Thermal behavior of the composite xerogels of zirconium oxyhydroxide and silicic acid

Gels were prepared via sol?Cgel method by addition of zirconium oxychloride solution into sodium metasilicate (SZ) and sodium metasilicate solution into zirconium oxychloride (ZS) at varying final pH. Si/Zr molar ratio equaled 1/1. Synthesized gels were dried with calcium chloride until they reached a constant mass. SEM and nitrogen adsorption analysis have shown that SZ gels have surface area 175?C200?m^2?g^?1, consist of 20?C30?nm grains. ZS samples have surface area about 1?m^2?g^?1, consist of grains smaller than 10?nm. Thermal and X-ray phase analysis have shown that transition of amorphous ZrO₂ to crystalline form shifts from 430 to 850?C870?°C for SZ gels. Unlike zirconia gels phase transitions that proceed in order: ??amorphous (430?°C)??tetragonal (800?°C)??monoclinic (1,000?°C) phases??, the monoclinic phase in ZS gels appears immediately after transition from amorphous to crystalline state; the tetragonal phase in SZ samples is stable until 1,000?°C.     

A simple method for the preparation of pure hafnium

The separation of zirconium and hafnium by fractional precipitation as pyrophosphate¹ has been extended for the preparation of pure hafnium. The favourable uptake of hafnium, in spite of the decreasing tendency of partition factor when hafnium concentration is high, is maintained for all concentration of hafnium (relative to zirconium). Particularly significant is the fact that at very high concentrations of hafnium (at≈84%) the uptake of zirconium sharply falls. So pure hafnium can be prepared from natural zirconium by a simple process of eight or nine stages of fractional precipitations as pyrophosphate. This process yields reactor grade zirconium on the one side and pure hafnium on the other side.     

Thermodynamics of oxidation of zirconium and hafnium borides

Gibbs thermodynamic potentials of oxidation of zirconium and hafnium diborides with molecular and atomic oxygen and nitrogen monoxide were calculated for a temperature range of 20–2500°C. Oxidation of zirconium and hafnium borides with atomic oxygen was found to be the most expected reaction. The probability of oxidation is lower for zirconium boride than that for hafnium boride.     

Effect of preparative conditions on the surface characteristics of mixed zirconium and titanium oxides

The surface characteristics of mixed zirconium and titanium oxides prepared from different starting materials are investigated. One mode of preparation entailed the use of zirconium sulfate and titanium oxysulfate as starting materials and ammonium hydroxide as precipitating agent. The produced oxides were washed to different extents to obtain samples with different sulfate content. A second preparative mode used zirconium oxychloride and titanous chloride as starting materials also with ammonium hydroxide as precipitating agent. The oxidation of the titanous to the titanic form for these oxides was carried out by means of oxygen gas. Resulting samples were heat treated at 400 °C and 600 °C, and textural characteristics determined from the adsorption of N₂ at 77 K, complemented by infrared and thermal studies. The samples precipitated from the oxychloride and chloride salts of zirconium and titanium, as well as those precipitated from the sulfate and oxysulfate salts and washed free of the sulfate ions displayed quite similar textural characteristics. The unheated samples and those heat-treated at 400 °C were mesoporous, with some microporosity, and relatively large surface areas in the order of 200–300 m²/g. Heat treatment to 600 °C led to a relative decrease in surface area, in the order of 100 m²/g, and to the disappearance of microporosity. The mixed zirconium and titanium oxides with a sulfate content of ≈17% displayed significantly lower surface areas, smaller than 10 m²/g, with a prevalence of micro and mesoporosity. Infrared and thermal studies indicated the presence of differently bounded sulfato groups, which seem to have a blocking effect on the pores, resulting in the observed smaller surface areas.     

Synthesis of nanosized group IV borides in ionic melts of anhydrous sodium tetraborate

The preparation of nanosized Group IV metal diborides by reacting powdery titanium, zirconium, and hafnium with fine-grained boron in Na₂B₄O₇ ionic melts in the temperature range 600–850°C has been studied. Nanosized titanium, zirconium, and hafnium diborides are formed at temperatures of at least 750°C.     

