Heat Treatment Of Gel Research Articles

Evolution of structural and optical properties in the course of thermal evolution of sol–gel derived cobalt-doped gahnite

Thermal evolution of sol–gel derived gahnite (ZnAl 2O 4) with 4, 8 and 12 at.% of Zn replaced with Co was studied by thermal analysis techniques (DTA/TGA), X-ray diffraction (XRD) and UV–vis diffuse reflectance spectroscopy (DRS). Zinc–cobalt spinel powders were produced by gel heat treatment at temperatures as low as 400 °C. Crystal structure was characterized using Rietveld refinement of X-ray diffraction patterns for the samples annealed at 800 °C, simultaneously with the analysis of diffraction line broadening. It was found out that the distribution of Co 2+ ions in tetrahedral and octahedral sites of zinc cobalt aluminate crystal lattice, crystallite size and lattice strain depend on Co loading. The green color of samples thermally treated at T < 800 °C has been explained as a consequence of partial oxidation of Co 2+ ions at lower temperatures and accommodation of Co 3+ ions in octahedral sites. Thermal treatment at higher temperatures promote gradual change of color from green to blue, characteristic for tetrahedrally coordinated Co 2+ ions. The spectra evolution could be interpreted as a progressive reduction of Co 3+ to Co 2+ ions at higher temperatures.

Synthesis and characterization of nanocrystalline zinc aluminate spinel by sol–gel technique using modified alkoxide precursor

Nanosized zinc aluminate spinel (gahnite, ZnAl2O4) powders were prepared by sol−gel technique at low sintering temperatures. Aluminium-sec-butoxide [Al(OsBu)3] and zinc nitrate hexahydrate Zn(NO3)2 . 6H2O were used as starting materials. Gels with and without chelating agent were prepared. Ethyl-acetoacetate (C6H10O3) was used as a chelating agent in order to control the rate of hydrolysis of Al(OsBu)3. The dried gels and thermally treated samples were characterized by means of Differential Thermal Analysis and Thermo-Gravimetric Analysis (DTA, TGA), X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and Transmission Electron Microscopy (TEM). The surface area was measured by Brunauer-Emmet-Teller (BET) adsorption–desorption isotherms. It has been established that chelation enables to obtain a transparent gel. The thermal evolution of gels was characterized by two crystallization processes in the range 200–400 °C and 600–700 °C. Both processes yielded pure ZnAl2O4 as evidenced by XRD, i.e. zinc aluminate spinel powders were produced by gel heat-treatment at temperatures as low as 300 °C. The average gahnite crystallite size for the samples sintered in the temperature range of 400–1000 °C has been calculated from the broadening of XRD lines revealing that nanocrystalline powders were prepared. The surface areas measured for the samples fired at 700 °C for 2 h were 43.1 and 62.6 m2 g−1, for sample without and with the chelating agent, respectively. TEM micrographs confirmed the nano-scale size of particles.

Preparation and Characterization of Binder for Inorganic Composites Made from Amorphous Mineral Raw Material

The preparation and characterization of a composite binder made through a sol-gel route from an amorphous mineral raw material has been studied. The amorphous mineral raw material is alumino-silicate based but contains also alkaline earth and some transition metal oxides. A stable sol can be prepared by dissolving the raw materials in formic acid. The drying and heat treatment of the subsequent gel has been studied by thermogravimetry, nuclear magnetic resonance, infrared spectroscopy as well as X-ray diffraction studies. The different constituents of the raw material have shown to play a major role during the drying and heat treatment of the gels. The binding effect of the binder was evaluated to be good by studying paper-binder composites as well as wetting properties.

Synthesis and structure of iron zirconium phosphate Fe1/3[Zr2(PO4)3

Iron zirconium phosphate Fe1/3 [Zr2(PO4)3] is synthesized by heat treatment of gels with the stoichiometric composition of components. The compound prepared is characterized using electron probe microanalysis, X-ray diffraction, and IR spectroscopy. The crystal structure of the Fe1/3[Zr2(PO4)3] phosphate is refined in space group P21/n by the Rietveld method in the isotropic approximation for atomic displacements of all elements.

