Thermal conductivity of nanocomposites based on diamonds and nanodiamonds

The thermal conductivity of composites sintered from natural microdiamond (5–7 and 10–14 μm) and nanodiamond powders under pressure of  6.0 to 6.5 GPa at the temperature  1000 to 2000 °C for 6–20 s was measured in a steady heat flow in the temperature range of 50–200 °C. It was found that the thermal conductivity of nanodiamond composites produced in these conditions was less than 10 W/(mК) while that of natural microdiamonds was as high as 500 W/(mК).     

Application of pulse discharge sintering (PDS) technique to rapid synthesis of Ti3SiC2 from Ti/Si/C powders

To synthesize Ti₃SiC₂ samples, pulse discharge sintering (PDS) technique was utilized to sinter elemental powders of Ti/Si/C with stoichiometric and off-stoichiometric ratios in a temperature range of 1200–1500 °C. The results showed that high purity Ti₃SiC₂ could not be obtained from the Ti/Si/C powder with molar ratio of 3:1:2, and Ti₃SiC₂ preferred to form at relatively low sintering temperature for a short time. When 5Ti/2Si/3C and 3Ti/1.5Si/2C powders were sintered for 15 min, the TiC content was respectively decreased to 6.4 and 10 wt.% at 1250–1300 °C. The corresponding relative density of the samples sintered from 5Ti/2Si/3C powder was calculated to be as high as 99% at the temperature above 1300 °C. It is suggested that low-temperature rapid synthesis of Ti₃SiC₂ would be possible through the PDS technique, provided that the composition of the starting powders should be adjusted to be off-stoichiometric ratio from 3:1:2.     

Synthesis and characterization of gadolinium phosphate neutron absorber

Hydrated gadolinium phosphate (GdPO₄·1H₂O) was synthesized by reacting high purity dissolved salts (gadolinium nitrates or chlorides) with phosphoric acid. The hydrated powders were shown to be extremely insoluble in water with a K_sp measured to be between 2.07 E-14 and 4.76 E-13. Calcination to between 800 and 1000 °C resulted in the formation of GdPO₄ in a monazite (monoclinic) crystal structure. This was correlated with the first exothermic differential thermal analysis (DTA) peak (864.9–883.4 °C). The DTA also showed small peaks in the 1200–1250 °C range, that could be associated with a change from the monazite (monoclinic) crystal structure to the xenotime (tetragonal) crystal structure. However, calcination of a sample to 1400 °C, followed by relatively rapid cooling and XRD, showed the structure was still monazite (monoclinic). DTA results showed a melting point at 1899–1920 °C (endothermic peak). It was therefore concluded that the melting point probably was the melting of the monazite (monoclinic) phase, but may have been xenotime if a phase change at 1200–1250 °C was reversible and very rapid. The higher part of the melting range was achieved with material derived using the slightly higher purity nitrate salt. The results show that GdPO₄ is an excellent candidate for a chemically stable, water-insoluble neutron absorber for inclusion in spent nuclear fuel canisters.     

Preparation of a porous nanocrystalline TiO2 layer by deposition of hydrothermally synthesized nanoparticles

In this work, a porous nanocrystalline anatase TiO₂ layer is prepared by tape casting a viscous dispersion of nanoparticles. Phase pure anatase titanium dioxide nanoparticles with a particle size of 10–20 nm are prepared by a very simple low temperature (100 °C) hydrothermal synthesis route in a pressure vessel, using only water as the medium and Ti(IV)-isopropoxide as starting material without additives. The size, shape and phase composition of the particles are studied by means of X-ray diffraction and transmission electron microscopy. A dispersion of the as-prepared nanoparticles with a narrow particle size distribution, confirmed by photon correlation spectroscopy, is prepared. After increasing the viscosity of this dispersion by addition of hydroxypropyl cellulose, anatase titanium dioxide layers are tape cast on a transparent conducting metal oxide substrate. Pores are induced by burning out the organic additive at 450 °C. The morphology and the final phase composition of the deposited TiO₂ layers are examined by X-ray diffraction and scanning electron microscopy.     