Structure and properties of poly-2,5-distyrylpyrazine

Poly-2,5-distyrylpyrazine (poly-DSP) was investigated by differential thermal analysis (DTA), thermogravimetric analysis (TGA), and measurements of dynamic viscoelastic and electrical properties. From DTA and TGA studies it was confirmed that poly-DSP melts at 321°C and depolymerizes rapidly to the monomer at temperatures between 335°C and 345°C in helium. The polymer is affected by oxygen above 200°C. The E′ value from dynamic viscoelasticity measurements on amorphous film is 2 × 10¹¹ dyne/cm² at room temperature. It decrease abruptly in the temperature range 140–150°C; but the net decrease of E′ within this temperature range is relatively small. The electrical properties of amorphous poly-DSP are characterized by a small temperature dependence of the dielectric constant between room temperature and 100°C. The dielectric loss tangent was observed to be small, and the dc conductivity was extremely small. It is concluded that rotation of the phenyl branches in the polymer occurs above ?30°C and the glass transition occurs at about 150°C. These properties are discussed in some detail in relation to the polymer structure.     

Carbothermal synthesis of ultra-fine zirconium carbide powders using inorganic precursors via sol–gel method

Ultra-fine zirconium carbide (ZrC) powders have been synthesized by carbothermal reduction reactions using inorganic precursors zirconium oxychloride (ZrOCl₂ · 8H₂O) as sources of zirconium and phenolic resin as the carbon source. The reactions were substantially completed at relatively lower temperatures (∼1400 °C/1 h) and the synthesized powders had a small average crystallite size (<200 nm) and a large specific area (54 m²/g). The oxygen content of the powder synthesized at 1400 °C/1 h was less than 1.0 wt%. The thermodynamic change process in the ZrO₂–C system and the synthesis mechanism were studied.     

Simultaneous thermal analysis of zirconium(IV) acetylacetonate in a helium atmosphere

TG, DTG, DTA, DDTA and ΔH analyses of zirconium(IV) acetylacetonate, Zr(C₆H₇O₂)₄ (= I), were performed in a helium atmosphere with a Netzsch Thermal Analyser STA 429. The enthalpies of the main steps of transformation were computed to be +42.182 J·g^?1 and ?21.113 J·g^?1. Pure I is thermally stable up to about 199°C in He gas, and melting too occurs at about 199°C. Four well-defined decomposition steps were observed over the range between ambient and 600 °C, accompanied by a weight loss of 61.59%. The final product contained pure ZrO. The unique shapes of the TG and DTA curves could be used for the identification of I.     

High-pressure studies of polymerization in sulfur

By x-ray and optical studies of thermally quenched products and by differential thermal analysis under pressure the effect of high pressure on the melting and polymerization of sulfur has been investigated to 31 kb and 500°C. At least four different liquid fields have been identified. DTA experiments indicate that pressure shifts the 159°C polymerization transition first toward higher temperatures and then toward lower temperatures until it finally coincides with the melting point at a pressure of about 0.7 kb. The depolymerization temperature was found by the same technique to increase with increasing pressure up to 0.4 kb. A liquid P—T boundary that may constitute the higher-pressure extension to the depolymerization transition has been traced up to 7.5 kb and 480°C. A very sharp, practically temperature-independent reaction in the liquid state has been located at about 9 kb extending from the liquidus to at least the limits of the apparatus at about 450°C. Evidence has been found for a possible second-order phase transformation extending from about 10 kb at 400°C to the liquidus at approximately 360°C.     

Solid State Nuclear Magnetic Resonance Studies of the Thermochromic Phase Transition of the Polydiacetylene from the Bis-n-Propylurethane of 5,7-Dodecadiyne-1,12-diol

The polydiacetylene (PDA) from the bis-n-propylurethane of 5,7-dodecadiyne-1,12-diol (PUDO) undergoes a first order phase transition near 135°C that is associated with a color change from blue at temperatures below the transition to red at temperatures above the transition. We have studied PDA-PUDO by solid state ¹³C nuclear magnetic resonance (NMR) spectra using cross polarization and magic angle spinning (CP-MAS) techniques at temperatures between 25° and 140°C. As observed previously, the acetylene carbon shift moves up field as the temperature is raised above the transition temperature. In addition, near 130°C, the oxymethylene carbon shows 3 resonances, indicating multiple side chain conformations as the PDA undergoes the phase transition.     