Regulation of the Structure of Coked Foams by Incorporation of Metal-Containing Carbon Nanostructures in Swellable Fireproofing Coatings

Metal-containing carbon nanostructures with a nano-size structure and large number of phase boundaries are cross-linking agents that stimulate formation of coked foam when added to swellable fireproofing composites. Metal-containing carbon nanostructures made from anthracene in nickel-containing eutectic melts affect ordering of the structure of coked foam to a greater degree. The products of heat treatment of gels of the composition PVA:NiCl2 1:1, 5:1 and PVA:CoCl2 20:1 are the best composites with respect to coked foam compressive strength indexes and swelling ratio.

Double Phosphates of Ce(IV) and Some Mono- and Bivalent Elements

A series of double Ce(IV) orthophosphates NaCe2(PO4)3 and B0.5Ce2(PO4)3 (B = Mg, Ca, Sr, Cd) was prepared by heat treatment of gels with the stoichiometric content of the components. These phosphates were characterized by powder X-ray diffraction, IR spectroscopy, and electron spin resonance. The com- pounds have the monazite structure. The unit cell parameters of the phosphates were determined. The crystal structure of the phosphates is stable at temperatures from 20 to 1600°C. The crystallographic parameters of the double Ce(IV) orthophosphates were compared with those of simple Ce(III) phosphate CePO4 which is artificial analog of monazite. Crystal chemical features of formation of double phosphates of tetravalent f elements and mono- and bivalent elements were discussed.

Porous Structure of Powders Prepared with Heat Treatment of Gels from Tin (IV) and Antimony (III) Hydroxides

The effect of heat treatment on the parameters of the porous structure of powders in the system Sn(IV) ― Sb ― O is studied. It is established that xerogels of coprecipitated tin and antimony hydroxides dried at 360 K are microporous adsorbents containing particles of both crystalline and amorphous phases. During powder heat treatment there is formation of solid solutions based on tin dioxide and there is redistribution of the microporous structure into a mesoporous structure. This is accompanied by a reduction in specific surface and an increase in pore size.

A new direction in the creation of ceramic materials with a specified phase composition and structure. part ii. theoretical foundations of low-temperature synthesis of sic from gels and practical realization of this process in the technology of ceramics and refractories 1. Physicochemical foundations of low-temperature synthesis of sic in heat treatment of gels

Results of a thermodynamic study of 108 phase diagrams of the system Si — O2 — C plotted in the coordinatesRT lnp Q2 —T are presented. Conditions of the gas medium that intensify SiC synthesis are established. According to the phase diagrams of Si — O2 — C, carbide formation can start at 700 K. The presence of a minimum amount of SiO vapor at a ratio \(p_{CO} /P_{CO_2 } = 9\) : 1 is the most favorable condition for synthesizing SiC from silica and carbon.

Low-temperature synthesis of SiC in heat-treatment of gels of hydrolyzed ethyl silicate

The problem of low-temperature synthesis of β-SiC in heat treatment of gels of hydrolyzed ethyl silicate is considered. Data obtained in DTA, IR, Raman spectroscopy, and EPR physicochemical investigations of the process of gel transformation in heat treatment in air are presented. A mechanism of the organization of the carbon system in a gel and possible ways of synthesizing SiC at low temperatures are suggested. Synthesis of SiC in heat treatment of gels of ethyl silicate at a rate exceeding 10 K/min is established by the x-ray method beginning with 773 K.

Infrared Spectral/Structural Investigation of the Heat Treatment of Tetramethoxysilane/Diethoxydimethylsilane-Derived Gels

Abstract Structural changes occurring during the heat treatment of gels that were prepared from binary tetramethoxysilane (TMOS)/diethoxydimethylsilane (DEDMS) mixtures were investigated using infrared spectra. Analysis of the spectral data indicated that the Si-O networks of the heat-treated gels increased their connectivity with increased dehydration due to increased treatment temperature or time. Four- and three-membered (Si-O)n rings were noted for appropriate heat-treated gel compositions. At higher heat-treatment temperatures, methyl groups were removed from the gel network.

