Strengthening of composite materials of the fluorohydroxyapatite–zirconia system by titanium nitride

Composite materials of the ZrO₂–fluorohydroxyapatite (FHA)–titanium nitride (TiN) system have been synthesized and studied. In the course of sintering even in a protective inert medium, the reaction between the components begins at 700°C to give calcium titanates, titanium oxides, and oxygen-deficient zirconia compounds. As a result of working through the procedure of synthesis of ultrafine powders and selecting sintering regimes, including the hot-pressing method, dense composite materials of the FHA–ZrO₂ system containing TiN were obtained with a bending strength of up to 273 MPa and a crack resistance of up to 3.3 MPa m^1/2.     

Structural evolution of Pt/ceria–zirconia TWC catalysts during the oxidation of carbon monoxide

The structural evolution of two Pt/ceria–zirconia catalysts, characterized by different amounts of supported Pt, was monitored by in situ X-ray diffraction during the anaerobic oxidation of CO at different temperatures. In a first phase, oxygen coming from the surface layers of the ceria–zirconia mixed oxide is consumed and no structural variation of the support is observed. After this induction time, bulk reduction of Pt/ceria–zirconia takes place as a step-like process, while the CO₂ production continues at a nearly constant rate. This behavior is totally different from that of the metal-free support in similar reaction conditions, that show a gradual bulk reduction. In repeated oxidation–reduction cycles, it was observed that the induction time in Pt/ceria–zirconia is a function of the thermal history, of the amount of supported Pt and of the structural evolution of the samples.     

Studies of temperature influence on adsorption behaviour of nonionic polymers at the zirconia–solution interface

The temperature influence (15–35 °C) on the adsorption mechanism and conformation of nonionic polymers (polyethylene glycol (PEG), polyethylene oxide (PEO) and polyvinyl alcohol (PVA)) on the zirconium dioxide surface was examined. The applied techniques (spectrophotometry, viscosimetry, potentiometric titration and microelectrophoresis) allowed characterization of the changes in structure and thickness of polymer adsorption layers with the increasing temperature. The rise of temperature favours more stretched conformation of polymer chains on the ZrO₂ surface, which results in higher adsorption and thicker adsorption layer. Moreover, these conformational changes of adsorbed macromolecules affect the electric (solid surface charge density) and electrokinetic (zeta potential) properties of the zirconia–polymer interface. The obtained data indicate that the polyvinyl alcohol adsorption has a greater influence on zirconia properties in comparison to that of PEG and PEO. It is due to the presence of acetate groups in the PVA macromolecules (degree of hydrolysis 97.5%), which undergo dissociation.     

Synthesis and FT–IR study of the acido–basic properties of the V2O5 catalysts supported on zirconia

In this paper we describe the synthesis and characterization of the acido–basic properties of catalysts containing varied amounts of vanadium supported on ZrO₂. The preparation of the zirconia was carried out using a hydrolysis method and the vanadium was introduced by impregnation with a porous volume in several stages, followed by calcinations under air at a temperature of 723 K. The obtained samples are characterized by adsorption–desorption of nitrogen and infrared spectral analysis of different species formed by acidic and basic probes. This adsorption on the surface of these compounds has been studied in order, in the hand to investigate information on their surface acidity and in the other hand to know particularly the nature and strength of acidic and basic sites. Among the molecular probes, we used carbon monoxide, carbon dioxide, pyridine and 2,6-dimethylpyridine. The adsorption of CO has shown that contrary to pure zirconia and oxidized V₂O₅/ ZrO₂, the reduced V₂O₅/ ZrO₂ samples favour the formation of CO co-ordinated on Lewis acidic sites of reduced V₂O₅ species (CO on V⁴⁺ or V³⁺). We also observe the creation of Brønsted acidic sites by means of the incorporation of vanadium.     

Sintering behavior and thermal expansion of zirconia–titanium composites

The dilatometric and thermogravimetric methods were used to investigate the sintering conditions of 3Y–ZrO₂ and 3Y–ZrO₂–Ti composites. For the materials preparation, the nanometric zirconia stabilized by 3 mol% Y₂O₃ powder and micrometric titanium powder (3 and 10 vol%) were used. The green body samples were formed by slip casting method. The morphology of samples microstructures was determined by SEM observations. The stereological analysis of zirconia and zirconia–titanium composites was carried out using computer program. The density was measured using the Archimedes method. The hardness of sinters was also investigated. Addition of Ti into ZrO₂ influenced the sintering behavior and thermal expansion of obtained composites. The analysis of the sintering process and characteristic temperatures confirmed the increase of onset and final temperature of shrinkage with the increase in Ti content. The changes of the thermal expansion curves for the pure zirconia and 3Y–ZrO₂–Ti composites were the result of the αTi → βTi transformation and the transition temperature of the zirconia m → t transformation. The zirconia and composite samples were characterized by relative density about 98%, close to theoretical density. The slight growth of zirconia grains was observed.     