Photoactivity of anatase–rutile TiO2 nanocrystalline mixtures obtained by heat treatment of homogeneously precipitated anatase

Nanosized titanium dioxide photocatalysts with varying amount of anatase and rutile phases have been synthesized. Homogeneous precipitation of aqueous solutions containing TiOSO₄ with urea was used to prepare porous spherical clusters of anatase TiO_2. Photoactive titania powders with variable amount of anatase and rutile phases were prepared by heating of pure anatase in the temperatutre range 800–1150 °C. The structure evolution during heating of the starting anatase powders was studied by XRD analysis in overall temperature range of phase transformation. The morphology and microstucture characteristics were also obtained by HRTEM, BET and BJH. The spherical particle morphology of TiO₂ mixtures determined by SEM was stable in air up to 900 °C. The photocatalytic activity of the sample titania TIT85/825 heated to 825 °C in air, contained 77.4% anatase and 22.6% rutile was higher than that nanocrystalline anatase powder. Titania sample TIT85/825 reveals the highest catalytic activity during the photocatalyzed degradation of 4-chlorophenol in aqueous suspension.     

Densification of nanocrystalline MgO ceramics by hot-pressing

Densification of pure nanocrystalline MgO powder with 10 nm particle size by hot-pressing was investigated in the temperature range 700–800 °C, applied pressure range 100–200 MPa, and for durations of up to 240 min. It was shown that significant densification under the pressure begins above 440 °C. Densities higher than 99.5% with grain size of 73 nm were achieved at 790 °C and 150 MPa for a 30 min duration. Remarkable densification from 90 to 99.5% was observed by temperature change from 700 to 790 °C, for which the grain size was doubled only. The final grain size decreased with increasing the applied pressure. Higher shrinkage rates and cumulative shrinkages were recorded by the application of pressure at 550 °C rather than from room temperature. The temperature at which the pressure was applied is crucial in determining the maximum shrinkage rate in the nanocrystalline compacts. This effect was related to the morphological changes of the particles caused by plastic deformation at lower temperatures. Analysis of the densification rate and its comparison to the literature data was in agreement with Coble creep, where self-diffusion of Mg²⁺ cations along the grain boundaries acts as a main densification mechanism.     

Electrochemical Synthesis and Sintering of Nanocrystalline Cerium(IV) Oxide Powders

Nanocrystalline CeO₂ powders were prepared electrochemically by the cathodic electrogeneration of base, and their sintering behavior was investigated. X-ray diffraction and transmission electron microscopy revealed that the as-prepared powders were crystalline cerium(IV) oxide with the cubic fluorite structure. The lattice parameter of the electrogenerated material was 0.5419 nm. The powders consisted of nonaggregated, faceted particles. The average crystallite size was a function of the solution temperature. It increased from 10 nm at 29°C to 14 nm at 80°C. Consolidated powders were sintered in air at both a constant heating rate of 10°C/min and under isothermal conditions. The temperature at which sintering started (750°C) for nanocrystalline CeO₂ powders was only about 100°C lower than that of coarser-grained powders (850°C). However, the sintering rate was enhanced. The temperature at which shrinkage stopped was 200°-300°C lower with the nanoscale powder than with micrometer-sized powders. A sintered specimen with 99.8% of theoretical density and a grain size of about 350 nm was obtained by sintering at 1300°C for 2 h.     

Nanocrystalline orthoferrite powders: Synthesis and magnetic properties

Nanocrystalline orthoferrite powders were synthesised at low temperatures by employing an aqueous sol–gel process. Colloidal sols and water re-dispersible gels of orthoferrite precursors were prepared by room-temperature processing of inexpensive metal salts. The average diameter (Z_av) of the precursor particles was in the size range from 4 to 7 nm; the diameters had a narrow size distribution. Water re-dispersible translucent gel monoliths were obtained by concentrating the aqueous sols followed by drying them under reduced pressure (10⁻² Torr) at room temperature. The sol–gel transition was found to be completely reversible. Nanocrystalline fine powders of orthoferrites of general formula, LnFeO₃ (Ln = La, Sm, Gd, Dy, Er, Yb and Y) having a crystallite size of about 25 nm were prepared by heating the gel precursors at 650–700 °C in air. Powder X-ray diffraction and thermogravimetry, respectively, were employed to identify perovskite phase formation and delineate thermal events that lead to gel to crystallite conversion. Magnetic measurements were carried out on the resultant powders at room temperature and down to 40 K. Nanocrystalline orthoferrite powders exhibited weak ferromagnetic behaviour, and reduced magnetic moments.     