Optimization of precursor fiber formation conditions from the Y-Ba-Cu-citric acid-H 2O sol

The formation and dissolution of hydroxides, carbonates and citrate complexes, which depend on the pH of the solution, are very important for the preparation of homogeneous and stoichiometric high- T c superconducting YBa 2Cu 3O 7−δ fibers from the Y-Ba-Cu-citric acid-H 2O system. Since the solubility of each complex ion is a linear function of pH, the solubility diagram can be obtained by plotting the logarithmic molar concentrations calculated from the values of the equilibrium constants and solubility products for hydroxides, carbonates, and citrates. The optimum pH for preparing homogeneous Y-Ba-Cu-citrate gel fiber is estimated to be 6.0 ± 0.2. The ceramic fiber (diameter: 0.1–2 mm, length: ∞) obtained after the heat treatment of gel was black colored and the corresponding superconducting transition temperatures ( T c, onset and T c, offset) have been determined to be approximately 95 and 86 K, respectively.

Sol-gel preparation and optical properties of silica glasses containing Cd and Zn chalcogenide microcrystals

The sol-gel process has been studied to prepare non-linear optics glasses doped with Cd and Zn chalcogenide microcrystals. CdS- or ZnS-doped SiO 2 glasses were prepared by heat treatment of gels containing thiourea complexes of corresponding metal ions. CdSe-doped glasses were alternatively formed by immersing sol-gel-derived porous CdO SiO 2 glasses in methanol solutions of selenourea. The size of microcrystals embedded in glasses changed with the amount of thiourea complexes in gels for the sulfides and with the immersion time of the porous glasses in selenourea solutions for the selenide. Glasses containing these chalcogenide microcrystals showed quantum size effects in optical absorption spectra.

Sol-Gel Synthesis of YBa2Cu3O7-&amp; Using Yttrium, Barium and Copper Polyether Alkoxide Precursors

ABSTRACTHomogeneous solutions of yttrium, barium and copper 2-(2-methoxy)-ethoxyethoxides in 2-(2-methoxy)ethoxyethanol of appropriate lY:2Ba:3Cu stoichiometry have been prepared and used in sol-gel synthesis of YBa2Cu3O7-δ. To our knowledge, this is the first report of the preparation of a homogeneous solution of yttrium, barium and copper homoleptic alkoxides dissolved only in the parent alcohol. These solutions provide an excellent starting point for sol-gel synthesis of YBa2Cu3O7-° since the metals are mixed on the molecular scale, and because the solutions are stable with no possibility of ligand exchange to give insoluble products. Hydrolysis and careful heat treatment of gels produced from these solutions yield nearly pure tetragonal YBa2Cu3O7-δ at temperatures as low as 725°C. The preparation of these solutions and their characterization by small-angle x-ray scattering measurements and TEM are reported, as well as preliminary data pertaining to their use in the sol-gel synthesis of YBa2Cu3O7-δ.

Adiabatic nucleation and crystallization of gels

Adiabatic Nucleation Theory (ANT) has been successfully applied to pure liquid metals, oxide glasses, metallic glasses and polymers. This paper shows that ANT gives an interesting correlation between the crystallization data of gels of both reluctant and good (dense) glass forming systems. For reluctant glass formers, one finds that T ch < T 14 −, whereas for good (dense) glass formers, T ch > T 14 −. T − 14 is the temperature predicted by ANT at which nucleation starts in cooling experiments or ceases during heating and T ch is the experimental temperature of crystallization on heating. For systems with T ch considerably lower than t − 14 , as observed for reluctant glass formers, it is doubtful that one can obtain dense glasses by heat treatment of gels. This conclusion is in diagreement with the often advanced idea that the gel route can lead to dense glasses of unusual, reluctant glass forming compositions.

Applications of zachariasen's rules to different types of noncrystalline solids

Zachariasen's rules for glass formation were originally considered to be applicable to all glass forming systems. Later research indicated the application of Zachariasen's concepts was limited to only some common glass-forming oxide systems. Glass formation from the liquid was governed not only by structure but by kinetics of cooling as well. Non-crystalline solids, however, can be prepared also by methods other than cooling the liquid. For instance, vapor deposition, heat-treatment of gels and precipitation from solutions. The structures of such amorphous phases, their tendency to crystallize and their relation to one another and to melt-formed glasses are important questions. Zachariasen's rules have now been applied to many of these non-crystalline solids. It is shown that such a treatment can furnish useful information regarding the stability of such amorphous phases and the structural relationship between them.