Sol–gel preparation and characterization of ceria stabilized zirconia minispheres

A novel processing technique based on sol–gel drop generation method has been developed to prepare fine zirconia minispheres for use as grinding media. Zirconium oxalate gel formation from the prepared sol was obtained in proper synthesis condition using Zirconium oxy-chloride octahydrate(ZrOCl₂·8H₂O) as starting material. The transparent oxalate gel was then added dropwise into the setting solution for the formation minispheres. To obtain the required fluidity and viscosity a suitable binder was mixed to the sol and stabilizing agent of required mol% was added to stabilize the phase formation. The addition of stabilizing agent transformation toughened the minispheres, with a complete retention of the tetragonal phase in the final product sintered at 1500 °C. Thermogravimetric analysis indicated the removal of most of the volatiles by 600 °C. Density and Crystallite size were found to be increasing linearly with sintering temperature. The phase identification, density variation, chemical decomposition, functional group specification and microstructural features for the dried and sintered final product were studied.     

Honey mediated microwave assisted sol–gel synthesis of stabilized zirconia nanofibers

Aim of the present work is to prepare zirconia nanofibers using microwave assisted sol–gel method. Both honey and microwave powers are employed as structure directing agents to improve the stability and reduce the crystallite size. Honey, acting as capping agent, prevents the particles from hard agglomeration. Soft agglomeration or less agglomeration results in smaller crystallite size that prevents the transformation of tetragonal to monoclinic phase resulting in stabilized tetragonal zirconia (t-ZrO₂). Zirconium oxychloride is used as precursor of zirconium and deionized water as solvent. Effect of microwave powers, in the range of 100–900?W with interval of 200?W, on zirconia stabilization is observed. X-ray diffraction analysis shows the presence of phase pure t-ZrO₂ at low microwave power ~?100?W with crystallite size ~?26?nm. Formation of phase pure t-ZrO₂ at low microwave power is due to the presence of sufficient amount of honey to coat the zirconia crystals. Relatively higher x-ray density has been observed in case of phase pure t-ZrO₂ at 100?W of microwave power. This high density and phase purity reveals the high value of hardness (~?1503?HV). Scanning electron microscopy analysis reveals the formation of well-separated nanofibers without agglomeration at 100?W. These nanofibers are purposed for bone implants and bone grafting. Structural transformation along with hard agglomeration is observed with increase in microwave powers from 500?W to 900?W. FTIR and Raman fundamental tetragonal bands, appearing at 490?cm^?1 and 148?cm^?1, respectively, confirm the formation of t-ZrO₂ at low microwave power. Sample with phase purity exhibits high grain boundary resistance (1.95?MΩ) along with high dielectric constant (~?74) and low tangent loss (at log f?=?4.0). It is worth mentioning here that phase pure t-ZrO₂ at very low microwave power (~100?W) with high density and well-separated nanofibers has been obtained without any post heat treatment.

Open image in new window

     

Stability and performance of porous silica–zirconia composite membranes for pervaporation of aqueous organic solutions

Porous silica–zirconia membranes were fabricated by the sol–gel techniques to study their stability against water and the pervaporation performance of aqueous solutions of organic solvents. Zirconia (10–70 mol%) was added to silica to obtain silica–zirconia composite membranes by firing at 400–500 °C for pervaporation tests with organic solvent/water mixtures, such as iso-propyl alcohol (IPA)/water and tetrahydrofuran (THF)/water mixtures at their normal boiling points.The membrane coatings have been done effectively by the hot-coating methods proposed previously. Boiling water treatments introduced in the coating processes have made the membranes quite stable even in the high water concentration region of aqueous organic solutions at their normal boiling points. Zirconia contents larger than about 40 mol% have made the silica–zirconia membranes quite stable. The membranes of zirconia contents less than about 30 mol% were found not stable in a dilute aqueous solution of IPA. The membranes fabricated by the conventional dip-coating methods with slow drying were not stable against water because of the probable segregation of silica and/or silica-rich phases during drying.The membranes fired at lower temperature (400 °C) gave a higher water flux of around 500 mol m⁻² h⁻¹ (9 kg m⁻² h⁻¹) with a separation factor larger than 1500 at 10 wt.% of water in the boiling feed of IPA/water mixture, for example.     