Glass transition in nylons

Differential thermal analysis (DTA) of some commercial nylons has disclosed some anomalous phenomena with respect to the glass transition, generally considered to occur at 40–50°C. On the first heat cycle the transition occurs normally. On cooling, however, no corresponding transition occurs, and on an immediate rerun the transition has disappeared. If another DTA thermogram is made after a few hours, the transition begins to reappear, but at a temperature lower by a few degrees. After about five days rest, the transition is again normal in size and temperature. On annealing at 75°C, the 43°C transition is pushed up to about 92°C. On resting after annealing, transitions appear at both 40 and 92°C. These phenomena are explained in terms of the slow formation of a hydrogen-bonded network in the amorphous regions of the polymer. It is the disruption of this network that is normally considered to be the glass transition in nylons. The network is slow in re-forming because of problems involved in matching up potential hydrogen-bonding sites, which are, of course, distributed at intervals along the polymer chain. The temperature at which the network is disrupted is apparently dependent not so much on the ratio of bonding to nonbonding sites, as on the temperature at which it was formed.     

Composite photocalysts based on the nanostructural titanium dioxide and zirconium phosphate

A series of composite photocatalysts based on titanium dioxide deposited on the surface of a zirconium phosphate support were synthesized under different synthesis and heat-treatment conditions. The study of the photodestruction kinetics of Rhodamine C showed that the synthesized composites possess high photocatalytic activity that is competitive with the activity of a commercial Hombikat UV100 photocatalyst. The composites based on zirconium phosphate treated with isopropanol at the precipitation stage whereupon heated at 550°C exhibit the highest photocatalytic activity after heating at 750°C. It was found that such zirconium phosphate support has the largest specific surface area (270 m²/g). After heating at 550°C, the surface becomes more stable to the subsequent heating to 750°C, which is necessary for the most complete crystallization of TiO₂ ensuring its high photocatalytic characteristics.     

Selective Determination of Hafnium with Solid Surface Room Temperature Phosphorescence Optosensing

Abstract

An optosensing method for selective determination of hafnium has be developed. It is based on the phenomenon that when the complex formed by 8-hydroxy-5-quinolinesulfonic acid with hafnium is absorbed on the strongly basic anion exchange resin, the phosphor can produce room temperature phosphora-scence (RTP) in aqueous medium. The hafnium can be determined selectively in the presence of zirconium. The RTP intensity is linear up to 4×10^-5 M of hafnium, the detection limit is found to be 5×10^-8 M of hafnium.     

Sur la thermogravimétrie des précipités analytiques: Dosage du hafnium

A zirconium-free oxychloride of hafnium was prepared. The hafnium can be determined gravimetrically from the thermolysis curves of the precipitate. In particular, the neutral selenate, the p-hydroxyphenylarsinate and the mandelate were studied, and the curves obtained were compared to those plotted of the homologous zirconium derivatives.     

Some Physico-chemical Properties of Doxazosin Mesylate Polymorphic Forms and its Amorphous State

Seven polymorphic modifications of doxazosin mesylate, designed as forms A, D, E, F, G, H, I, and the amorphous state were studied by thermal methods (TG and DSC), temperature resolved X-ray powder diffractometry, hot stage and scanning electron microscopy and by FT-IR spectroscopy. Amorphous form was obtained either by fast evaporation of the solvent or by fast cooling of the melt in the DSC. Polymorphs A and F were found to be stable in the temperature range from room temperature to their melting points at 277.9 and 276.5°C, respectively. Form G, which melts at 270.8°C, was found to be hygroscopic. Polymorph D undergoes irreversible solid–liquid–solid phase transition at 235.5°C to polymorph I which melts at 274.9°C. Form H, which melts at 258.0°C, was found to be unstable at high temperatures. DSC examinations revealed that form H is irreversibly transformed to polymorph F during heating above the temperature of about 240°C. The amorphous state was found to be stable at room temperature but when heating above the glass transition (T _g=144.1°C) it crystallizes at 221.6°C, what leads into a mixture of polymorphic forms. The new polymorphic form designed as E was identified in the mixture. The polymorph E is converted by heating to the more stable form F. The solubilities at 25°C for forms A, and F in methanol are 3.5 and 7.7 mg mL⁻¹and in water they are 3.8 and 6.2 mg mL⁻¹, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solvothermal synthesis of large surface area zirconia

