Showing papers on "Rutile published in 1987"

Structural-electronic relationships in inorganic solids: powder neutron diffraction studies of the rutile and anatase polymorphs of titanium dioxide at 15 and 295 K

Abstract: Rietveld analysis of time-of-flight pulsed neutron diffraction of powders shows a nearly isotropic shrinkage of the structures of both the rutile and anatase polymorphs of TiO/sub 2/ upon cooling from 295 to 15 K and no change in the sense of the distortion of the TiO/sub 6/ octahedra (two long and four short Ti-O distances in both): rutile at 295 (first) and 15 K (second), a/sub 0/ 4.593 08 (4), 4.586 66 (4), c/sub 0/, 2.958 89 (3), 2.954 07 (3), x/sub oxygen/, 0.304 76 (6), 0.304 69 (6), Ti-O(4x), 1.9486 (3), 1.9459 (3), Ti-O(2X), 1.9796 (4), 1.9764 (4); anatase at 295 and 15 K, a/sub 0/, 3.784 79 (3), c/sub 0/, 9.502 26 (12), 0.504 65 (12), x/sub oxygen/, 0.16686 (5), 0.166 75 (4), Ti-O(4x), 1.9338 (1), 1.9322 (1), Ti-O(2X), 1.9799 (5), 1.9788 (4): all distances (A) referenced to silicon (a = 5.430 88 A). Both tight-binding calculations on the crystalline solids and molecular mechanics computations on the oxide lattice alone lead to a model in which the balance of attractive Ti-O and repulsive O-O interactions control the details of the overall structures. The relative bond lengths around metal centers in some other systems are predicted.

Structural-Electronic Relationships in Inorganic Solids: Powder Neutron Diffraction Studies of the Rutile and Anatase Polymorphs of Titanium Dioxide at 15 and 295 K

Abstract: Rietveld analysis of time-of-flight pulsed neutron diffraction of powders shows a nearly isotropic shrinkage of the structures of both the rutile and anatase polymorphs of TiO/sub 2/ upon cooling from 295 to 15 K and no change in the sense of the distortion of the TiO/sub 6/ octahedra (two long and four short Ti-O distances in both): rutile at 295 (first) and 15 K (second), a/sub 0/ 4.593 08 (4), 4.586 66 (4), c/sub 0/, 2.958 89 (3), 2.954 07 (3), x/sub oxygen/, 0.304 76 (6), 0.304 69 (6), Ti-O(4x), 1.9486 (3), 1.9459 (3), Ti-O(2X), 1.9796 (4), 1.9764 (4); anatase at 295 and 15 K, a/sub 0/, 3.784 79 (3), c/sub 0/, 9.502 26 (12), 0.504 65 (12), x/sub oxygen/, 0.16686 (5), 0.166 75 (4), Ti-O(4x), 1.9338 (1), 1.9322 (1), Ti-O(2X), 1.9799 (5), 1.9788 (4): all distances (A) referenced to silicon (a = 5.430 88 A). Both tight-binding calculations on the crystalline solids and molecular mechanics computations on the oxide lattice alone lead to a model in which the balance of attractive Ti-O and repulsive O-O interactions control the details of the overall structures. The relative bond lengths around metal centers in some other systems are predicted.

Structural effects on the adsorption of alcohols on titanium dioxides

Abstract: The infrared spectra of methanol, isopropanol and t-butanol adsorbed on TiO2 anatase and rutile are reported and discussed. The irreversible adsorption at room temperature is completely dissociative in all cases on rutile, while undissociatively coordinated molecules are also formed together with alkoxide groups on anatase. This shows the presence on the anatase surface of particular sites whose presence might explain the different behaviour in some surface phenomena reported in the literature of the two crystal modifications.

Lowering crystallization temperature of zircon by nanoheterogeneous sol-gel processing

Abstract: The use of deliberate heterogeneity on the nanometre scale via the sol-gel technique was introduced by this laboratory in 1982. In this study the thermal behaviour of different zircon (ZrSiO4)-composition precursors obtained via a sol-gel route and seeded with sols of zircon, rutile or thorite (α-ThSiO4) have been studied. Both compositional diphasicity and isostructural seeding were found to lower the zircon crystallization or formation temperature. As anticipated from the “nucleation and epitaxial growth” mechanism model, rutile seed crystals, with a structure different from that of the equilibrium phase did not show any effect. An unexpected counter-effect was observed when the isostructural thorite seeds were used. These results suggest that crystallization temperatures in zircon can be lowered either by utilizing the heat of reaction in compositionally diphasic gels, or by nucleation and epitaxial growth in structurally diphasic gels.[/p]

Effect of Antimony Oxide on the Anatase‐Rutile Transformation in Titanium Dioxide

Abstract: For both pure and doped titanium dioxides, the addition of antimony oxide lowers the temperature at which the anatase-to-rutile transformation occurs.Its presence also affects the color obtained in doped systems.