Surface Characterization of Ti and Ti (6% AI-4%, V) Metal Powders and Interaction with Primer Solutions

The surfaces of Ti and Ti (6% Al-4% V) powders were characterized by several techniques. BET surface areas as a function of temperature were measured using nitrogen adsorption. Heats of immersion (δ_wH) of these metal powders in water were measured after evacuation over the temperature range 100°-400°C. The δ_wH in water increased with increasing evacuation temperature and an anomalous increase was observed between 300° and 400°C. This was attributed to exposure of water to elemental titanium by cracking of the oxide layer at 400°C. XPS analysis did not support the possibility of metal migration through the oxide layer. Higher heats of immersion in water were determined for chemically pretreated compared to untreated Ti 6-4 powders. Water vapor adsorption isotherms were measured after evacuation of the metal powders at 100°C. Partial irreversibility of the water adsorption was observed on both powders. Water adsorption on Ti was temperature dependent. Heats of immersion measurements were used to study the interaction of primer solutions with these metal powders. Polyimide (LARC-13) and polyphenylquinoxaline (PPQ) interacted preferentially compared to the solvents with both powders. This polymer-metal interaction improved significantly after pretreatment of the Ti 6-4 powder by the Turco® 5578 and phosphate-fluoride processes. Again, an anomalous increase in the heat of immersion of Ti 6-4 evacuated at 400°C in the primer solution/solvents was observed. Anatase and rutile TiO₂ powders are not satisfactory models for the surface oxide layer on either Ti or Ti 6-4 powders.     

Effect of pre-sintering temperature on the structural and dielectric properties of (Ba0.5Sr0.5)TiO3 thin films deposited by sol–gel technique

(Ba_0.5Sr_0.5)TiO₃ thin films have been deposited by sol–gel technique and the effect of pre-sintering temperature on the structural and dielectric properties has been studied. The sol was prepared from barium acetate and strontium acetate powders by dissolving them in acetic acid; while titanium isopropoxide was used as titanium source. Acetyl acetone, 2-methoxyethanol, and formamide were used as chelating agent, diluting reagent, and for getting crack free films, respectively. Two sets of films were prepared; one set pre-sintered at 400 °C while the other one at 600 °C. In all the cases, the final sintering temperature was kept fixed at 700 °C for 2 h. These films were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), dielectric constant, and loss measurements and AC conductivity studies. It has been found that with an increase in the pre-sintering temperature from 400 to 600 °C, the dielectric constant increases from 225 to 383 (measured at 100 kHz); whereas, the loss tangent remains nearly constant at 0.03–0.05. The XRD results show better crystallinity and enhanced grain growth in case of films pre-sintered at 600 °C. The FTIR spectra reveals that there is significant removal of organic materials in films with higher pre-sintering temperature as compared to that with lower pre-sintering temperature. The AC conductivity studies show a decrease in the frequency exponent ‘s’ with an increase in the pre-sintering temperature which has been correlated with the reduction in oxygen vacancy densities in the sample with higher pre-sintering temperature.     

Hydrothermal Synthesis of Cerium(IV) Oxide

CeO₂ powders have been prepared from cerium(III) nitrate, cerium(IV) sulfate, and cerium(IV) ammonium sulfate under hydrothermal conditions at 120° to 200°C for 5 to 40 h. The effects of the starting cerium compounds, hydrothermal treatment temperature, and the concentration of the solutions on the crystal growth of CeO₂ were investigated. CeO₂ powders hydrothermally synthesized at 180°C for 5 h from cerium(IV) salts had very fine particle sizes (30 Å); on the other hand, the powder from the cerium(III) salt had a relatively coarse particle size (160 Å). Although the crystallite size of the powder synthesized from the cerium(IV) compounds depended on the treatment temperature, that from the cerium(III) compound was insensitive to the treatment temperature. The mechanisms for the growth of CeO₂ particles under hydrothermal conditions are discussed.     

Propylene epoxidation over Ti/MCM-41 catalysts prepared by chemical vapor deposition

Ti-containing mesoporous catalysts were prepared by chemical vapor deposition (CVD) of TiCl₄ on silica MCM-41 in the 700–900 °C temperature range. These samples were characterized (with XRD, ICP, nitrogen adsorption, FT-IR, ESCA, and TEM) and evaluated for the epoxidation of propylene with two alkyl hydroperoxides. The increase of CVD temperature resulted in the decrease of titanium content, catalyst hydroxyl population, crystallinity, and surface area. Catalyst selectivity to the desired product – propylene oxide – was highly sensitive to the deposition temperature. The best Ti/MCM-41 catalyst was prepared at the temperature of 800 °C, which had the maximum propylene oxide yield of 94.3%.     

