Nanocomposites in mullite-ZrO2 and mullite-TiO2 systems synthesised through alkoxide hydrolysis gel routes: microstructure and fractography

The sol-gel process allows preparation of very homogeneous and reactive monolithic, optically clear gels. Low-temperature thermal treatments (700–1000 °C) lead to amorphous optically clear samples (glass). Amorphous mullite compositions (0.4Al₂O₃-0.6SiO₂ to 0.8Al₂O₃-0.2SiO₂) retain large amounts of Ti and Zr elements. The crystallization has been studied by differential thermal analysis, dilatometry, X-ray and electron diffraction and Raman scattering. The nucleation begins above 1000 °C with the departure of the last protonic species, the amorphous matrix being completely crystallized only above 1400 °C. The addition of Zr and Ti elements leads to a homogeneous nucleation of phases with a composition close to ZrO₂ and Al₂Ti₃O₉ (EDX analysis) above the solubility limit. TEM and SEM analyses show that the precipitate size remains submicrometric over a wide temperature range (1000–1400 °C) and consequently glass-like mechanical properties, as well as toughening effects, caused by the presence of nanoprecipitates, are observed.     

Effects of mono-substituting chelating agents on BaTiO3 prepared by the sol-gel process

BaTiO₃ of various grain size was prepared by the sol-gel process from Ti (OR)₄ (R = isoC₃H₇ or C₄H₉)+Ba(CH₃COO)₂+chelating agent CH₃COCH₂COR(R = CH₃ or OC₂H₅) in a composition of equal molar ratio. Fourier transform infrared and fast atom bombardment mass spectrometry analyses suggested that the chelating agent substituted for one of the OR groups in Ti (OR)₄ to form Ti (OR)₃ (CH₃COCHCOR). The gelation time varied from 3 to 5 months and diminished with increasing steric hindrance. The amorphous gel was crystallized into cubic phase BaTiO₃ upon heating above 650°C. The tetragonal phase was obtained after heating for 1 h at 1350°C with the theoretical Ba/Ti ratio and 1.0096 c/a value. The measured dielectric constants diminished with increasing grain size. The results illustrated the merits of altering the chemistry of the precursors to control the properties of the BaTiO₃.     

Stress relaxation in hot-drawn low density polyethylene

The tensile stress relaxation behaviour of hot-drawn low density polyethylene, (LDPE), has been investigated at room temperature at various draw ratios. The drawing was performed at 85° C. The main result was an increase in relaxation rate in the draw direction, especially at low draw ratios when compared to the relaxation behaviour of the isotropic material. This is attributed to a lowering of the internal stress. The position of the relaxation curves along the log time axis was also changed as a result of the drawing, corresponding to a shift to shorter times. The activation volume, , varied with the initial effective stress ₀ ^* according to ₀ ^* 10kT, where ₀ ^* =₀ — _i, is the difference between the applied initial stress, ₀, and the internal stress _i. This result supports earlier findings relating to similarities in the stress relaxation behaviour of different solids.     

Steric stabilization of Stöber silica dispersions using organosilanes

Organosilanes of the general formula R _x Si(OR)_4–x (where R is an alkyl group and R = CH₃ or CH₂CH₃) were used to sterically stabilize hexane dispersions of submicrometre silica spheres. The dispersions were characterized according to sediment volume results. For 0.5m silica particles, the sediment density increased by more than a factor of three up to 50 to 55% of theoretical in the presence of organosilanes with 12 or more carbons in the R group. Solid-state¹³C nuclear magnetic resonance was used to characterize the powder-dispersant interaction; this technique can distinguish between carbons in the R group of the organosilane and residual organic groups in the silica. Scanning electron micrographs of filter compacts were used to further characterize the dispersions and indicated the presence of primary particles as well as small agglomerates.     

