compared to an N₂ stream. LiCaN forms from two possible pathways: one is a modified metathesis chemical reaction represented by Li₃N+Ca→LiCaN+2Li, and the other is a combination chemical reaction represented by Li₃N+Ca₃N₂→3LiCaN. The formation of LiCaN by the metathesis reaction is thermodynamically favored over the other pathway. In addition, the formation of LiCaN might benefit from a slightly larger initial amount of Li₃N compared with Ca or Ca₃N₂.     

Hydrolytic resistance of Na2O---B2O3---SiO2 gels prepared by sol-gel process

Alkoxide derived gels were prepared in the system Na₂O---B₂O₃---SiO₂. The gel compositions were situated in the liquid-liquid immiscibility area of the phase equilibrium diagram.

Hydrolytic resistance tests were performed on the gels heat-treated at temperatures ranging between 120 to 850 °C. The Na₂O, B₂O₃ and SiO₂ extracted from the attack gels were analyzed. The experimental results indicate that the amount of B₂O₃ has a significant influence on the chemical durability of the heat-treated gels. At temperatures of 850 °C the greater the B₂O₃ mol% the greater are the amounts of Na₂O and B₂O₃ extracted. Different behaviour was observed for gels heat-treated at 600 °C where the amounts of B₂O₃ and Na₂O extracted slightly increases as the B₂O₃ mol% increases. Small amounts of extracted SiO₂ were always observed.

These results are complemented with other measurements so that an explanation of the controlling mechanism is given.     

Al2O3-TiO2 and Al2TiO5 ceramic materials by the sol-gel process

Ceramic materials with a very low thermal expansion coefficient are synthesized by the sol-gel process. The binary gel is obtained by hydrolysis and polycondensation reactions of organometallic compounds of aluminium and titanium. The thermal evolution of the amorphous powder is followed by DTA and TGA measurements. Structural evolution is followed using X-ray diffraction. The crystallization of the TiO₂ rutile and Al₂O₃ corindon starts at 700 and 900°C respectively. The transformation of Al₂O₃ and TiO₂ into Al₂TiO₅ appears between 1200 and 1300°C. The densification of the powder is performed by the hot pressing process. The shrinkage of the powder was previously followed by dilatometric measurements. The physical properties of the final material are studied as a function of pressing parameters.     

Growth mechanisms in excimer laser photolytic deposition of gallium nitride at 500°C

The mechanisms of controlling laser-induced chemical vapour deposition of GaN at substrate temperatures between 350 and 650°C have been investigated. Ultraviolet (193 nm) photolytic decomposition of trimethylgallium (TMGa) and ammonia (NH₃) precursors was examined in this range. Laser-induced fluorescence studies support the view that the dissociated intermediate fragments GaCH₃ and NH are the reacting species in GaN film formation, irrespective of substrate temperature. It was found that two crystal phases coexist in films grown at substrate temperatures below 500°C, wurtzite crystal structure with (0002) orientation forms at substrate temperatures above 500°C. The growth rate increases with both NH₃/TMGa ratio, and TMGa flow rate, while the temperature dependence shows a thermal activation energy of 0.2 eV which is smaller by a factor of five than that of films prepared by conventional thermal CVD. The large NH₃/TMGa ratios needed to achieve stoichiometry are interpreted in terms of the two-photon dissociation cross section of NH₃.     

Sodium borosilicate glasses prepared by the sol-gel process

A sodium borosilicate gel of composition 80SiO₂·15B₂O₃·5Na₂O (wt%) was prepared from tetraethyl orthosilicate, trimethyl borate, sodium methylate, H₂O, and HCl as the catalyst. Variation of specific surface area and porosity as a function of heating temperature indicated that closed pores were opened at temperatures lower than 400°C and collapsed above 450°C. From TG and DTA curves, about 19% Si and B atoms are evaluated to have −OH bonds. X-ray diffraction patterns indicated crystallization of low-cristobalite out of the gel when it was heated at 700°C for 5 h, showing a difference from a melt-quenched glass of the same composition.     

Energy transfer and annealing behavior of luminescence in Dy-doped PbWO4 single crystal

The Dy³⁺: PWO single crystal was subsequently annealed in air atmosphere at a temperature of 500°C, 550°C, 600°C, 700°C, 800°C, 900°C, and 1000°C, respectively. X-ray excited luminescence spectra were measured before and after each step of annealing. Annealing experiments confirmed the energy transfer-taking place from PbWO₄ (PWO) host to Dy³⁺ ions, followed by the enhancing characteristic emission of Dy³⁺ ions. In the process of annealing, the luminescence of PWO host was significantly reduced while that of Dy³⁺ was increased simultaneously. Annealing at a temperature below 700°C suppressed the blue luminescence of the PWO host and enhanced its green components, while the emission of Dy³⁺ is increased to some extent. Further annealing at higher temperature strongly reduced the luminescence of the PWO host, while the emission of Dy³⁺ was greatly increased. Interstitial oxygen O_i could play an important role in the luminescence. Annealing could modify the luminescence of Dy³⁺ ions in PWO, which may be useful in terms of some application purposes.     

Homogeneity of a ZrF4-based glass at the nano-scale

A glass of composition 53ZrF₄–20BaF₂–4LaF₃–3AlF₃–20NaF (T_g=260°C) was prepared by careful crucible melting. High-resolution atomic force microscopy of fracture surfaces displayed the presence of nano-pores with diameters of 20–50 nm, being 4–10 nm deep, in all glasses. It was further found that only glasses without annealing and glasses with an annealing step considerably below T_g showed a distinct pattern, i.e. ripples of ≈20 nm in diameter and an rms roughness of ≈0.6 nm. Glasses annealed either near T_g or at the temperatures of maximum nucleation or maximum crystal growth rates showed both regions with the ripple pattern and regions with nano-hillocks, growing in size with increasing annealing temperature and time. Thus these hillocks nearly reach micro-dimensions of ≈270 nm in diameter and ≈65 nm in height following a 90 min annealing step at 343°C, the temperature of maximum crystal growth. These findings give evidence that the glass system, which is thought to be one of the most suitable for fiber drawing, is much less stable against nucleation and crystallization than anticipated.