X-ray-absorption near-edge structure of titanium and vanadium in (Ti,V)O2 rutile solid solutions.

Abstract: An improved smoothing procedure is used to extract more information from x-ray-absorption spectra by second-derivative analysis. The true energy position of the maxima and full width at half maximum are measured for all features. The excitonic peak is present for all compositions and is well correlated with the 3d-electron density. A weak structural change appears around the titanium atom whereas a structure change is more detectable around the vanadium atom. The crystal-field splitting seems to be largely determined by V-V interactions. No electron transfer occurs between titanium and vanadium as the V 3d level is about 1 eV below the Ti 3d level. This is done by aligning the two edges with a 498.3-eV shift.

An electron spin resonance study of rutile and anatase titanium dioxide polycrystalline powders treated with transition-metal ions

Abstract: Rutile and anatase forms of titanium dioxide polycrystalline powders have been treated with low levels, ca. 200 ppm wt/wt, of Cr3+ and Fe3+ ions. The effects of calcination temperature upon these powders are described. At temperatures below 500 °C, both rutile and anatase forms of doped powder exhibited e.s.r. resonances at g= 1.97 and g= 4.3 ascribed to surface chromium and iron species, respectively. At higher temperature migration of these surface bound ions into the bulk of the rutile powder is observed. In the case of both chromium and iron substitutionally incorporated within the rutile lattice, photochromic activity is observed. Diffusion of ions into the bulk upon calcination is also demonstrated (using e.s.r.) by the corresponding anatase powder, although the results appear to indicate that the process is more difficult for the anatase case. Trapping of transition-metal ions at substitutional sites within the anatase structure is also possible by impregnation of a hydrous precursor pulp of TiO2 prior to calcination.

The gel route to Cr3+-doped TiO2, an ESR study

Abstract: Cr 3+ ions have been incorporated as Cr(acac) 3 to the titanium alkoxide precursor in order to follow hydrolysis and polycondensation by ESR. X-band and Q-band spectra have been recorded and analyzed all the way from the molecular precursors to the rutile TiO 2 , giving detailed information about the different steps involved in the sol-gel process. Our experiments show that a mixed Cr-Ti alkoxide is obtained upon mixing and heating the molecular solution. They also point out the role of acetate as ligands chemically involved in the process. No modification is observed during the gel-xerogel transformation and short range order in the amorphous xerogel appears to be completely different than in TiO 2 crystalline phases. Upon heating anatase and then rutile are successively obtained.

Process for preparing rutile-coated micaceous pigments

Abstract: In mica/titanium dioxide pigments reliable rutilization is obtained with small amounts of tin by precipitating tin dioxide directly on the mica and completing this precipitation before the precipitation of titanium dioxide is started.

Laser-Driven Pyrolysis: Synthesis of TiO2 from Titanium Isopropoxide

Abstract: Laser-driven pyrolysis of the titanium alkoxide Ti(OCHMe2)4 yields TiO2 powder. The pyrolysis can be conducted using the pure alkoxide vapor or by using argon, ethylene, or ethylene| oxygen carrier gases. The powder is principally anatase, but contains some rutile when the pyrolysis is conducted at very high temperatures.

Thermal and chemical properties of TiO2-SiO2 porous glass-ceramics

Abstract: The thermal and chemical stability of porous glass-ceramics of the TiO2-SiO2 system have been investigated. Porous glass-ceramics containing both anatase and rutile had thermal expansion coefficients of 40 to 55 x 10−7 K−1 in the range 0 to 700° C. They remained porous up to 1000° C, while the pores of high-silica porous glass of Vycor type collapsed completely at that temperature. The high thermal stability of the porous glass-ceramics may be attributed to a high viscosity due to dispersed crystallites of anatase and rutile in the skeleton. Most of the anatase in the skeleton was transformed to rutile by heat treatment at 900° C, but some of it remained untransformed even after 6 h. Surface -OH groups identified by IR spectroscopy were removed by dehydration polycondensation with heating up to 900° C. The porous glass-ceramics were quite durable to alkali solution compared with high-silica porous glass. The excellent durability of these porous glass-ceramics was attributed to the large amount of TiO2 contained in the skeletal structure.