Reactivity of perovskites on oxidative coupling of methane

Well-defined structures like ABO₃ (A = Ba, Sr, Ca; and B = Ti, Zr) perovskites were prepared by a sol-gel method which permits the formation of a crystalline structure at low-calcination temperatures (700 °C). The perovskite structures were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) methods and the carbonate present at the surface was determined by Fourier transform infra-red spectroscopy. Reactivity studies showed that the perovskite structures are stable under oxidative coupling of methane (OCM) conditions. The yield increases followed the sequence Ca < Sr < Ba and Zr < Ti, and a stable BaTiO₃ gives a C₂ yield of 15% at 800 °C.     

Damped response of an axially excited rotating liquid bridge in zero-gravity

Summary The response of a solidly rotating finite liquid bridge due to axial excitation exhibits for frictionless liquid at the resonances singularities. For the experimenter in a spacelabmission the actual resonance amplitude is of quite some importance. For this reason damping, that has to be measured in ground tests, has been introduced into the results of the response.Notation a radius of the liquid bridge - h length of the liquid bridge - I ₀,I ₁ modified Besselfunctions - J ₀,J ₁ Besselfunctions - r, ,z polar coordinates - t time - excitation amplitude - elliptic case - hyperbolic case - abbreviation - damping factor of liquid - (z, t) free surface displacement - =² – ² surface tension - surface tension - liquid density - ₀ rotational speed of liquid bridge - forcing frequency of axial excitation - natural frequency of liquid bridge With 2 Figures     

Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB₂ powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB₂ formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB₂ powder with an average grain size of 15m was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB₂ powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB₂ powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.     

Reactivity and interface characteristics of titanium-alumina composites

The reaction kinetics between -Ti alloys and single crystal sapphire, the phase composition and morphology of the reaction-zone, and the phase compatibility in the system Ti-Al-O were investigated as part of a study to determine the feasibility of fabricating useful Al₂O₃-reinforced titanium matrix composites. In the temperature range 650 to 1000° C titanium reduces Al₂O₃ to form a complex reaction layer consisting of two distinct zones; an inner zone adjacent to the Al₂O₃ of a TiO phase containing isolated particles of (Ti, Al)₂O₃, presumably, and an outer zone of a Ti₃Al phase adjacent to the Ti matrix. The isothermal growth of the reaction layer follows a parabolic rate law. The temperature dependence of the rate constants fits an Arrhenius equation yielding activation energies of 50 to 52 kcal/mol. The high Al alloys, except Ti-6Al-2Sn-4Mo-2Zr, reacted more rapidly than pure Ti indicating that Al diffusion through the reaction zone may be the rate-limiting step.     

Coating of CaTiO3 on titanium substrates by hydrothermal reactions using calcium-ethylene diamine tetra acetic acid chelate

Titanium plates were treated in [Ti(O₂)EDTA]^2-– -Ca(EDTA)^2- mixed solutions and/or Ca(EDTA)^2- solutions (where EDTA is ethylene diamine tetra acetic acid) at pH 9–13 and 150–250 °C for 0.5–12 h. The film, about 50 m thick, and consisting of mixtures of CaTiO₃ and TiO₂ was formed in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution at pH 13 and 250 °C for 6 h. The film consisted of large icosahedral and hexagonal particles, of about 10 m diameter, and small aggregated particles, of about 1 m diameter. On the other hand, the film, about 20 m thick, consisted of hexagonal plate-like CaTiO₃ particles, of about 1 m diameter, was formed in 0.01 M Ca(EDTA)^2- solution at pH 13 and 250 °C for 6 h. The thickness of both films increased with time, where the film formation rate in 0.01 M [Ti(O₂)EDTA]^2- – 0.01 M Ca(EDTA)^2- mixed solution was much faster. The CaTiO₃ film formed on the surface of titanium promoted the precipitation of hydroxyapatite on the substrate by the hydrothermal reactions in Ca(EDTA)^2-–PO ₄ ^3- mixed solutions.     