Preliminary Characterization of Gel Precursors and Their High‐Temperature Products by 27Al Magic‐Angle Spinning NMR

The environment of the Al atom in alumina gels, single‐phase and diphasic mullite gels, and their transformation products was studied by 27Al magic‐angle spinning nuclear magnetic resonance (MASNMR). Alumina gels made from inorganic and organic precursors showed exactly the same 27Al MASNMR spectra. Heat treatment of gels at 500°C caused artial conversion of octahedral Al to tetrahedral Al. The Al in single‐phase mullite gel appears to be highly ordered compared to that in diphasic mullite gel.

Crystallization of silica in SiO 2-Fe 2O 3 glasses obtained from gels

The effect of iron oxides on silica crystallization was investigated on SiO 2 -Fe 2O 3 glasses prepared by the sol-gel method. Heat treatment of gels under oxygen and hydrogen fluxes gave oxidized and reduced iron in glass, while different redox ratios were achieved under nitrogen in different compositions. Crystallization of these samples at different temperatures was studied by X-ray diffraction. quartz was the only ordered phase to be detected and the influence of redox ratio on its formation was verified.

Preparation of High‐Silica Glasses from Colloidal Gels: III, Infrared Spectrophotometric Studies

The gel process for making silica was studied by infrared spectrophotometry of solid samples in various stages of dehydration. Spectra were recorded in the high‐frequency overtone region (2500 to 8000 cm‐1) as well as in the region of fundamental absorption (200 to 4000 cm‐1), depending on the nature of the sample. Molecular water was distinguished from silanol groups and the effects of hydrogen bonding were observed. Drying and heat treatment of gels at low temperatures (&lt;150°C) causes a loss of unbonded included water, but significant loss of hydrogen‐bonded water occurs only with relatively high‐temperature heat treatments (&gt;800°C). Hydration of siloxane groups on pore surfaces and attachment of water molecules by hydrogen bonding to these surface silanol groups is reversible for heat treatments up to 1025°C. At higher temperatures (&gt;1025°C), active sintering takes place, converting the porous structure into a homogeneous silica network. In this process, surface silanol groups become isolated within the silica network, giving rise to vibrational frequencies characteristic of internal ‐OH groups. Also, at the higher temperatures silanol groups react to form siloxane groups with loss of H2O. A model is presented for formation of a gel in which hydrogen bonding initiates association of colloidal particles, followed by strengthening through interparticle solid silica precipitation.

Micropore structure of chromium oxide gels

The adsorption of nitrogen, carbon tetrachloride and water vapour was studied on various well-characterized chromium oxide gels in an attempt to relate the adsorptive properties with the texture and surface chemistry of the gels. Standard adsorption data obtained on non-porous chromium oxides provided a basis for the analysis of the isotherms on the porous gels by the application of the αs-method. The pore structures identified were within both the micropore and the mesopore range. For the microporous gels it was possible to use the nitrogen αs-plot to assess the pore volume and the external surface area; comparison of the micropore volumes obtained with the three adsorbates gave an indication of the micropore size distribution.Certain of the gels outgassed at temperatures below 200°C exhibited molecular sieve properties; in these cases, the water sorption capacity was high, whereas the pore volume available for nitrogen and carbon tetrachloride sorption was small. It is suggested that cavities were formed in the gel in the vicinity of the Cr3+ ions through the removal of water ligands under conditions where the hydroxide, oxide-hydroxide or oxide structures were slow to develop. The molecular sieve character was not found with gels prepared at high pH, presumably because of replacement of H2O ligands by OH– ions. Heat treatment of gels in air and nitrogen at temperatures between 200 and 600°C led in some cases to the closure of micropores and in others to pore widening with the development of mesopores. The results show that the pore structure may be stabilized to higher temperatures if the formation of the thermally unstable CrO3 is prevented. Crystalline α-Cr2O3 was non-porous, but the water sorption results indicate that the surface structure was not perfected until the calcination temperature was above 900°C.