Preferential oxidation of carbon monoxide in a hydrogen-rich gas stream over supported gold catalysts: the effect of a mixed ceria–zirconia support composition

Research on Chemical Intermediates - The effect of the ceria–zirconia (CeO2–ZrO2) support composition of a gold (Au) catalyst (Au/Ce1?xZrxO2, where x is the molar ratio), prepared...     

Phase transformation and crystalline growth of 4 mol% yttria partially stabilized zirconia

The phase transformation and crystalline growth of 4 mol% yttria partially stabilized zirconia (4Y-PSZ) precursor powders have been investigated using the coprecipitation route, using zirconium oxide chloride octahydrate (ZrOCl₂·8H₂O) and yttrium nitrate (Y(NO₃)₃·6H₂O) as the initial materials. Differential thermal analysis (DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano beam electron diffraction (NBED), and high resolution TEM (HRTEM) were utilized to characterize the behavior of phase transformation and crystalline growth of the 4Y-PSZ precursor powders after calcined. Tetragonal ZrO₂ crystallization occurred at about 718.2 K. The activation energy of tetragonal ZrO₂ crystallization was 227.0 ± 17.4 kJ/mol, obtained by a non-isothermal method. The growth morphology parameter (n) and growth mechanism index were close to 2.0, showing that tetragonal ZrO₂ had a plate-like morphology. The crystalline size of tetragonal ZrO₂ increased from 7.9 to 27.6 nm when the calcination temperature was increased from 973 to 1,273 K. The activation energies of tetragonal ZrO₂ growth were 14.97 ± 0.33 and 84.46 ± 6.65 kJ/mol when precursor powders after calcined from 723–973 and 973–1,273 K, respectively.     

Total oxidation of ethanol and toluene over ceria—zirconia supported platinum catalysts

The effect of platinum loading (0.09–1.00 mass %) on the performance of ceria-zirconia supported catalysts in the total oxidation of ethanol and toluene in air was investigated. The introduction of platinum promoted the reduction of surface cerium and decreased the acidity of the catalysts. In ethanol oxidation, the temperature of 50 % conversion decreased with increasing platinum content. This increase in catalytic performance was more pronounced for the catalysts with 0.59 mass % and 1.00 mass % Pt. On the other hand, higher amount of by-products (mainly acetaldehyde) was observed at increased platinum loadings. For all catalysts, a correlation between their H₂-TPR profiles and catalytic performance was revealed. In toluene oxidation, only the catalysts with 0.59 mass % and 1.00 mass % Pt exhibited a lower temperature of 50 % conversion than pristine ceria-zirconia. The effect of platinum loading was less pronounced than in ethanol oxidation and a correlation between reduction behaviour and catalytic performance was not observed. The superior catalytic performance of the catalysts with 0.59 mass % and 1.00 mass % of Pt in both ethanol and toluene oxidation was ascribed to the presence of large platinum nanoparticles, which were not observed at lower Pt loadings.     

Silica–zirconia membranes for nanofiltration

Inorganic nanofiltration membranes were fabricated from silica–zirconia composite colloidal sol (molar ratio Si/Zr=9/1) using a sol–gel process. Molecular weight cut-off (MWCO) was successfully controlled between 200 and 1000 Da by regulating the colloidal diameters of sol solutions in the final coating stage. The pure water permeabilities ranged from 0.15×10⁻¹¹ to 1.5×10⁻¹¹ m³ m⁻² s⁻¹ Pa⁻¹. Pore size and pore size distribution were estimated based on the dynamic method of humid air permeation, and found to be from 1.0 to 2.9 nm. The MWCO obtained from NF experiments using neutral organic solutes corresponds well with the pore diameters estimated from the dynamic permeation method. Silica–zirconia membranes were found to be stable in aqueous solution for periods in excess of four months.     

Anionic polymerization of ϵ-caprolactam-lix effect of the ratio of reacting components,of the medium and of the ring size on the initial stage of the anionic polymerization of lactams

Effects of the concentrations and ratio of reacting components and of temperature on the kinetics of reaction of N-propionyllactam (I) with potassium salt of lactam (KL) were studied for derivatives of ϵ-caprolactam, 8-octanelactam and 12-dodecanelactam. For ϵ-caprolactam, the initial rates of propagation, acylation of open-chain amide groups and condensation of growth centres were estimated. At the ratio of the starting components [I]₀/[KL]₀ = 0.5-3, the participation of the polymerization reaction is constant, amounting to ca 45% of the overall consumption of 1. The condensation reaction is 14–36% of the total consumption of I; its initial rate passes through a maximum at [I]₀/[KL]₀ = 2. With increasing permittivity of the medium, the total rate of consumption of I increases; in two media with the same bulk permittivity, however, the rates may differ by as much as one order of magnitude.     