THE EFFECT OF PRECIPITATION PARAMETERS ON PREPARATION OF LITHIUM FLUORIDE (LIF) NANO-POWDER

Lithium fluoride powder (LiF) is a white powder with a density of 2.64 gr/cm³ and a melting point of 848°C. This powder has several applications such as flux, glaze, soldering, and aluminum melting process, but one of the most important uses of this powder is its application in dosimetry. The commercial powders currently used for this purpose have average sizes of 5 to 10 micrometers; the objective of this research is to produce LiF powder with nano-metric particle size. In this study, the reaction of LiOH + HF → LiF + H₂O has been selected from among several reactions that were able to produce LiF powder, and some precipitation parameters such as temperature, time, agitation type, and supersaturation degree have been controlled. The morphology, phase analysis, and particle size distribution of the resulting powders were analyzed by SEM, XRD, and LPSA. Finally, lithium fluoride nano-powder was synthesized at a temperature of 25°C, pH about 2-3, reaction time less than 1 s, and agitation by ultrasonic bath.     

Preparation of three-dimensional ordered macroporous SiCN ceramic using sacrificing template method

Three-dimensional (3D) long range well ordered macroporous SiCN ceramics were prepared by infiltrating sacrificial colloidal silica templates with the low molecular weight preceramic polymer, polysilazane. This was followed by a thermal curing step, pyrolysis at 1250 °C in a N₂ atmosphere, and finally the removal of the templates by etching with dilute HF. The produced macroporous SiCN ceramics showed high BET surface areas (pore volume) in the range 455 m²/g (0.31 cm³/g)–250 m²/g (0.16 cm³/g) with the pore sizes of 98–578 nm, which could be tailored by controlling the sizes of the sacrificial silica spheres in the range 112–650 nm. The sphere-inversed macropores were interconnected by 50 ± 30 nm windows and 3–5 nm mesopores embedded in the porous SiCN ceramic frameworks, which resulted in a trimodal pore size distribution. The surface of the achieved porous SiCN ceramic was then modified by Pt–Ru nanoparticle depositing under mild chemical conditions.     

Synthesis of nano-crystalline Gd0.1Ce0.9O2−x for IT-SOFC by aerosol flame deposition

Nano-sized gadolinia-doped ceria (GDC) can be used as an IT-SOFC electrolyte, oxygen gas sensor or abrasives. In this study, nano-sized GDC powders with bimodal particle distribution of about 10 nm and 200 nm particle size were successfully synthesized by aerosol flame deposition (AFD). The resulting effects of sintering temperature on microstructure and electrical properties were investigated in the sintering temperature range 1100–1400 °C. The pellet had a completely dense microstructure after sintering at 1400 °C for 10 h. Raman measurement showed an increase of oxygen vacancy due to shift between reduced and oxidized states (Ce³⁺ ↔ Ce⁴⁺) with increasing sintering temperature. The formation of oxygen vacancies noticeably increased the ionic conductivity above 1300 °C.     

Characterization and Sintering of Reactive Cerium(IV) Oxide Powders Prepared by the Hydrazine Method

Nanocrystalline cerium(IV) oxide (CeO₂) powders have been prepared by adding hydrazine monohydrate to an aqueous solution of hydrous cerium nitrate (Ce(NO₃)₃·6H₂O), followed by washing and drying. The lattice parameter of the as-prepared powder is a = 0.5415 nm. The powder characteristics and sinterability of reactive CeO₂ have been studied. The surface areas of powders that have been heated at low temperatures are high, and these surface areas do not decrease to 10 m²/g until the temperature is >1200°C. Crystallite size and particle size are strongly dependent on the heating temperature. Optimum sintered densities are obtained by calcining in the temperature range of 700°–800°C. Ceramics with almost-full density can be fabricated at a temperature as low as 1150°C.     