In-situ growth of oriented PbTiO3 thin films by low temperature metalorganic chemical vapor deposition

Oriented PbTiO₃ thin films were successfully grown on (200)-oriented Pt/SiO₂/Si by metalorganic chemical vapor deposition at low temperature range from 350°C to 400°C, using -diketonate complex of Pb(tmhd)₂ and titanium isopropoxide as source precursors. Dependences of orientation and formation of crystalline PbTiO₃ phase on Pb/Ti ratio and substrate temperature was investigated. Crystalline phases and preferred orientations were determined by X-ray diffraction technique, and surface morphology was identified with scanning electron microscopy. As the deposition temperature was raised from 350°C to 400°C at two fixed Pb/Ti ratios of 3.3 and 5.0, structures of PbTiO3w₃ films transformed from amorphous to polycrystalline and preferred orientation changed from random to [100] parallel to the surface. Similar results were also observed in the films deposited at 400°C with the increase of Pb/Ti ratio from 1.1 to 5.0. As the Pb/Ti ratio increased, the dielectric constant and current density increased due to crystallization of the PbTiO₃ films. It is found that the control of excess Pb precursor amount through Pb/Ti ratio change is the key process parameter for the formation of crystalline PbTiO₃ phase in the low temperature MOCVD process.     

Study of the system FePO4-FeVO4 prepared from the solution

The samples of the pseudobinary system (FePO₄)_1–x -(FeVO₄) _x with x=0–1 were prepared by precipitation from a solution of iron nitrate and ammonium phosphate/vanadate, and the evolution of their structure during the heating of the precipitate within the temperature range of 20–1000 °C was studied by thermal analysis, X-ray diffraction and infrared spectroscopy. The samples dried at 120°C were amorphous having a high specific surface area of 160–222 m²g^–1 and their crystallization took place at 440–560 °C. The regions of solid solutions between the two boundary phases were smaller than 10 mol % of the second phase and a new crystalline phase was formed in the system at the composition of FePO₄FeVO₄=11.     

Oxidation behavior of FeAl alloys with and without titanium

The influence of Ti addition on the high temperature oxidation behavior of FeAl intermetallic alloys in air at 1000°C and 1100°C has been investigated. The oxidation kinetics of FeAl alloys was examined by the weight gain method and oxide products were examined by XRD, SEM, EDS and EPMA. The results showed that the oxidation kinetic curves of both Ti-doped and binary Fe-36.5Al alloys could be described as different parabolas that followed the formula: (W/S)² = K _p t + C. The parabolic rate constant, K _p values are approximately 2.4 and 3.3 mg² cm^–4 h^–1 for Fe-36.5Al alloy and about 1.3 and 2.0 mg² cm^–4 h^–1 for Fe-36.5Al-2Ti alloy when oxidizing at 1000°C and 1100°C respectively. The difference between Fe-36.5Al and Fe-36.5Al-2Ti alloy is not only in the surface morphology but also in the phase components. In the surface there is only -Al₂O₃ oxide for Fe-36.5Al alloy while there are -Al₂O₃ and TiO oxides for Fe-36.5Al-2Ti alloy. The effects of Ti addition on the oxidation resistance of FeAl alloy were addressed based on the microstructural evidence.     

Formation of monodispersed Ta2O5 powders

The Ta₂O₅ powders synthesized by the hydrolysis of tantalum pentaethoxide, Ta(OC₂H₅)₅ in alcoholic solution were monodispersed fine oxide particles, which were a uniform, spherical shape, non-agglomerate, and had a narrow size distribution. They grew to 1.2m after ageing for 1 h after hydrolysis. Powder X-ray diffraction and differential thermal analysisthermogravimetric analysis showed the particles were amorphous and hydrated. These particles lost the water at 290° C and gave well-crystalline Ta₂O at 740° C. Throughout these thermal processes, the particle morphology was kept almost the same.     