Construction of the Skeleton of Phthalascidin, Mechanism of the Formation of the Key Tricyclic Lactam Intermediate

Phthalascidin is a structurally simplified version of Et-743, which is a potent anti-tumor marine natural product isolated from Ecteinascidia turbinata. Its antiproliferative activity is greater than that of the agents taxol, camptothecin, adriamycin, mitomycin C, cisplatin, bleomycin, and etoposide by 1-3 orders of magnitude. An elegant synthesis of Et-743 and phthalascidin has been reported by E. J. Corey and co-workers1,2. As part of our continuing program, we have also engaged in dev…     

Synthesis and growth mechanism of zirconia nanotubes by anodization in electrolyte containing Cl−

The formation and growth of a self-organized zirconia porous layer can be achieved directly by anodization of Zr in chloride containing electrolytes. The morphology of the porous layers is affected by electrochemical conditions such as Cl⁻ concentration. Zirconia nanotubes with diameters ranging from 250 to 300 nm and a length of 33 μm were formed under proper conditions. The nanotubes have smooth and straight walls. The composition of the nanotubes was characterized by using an energy dispersive spectrometer. Selected area electron diffraction investigation reveals that the as-anodized zirconia nanotubes have an amorphous structure. Crystal phase transition and structural stability of the ZrO₂ nanotubes after heat treatment were characterized. A possible growth mechanism is presented.     

Synthesis of two peptides of α-bungarotoxin and the participation of the amino acid residue Trp-28 of the neurotoxin in the antigenicity of the molecule

Using the method of solid-phase peptide synthesis, two peptides have been synthesized, one of which corresponds to the central ring structure of -bungarotoxin (-BTX), while the second has in position 28 a Gly residue in place of the Trp in the first peptide, and their interrelationship with antitoxin antibodies has been investigated. It has been shown that the amino acid residue Trp-28 of -BTX, which is the contact residue in binding with the acetylcholine receptor, also participates directly in binding with the active centers of antibodies to -BTX.Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 628–631, July–August, 1997.     

Studies of the dissociation and the protonation of TB-chlorosulphophenol

The dissociation constant of each step for TB-chlorosulphophenol has been determined by potentiometric method, and the thermodynamic constants, △G°, △H° and △S°, of the dissociation process have been calculated. The protonation constants were measured by the spectrophotometric method. The pH values of various forms of anions of the chromogenic reagent at their concentrations were also calculated.     

Determination of the equilibrium melting point of the β-form of polypropylene

The melting behavior of the -form of isotactic polypropylene (-iPP) was investigated as a function of crystallization time and temperature. Calcium suberate, a selective -nucleating agent was used to produce samples that consist entirely of -form i-PP. The experimental melting points were recorded at different crystallization times and were extrapolated to the start of the crystallization process in order to eliminate the effect of lamellar thickening. Using the non-linear Hoffman—Weeks approach to correlate these extrapolated experimental melting temperatures with the corresponding crystallization temperatures, an equilibrium melting point of 209°C was obtained for -iPP. The equilibrium melting point estimated through the non-linear Hoffman—Weeks analysis is about 30°C higher than that (T _m ⁰=177°C) obtained on the basis of the linear extrapolation. These results are consistent with earlier claims that a linear extrapolation of T _m–T _c data leads to an underestimation of the equilibrium melting point. The results obtained for -iPP exemplify the importance of accounting for both the isothermal lamellar thickening effects and the non-linearity in the T _m–T _c correlation, when the determination of an equilibrium melting point is carried out using a procedure based on the predictions of the Lauritzen—Hoffman secondary nucleation theory.This revised version was published online in November 2005 with corrections to the Cover Date.     

The applicability of the “straight-line method” of asmus in the determination of the composition of polynuclear complexes

The straight-line method of Asmus was originally developed for the determination of n in mononuclear complexes of the general form AB_n (n≧ 1). In the present investigation it is demonstrated that the method also can be used for determining the value of n in polynuclear complexes of the form A[_mB_n (m > 1). The method as suggested by Asmus, is, however, not capable of distinguishing between mono- and polynuclear species. It is further shown, that the straight-line method can be applied for the determination of the value of m.