The reaction of zirconium n-propoxide in glycol at 300°C yielded microcrystalline tetragonal zirconia (ZrO₂). The crystallite size of the product depended on the carbon number of the glycol and increased in the following order (carbon number of glycol): 2<6<4, which suggested that the heterolytic cleavage of O-C bond of gylcoxide formed by transesterification is the prime factor for the formation of the product. In toluene, zirconium isopropoxide also gave tetragonal zirconia at 300°C, and zirconium tert-butoxide decomposed at 200°C yielding amorphous zirconia, while zirconium n-propoxide was stable at 300°C. These results suggest that the reaction in toluene depends on the structure of the alkyl group of the alkoxides. Thus-obtained tetragonal zirconias maintained large surface areas (90–160 m²/g) even after calcination at 500°C.     

Differential scanning calorimetry of RbH2PO4 and CsH2PO4

The high-temperature phase behaviour of RbH₂PO₄ and CsH₂PO₄ have been studied. RbH₂PO₄ undergoes a single quasi-irreversible phase transition with an enthalpy of 4.665 kJ mol^?1. The transition is found to occur over the temperature range 86–111°C. CsH₂PO₄ undergoes two transitions at 149 and 230°C. The lower one is quasi-irreversible and has an enthalpy of 4.284 kJ mol^?1. The one at 230°C is reversible and has an enthalpy of 1.071 kJ mol^?1.     

Synthesis of MgHf(WO4)3 and MgZr(WO4)3 using a non-hydrolytic sol–gel method

Magnesium zirconium tungstate and magnesium hafnium tungstate were successfully synthesized using a non-hydrolytic sol–gel method. Crystalline materials could be obtained at temperatures as low as 540 °C after as little as 3 h. The samples were composed of micron-size particles with defined morphology. Highly crystalline material was formed after a 0.5 h heat treatment of 1050 °C.     

Reaction of chromium trifluoride with zirconium in NaF + ZrF4 melts

We report on the reaction of chromium trifluoride with zirconium in NaF:ZrF₄ = 50:50 (mol/mol) mixed melts. Chemical analysis, X-ray powder diffraction, differential thermal analysis (DTA), and IR spectroscopy show that at 400–600°C zirconium reduces chromium(III) fluoride to Cr²⁺ or Cr⁰ compounds and reduces zirconium tetrafluoride to ZrF_2–x , where 0 < x < 0.2; the particular products depend on the zirconium concentration in the batch.     

Low-temperature synthesis of nanodispersed titanium,zirconium, and hafnium carbides

Nanosized refractory titanium, zirconium, and hafnium carbides were manufactured from highly dispersed metal dioxide-carbon starting mixtures at moderate temperatures of 1200°C or lower. The products were characterized by powder X-ray diffraction, elemental analysis, and transmission electron microscopy. The average size of particles (in nanometers) manufactured at 1200°C was as follows: for TiC, 13 ± 4; for ZrC, 17 ± 3; and for HfC, 16 ± 3; the average crystallite size (in nanometers) was as follows: for TiC, 8 ± 2; for ZrC, 5 ± 2; and for HfC, 8 ± 3. Thermodynamic modeling was performed for the synthesis of Group IVB carbides via carbothermal reduction of the corresponding oxides. The results show that in the titanium dioxide-carbon system, for example, titanium monocarbide formation is possible at a temperature as low as 850°C (p = 10⁻⁴ MPa). Highly dispersed metal dioxide-carbon starting mixtures were prepared using solgel technology from metal alkoxyacetylacetonates in the presence of a polymeric carbon source.