Microsphere-filled lightweight calcium phosphate cements

The incorporation of inorganic and organic microsphere fillers into calcium phosphate cement (CPC) to produce lightweight cementitious materials that could be used under hydrothermal conditions at high temperatures between 200 and 1000 °C was investigated. An aluminosilicate based hollow microsphere, with a density of 0.67 gcm^–3 and a particle size of 75–200 m, was the most suitable having a low slurry density of 1.3 gcm^–3, and a compressive strength greater than 6.89 M Pa. This microsphere-filled lightweight CPC exhibited the following characteristics: 1. after autoclaving at 200 °C, amorphous ammonium calcium orthophosphate (AmCOP) salt and Al₂O₃·xH₂0 gel phases, formed by the reaction between calcium aluminate cement and an NH₄H₂P0₄ based fertilizer, were primarily responsible for the development of strength; 2. at a hydrothermal temperature of 300 °C, the microsphere shell moderately reacted with the CPC to form an intermediate reaction product, epistilbite (EP), while crystalline hydroxyapatite (HOAp) and boehmite (BO) were yielded by the phase transformations of AmCOP and Al₂O₃·xH₂O, respectively; 3. at an annealing temperature of 600 °C, the HOAp phase remained in the cement body, even though an EP anorthite (AN) phase transition occurred; 4. at 1000 °C, the phase conversion of HOAp into whitlockite was completed, while the AN phase was eliminated; and 5. the microsphere demonstrated excellent thermal stability up to temperatures of 1000 °C.This work was performed under the auspices of the US Department of Energy, Washington, DC, under Contract No. DE-AC02-76CH00016.     

Synthesis and structural and optical properties of metastable ZrO2 nanoparticles with intergranular Cr3+/Cr4+ doping and grain surface modification

Nanoparticles of stabilized ZrO₂ in a single cubic (c) phase are obtained with an intergranular doping of 4 to 20 at.% Cr³⁺/Cr⁴⁺ additives by a chemical method using a high energy amorphous precursor with polymer molecules of sucrose and polyvinyl alcohol. In the polymer, the metal cations disperse and rearrange in a specific network structure with local symmetry probably similar to that in c-ZrO₂. On heating at 250 to 800°C in air, the polymer network decomposes and burns out spontaneously (with a strong exothermic peak over 350 to 500°C in thermal analysis) in a refined microstructure in 10 to 20 nm diameter particles of near spherical shape. Those are identified to be of c-ZrO₂ by x-ray diffraction. A modified microstructure of 15 to 30 nm crystallites of dispersed tetragonal (t) and/or monoclinic (m) phases in stabilized c-ZrO₂ develops on a prolong heating at 900 to 1000°C from a polymer precursor for 2 h or longer. Particles in t-ZrO₂ are in acicular shape, as long as 450 nm with aspect ratio 5 to 20, while in the shape of platelets in m-phase in an average 300 nm size. It is found that the Cr³⁺/Cr⁴⁺ additives promote formation of c-ZrO₂ by a controlled decomposition and combustion of precursor in small particles at 250 to 800°C temperature. Part of the additives form a thin amorphous surface layer in individual c-ZrO₂ grains so that it prevents them to grow or transform in the equilibrium m-ZrO₂ bulk structure as long as the temperature lies below 900°C. The x-ray diffraction in light of the optical spectrum reveals that part of the Cr⁴⁺ cations occupy Zr⁴⁺ sites in a distorted c-ZrO₂ lattice.     

The second phases in a burn resistant stable beta titanium alloy—Ti40

The second phases in Ti40 (Ti25V-15Cr-0.2Si) burn resistant titanium alloy are studied. The higher solution temperature, the more second phases in Ti40 alloy. There are not second phases if the solution temperature is below 850°C, while there are rod-like and little Ti₅Si₃ phases if temperature is over 850°C. , Ti₅Si₃ and oblique phases emerge after Ti40 solution at 910°C followed by aging at 600°C, while only Ti₅Si₃ phases emerge after solution at 860°C followed by aging at 600°C. The effect of the second phases on properties at RT are not obvious. There are different shapes of and Ti₅Si₃ precipitates after Ti40 alloy exposure at 540°C for 100 h, which decreases the thermal stability.     