Role of oxygen and nitrogen in the titanium‐silicon reaction

Abstract: X‐ray diffraction and Rutherford backscattering measurements were performed on titanium silicide, titanium oxide, and titanium nitride. These compounds were formed by the heat treatment of Ti film on single‐crystal Si in O2, N2, or O2+N2 ambient. The heat treatment in O2 and O2+N2 at 600 °C resulted in the formation of TiO2 and TiSi2, whereas the heat treatment in N2 resulted in the formation of TiN, Ti5Si3, and TiSi. It was also observed that Ti5Si3 and TiSi were converted to TiSi2 during the post‐annealing above 800 °C and that the resistivity of TiSi2 was lowered to about 25% that of TiSi and Ti5Si3. The refractive index of TiO2 formed in O2 ambient was similar to that of rutile bulk.

Surface character and membranolytic activity of rutile and anatase: two titanium dioxide polymorphs.

Abstract: Biological studies of two titanium dioxide polymorphs, rutile and anatase, have produced conflicting results. Generally, the in vivo and in vitro methods used to evaluate pneumoconiotic dusts have shown the polymorphs to be inert, but occasionally both minerals have been reported to produce effects consistent with biologically active minerals. Many of these reports failed to specify which polymorph was used experimentally. While this limited the value of the data, the problem was further compounded by the variation in the surface properties of each polymorph depending on whether the specimen was a naturally occurring mineral or was made synthetically. Five naturally occurring and 11 synthetically produced titanium dioxide specimens were studied. The physical characterisation of each specimen entailed the determination of the polymorph type(s) by continuous scan x ray diffraction and the size distribution by transmission electron microscopy. The ability of each specimen to lyse erythrocytes was determined and compared with quartz. Only two, both synthetic rutiles, were found to be active. The hydrogen bonding ability of the surfaces of these rutiles were compared with inert rutile and quartz. The binding properties of the active rutile have been found to be consistent with those properties associated with biologically active quartz. The surface properties of rutile are the determinants of its activity. Because natural and synthetic rutiles possess different surface properties, they display different activities.

Grain-boundary and surface segregation of Ba-Ti-O phases in rutile

Abstract: The solubility of barium in rutile and the formation and morphology of second phase at external and internal surfaces of single-crystal and polycrystalline rutile (TiO,2) have been studied for various heat treatments up to 1450°C and in several configurations of the barium source. It has been found that barium is not incorporated into the rutile lattice but instead forms an amorphous, quasi-liquid film of Ba-Ti-O at high temperature (=1400°C) which covers the grain boundaries and the external surfaces. On cooling or annealing at lower temperature (=1100°C) the continuous film breaks up to form isolated segregation patterns. Transport of the second phase by surface and grain-boundary diffusion is rapid and reversible by heat treatment.

Use of mixed-phase rutile pigments for colouring plastics

Abstract: Temperature stable rutile mixed phase of the ternary system titanium, antimony and chromium containing O, 1 to 2, O wt .-% magnesium.

Synthesis of star sapphire artificial crystal

Abstract: PURPOSE:To obtain a bright blue crystal containing rutile needlelike crystals finely distributed in the interior, by growing star sapphire in a reducing atmosphere, e.g. hydrogen or CO, etc., by a floating zone (FZ) method and heat- treating the resultant crystal in an oxidizing atmosphere after the growth. CONSTITUTION:A star sapphire crystal is grown in a reducing atmosphere consisting of hydrogen or carbon monoxide by using a floating zone (FZ) method. Since the reducing gas consisting of the hydrogen or carbon monoxide reduces iron oxide and titanium oxide which are raw materials for the star sapphire respectively into metallic iron and metallic titanium, a colorless transparent crystal is obtained. The resultant crystal is then placed in a furnace of an oxidizing atmosphere and heat-treated at 1,000-1,500 deg.C for about 10-50hr. The metallic iron and metallic titanium in the crystal are oxidized, ionized and uniformly colored to finely and densely grow rutile needlelike crystals at the same time and afford the aimed bright blue star sapphire without uneven color.