Preparation and characterization of TiO2–SiO2 nano-composite thin films

TiO₂ nanocrystalline particles dispersed in SiO₂ have been prepared by the sol-gel method using titanium- and silicon-alkoxides as precursors. Nano-composite thin films were formed on the glass substrates by dip-coating technique and heat treated at temperatures up to 500 °C for 1 h. The size of the TiO₂ nanocrystalline particles in the TiO₂–SiO₂ solution ranged from 5 to 8 nm. The crystalline structure of TiO₂ powders was identified as the anatase phase. As the content of SiO₂ increased, the anatase phase tended to be stabilized to higher temperature. TEM results revealed the presence of spherical TiO₂ particles dispersed in a disk-shaped glassy matrix. Photocatalytic activity of the TiO₂–SiO₂ (1:1) thin films showed decomposition of 95% of methylene blue solution in 2 h and a contact angle of 10°. The photocatalytic decomposition of methylene blue increased and the contact angle decreased with the content of TiO₂ phase. TiO₂–SiO₂ with the molar ratio of 1:1 showed a reasonable combination of adhesion, film strength, and the photocatalytic activity.     

Dynamic mechanical studies of oriented p-oxybenzoate (POB)-poly(ethylene terephthalate) (PET) copolyester films

The effects of orientation on the dynamic mechanical properties of p-oxybenzoate/poly(ethylene terephthalate) (POB/PET (60/40)) copolyesters were studied using films prepared by extrusion drawing and by uniaxially drawing sections of compression moulded sheets. The extrusion drawn sample ED-1 showed a higher degree of orientation than uniaxially drawn samples U-2S and U-8S. The loss tangent curves for these oriented samples are characterized by a β-relaxation at 62°C and a high temperature (′) relaxation. The position of the ′-relaxation peak is dependent on the degree of orientation. Samples U-8S (f_H = 0.05), U-2S (_H = 0.06) and ED-1 (_H = 0.21), have their ′ relaxation peak at 135, 140 and 156°C, respectively. The dynamic storage modulus (E′) below 50°C increases with the degree of orientation. The highly oriented sample ED-1 maintains its high storage modulus (4 GPa) over a larger temperature range (25–125°C).     

Crystallization and Photoactivity of Tio2 Films Formed on Soda Lime Glass by a Sol-Gel Dip-Coating Process

Transparent nanophase TiO₂ thin films on soda lime glass were prepared from titanium tetraisopropoxide (TTIP) by a sol-gel dip-coating method. The TiO₂ films had amorphous phase up to 400°C and anatase phase at 500°C. The amorphous TiO₂ films obtained at 300-400°C showed considerable photoactivity for the degradation of formic acid. The photoactivity of the TiO₂ films was enhanced with increasing calcination temperature from 300° to 500°C. The crystallinity of the anatase films at 500°C was improved with increasing calcination time up to 2 h and reduced with a further increase in calcination time to 4 h due to the significant formation of sodium titanate phase as a result of sodium diffusion. The four-time-dipping anatase films at 500°C exhibited the greatest photoactivity at the calcination time of 2 h. Sodium diffusion into TiO₂ films was retarded by a SiO₂ underlayer of 50 nm in thickness.     

Effects of hydrothermal temperature and time on the photocatalytic activity and microstructures of bimodal mesoporous TiO2 powders

Bimodal nanocrystalline mesoporous TiO₂ powders with high photocatalytic activity were prepared by a hydrothermal method using tetrabutylorthotitanate (TiO(C₄H₉)₄, TBOT) as precursor. The as-prepared TiO₂ powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and N₂ adsorption–desorption measurements. The photocatalytic activity of the as-prepared TiO₂ powders was evaluated by the photocatalytic degradation of acetone (CH₃COCH₃) under UV-light irradiation at room temperature in air. The effects of hydrothermal temperature and time on the microstructures and photocatalytic activity of the TiO₂ powders were investigated and discussed. It was found that hydrothermal treatment enhanced the phase transformation of the TiO₂ powders from amorphous to anatase and crystallization of anatase. All TiO₂ powders after hydrothermal treatment showed bimodal pore-size distributions in the mesoporous region: one was intra-aggregated pores with maximum pore diameters of ca. 4–8 nm and the other with inter-aggregated pores with maximum pore diameters of ca. 45–50 nm. With increasing hydrothermal temperature and time, the average crystallite size and average pore size increased, in contrast, the Brunauer-Emmett-Teller (BET) specific surface areas, pore volumes and porosity steadily decreased. An optimal hydrothermal condition (180 °C for 10 h) was determined. The photocatalytic activity of the prepared TiO₂ powders under optimal hydrothermal conditions was more than three times higher than that of Degussa P25.