Impurity control in sapphire whiskers

The effect of selective chemical removal of impurity elements on the structure and high temperature stability of sapphire whiskers has been investigated. For Compagnie Thomson Houston (CTH) sapphire whiskers (with an as-grown impurity concentration of 6% silicon and approximately 2% (sodium + potassium + calcium)), treatment in a 20% H₂SO₄-20% HF solution (Solution A) effectively removed the grown-in second phase particles and formed a surface reaction product. The reaction product could be removed from the whiskers with distilled water and the cleaned whisker annealed at 1100° C for 17 h (in high purity argon) without attendant whisker disintegration. A similar effect was noted for Thermokinetic Fibers Inc. (TFI) whiskers (approximately 0.2% silicon) after treatment in Solution A. In both cases the improvement in high temperature stability is correlated with a reduction in silicon concentration to < 0.15%.An alternative treatment in H₃PO₄ (Solution B) resulted in preferential removal of (sodium + potassium + calcium) and aluminium from CTH whiskers. The grown-in second particles were retained in the whiskers and coarsened into a variety of configurations during an 1100° C/17 h anneal, which may be attributed to the geometry produced by differential whisker thinning.     

Hydrothermal synthesis of fibrous lead titanate powders

A fibrous lead titanate (PbTiO₃) powder with light-yellow colour has been prepared by hydrothermal synthesis. The influences of Pb/Ti ratio (0.3 to 1.0) in the mixture and reaction time on the formation of fibrous PbTiO₃ under hydrothermal conditions have been investigated. The preferable conditions for preparing fibrous perovskite-type PbTiO₃ from fibrous potassium titanate are that the Pb/Ti ratio is 1.0, reaction temperature 150 °C and time 72 h. The particles of fibrous powder of perovskite type are usually less than 2 m in diameter and more than 50 m in length. The fibrous morphology is essentially unchanged up to about 650 °C, but it disappeared after heating to 1000 °C.     

Diaphyseal fractures treated by polylactide and hydroxyapatite pins. Experimental study in rat

Hydroxyapatite (HAp) coatings were deposited onto substrates of metal biomaterials (Ti, Ti6Al4V, and 316L stainless steel) by electrophoretic deposition (EPD). Only ultra-high surface area HAp powder, prepared by the metathesis method 10Ca(NO₃)₂ + 6(NH₄)₂HPO₄ + 8NH₄OH), could produce dense coatings when sintered at 875–1000°C. Single EPD coatings cracked during sintering owing to the 15–18% sintering shrinkage, but the HAp did not decompose. The use of dual coatings (coat, sinter, coat, sinter) resolved the cracking problem. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) inspection revealed that the second coating filled in the valleys in the cracks of the first coating. The interfacial shear strength of the dual coatings was found, by ASTM F1044-87, to be 12 MPa on a titanium substrate and 22 MPa on 316L stainless steel, comparing quite favorably with the 34 MPa benchmark (the shear strength of bovine cortical bone was found to be 34 MPa). Stainless steel gave the better result since -316L (20.5 m mK^-1) > -HAp (14 m mK^-1), resulting in residual compressive stresses in the coating, whereas -titanium (10.3 m mK^-1) < -HAp, resulting in residual tensile stresses in the coating. © 1999 Kluwer Academic Publishers     

Thermal stability in the Al-Al3Ti system

Directionally solidified samples of an Al-2 wt% Ti alloy were annealed at temperatures between 435° C and 660° C to investigate the thermal stability of phases formed during an incomplete peritectic transformation. The proportion of Al₃Ti present in the assolidified alloy is less than equilibrium up to about 480° C, and more than equilibrium at higher temperatures. Hence, Al₃Ti particles will be stable up to about 500° C and will tend to dissolve at higher temperatures. Diffusion due to non-equilibrium composition of the phase continues at all temperatures but is sluggish up to about 600° C. The diffusion coefficient of Ti in Al at 635° C is estimated to be 2×10^–11 cm² sec^–1.