Heterogeneous Precipitation Method Research Articles

Mechanical properties and electrical conductivity of TiN–Al2O3 nanocomposites

Abstract TiN–Al2O3 nanocomposites with various additions of TiN were prepared by hot pressing of nano TiN–Al2O3 powder that was produced by in-situ nitridation of nano TiO2–Al2O3 powder. TiO2–Al2O3 powder mixtures are prepared by a heterogeneous precipitation method. The effects of nano TiN particles on the mechanical properties and electrical conductivity of TiN–Al2O3 nanocomposites were studied. Experimental results show that the addition of 20 vol.% TiN could increase the bending strength of hot-pressed Al2O3 from 370 to 725 MPa and the fracture toughness from 3.40 to 5.27 MPa m1/2, respectively. The resistivity of nanocomposites decreases with increasing amount of TiN and reaches to a minimum value of 5.5×10−3 Ω cm for the nanocomposite with 25 vol.% TiN.

Synthesis of HAp-ZrO<sub>2</sub>(3Y) Nano-Powder by Heterogeneous Precipitation Method

Synthesis of HAp-ZrO<sub>2</sub>(3Y) Nano-Powder by Heterogeneous Precipitation Method

Fabrication of YAG–SiC nanocomposites by spark plasma sintering

Abstract The heterogeneous precipitation method was used to prepare YAG–5 vol.% SiC powder. Hot-press sintering and spark plasma sintering were used to obtain dense sintered bodies. The present results revealed that nanoscale-sized SiC particles were well distributed within the composites, with most of the particles located within the YAG grains. The bending strength of spark-plasma-sintered YAG–5 vol.% SiC nanocomposites was 565 MPa, much higher than that of monolithic YAG ceramics. The fracture mode of the YAG–5 vol.% SiC nanocomposites was transgranular. The present results indicate that YAG–5% SiC nanocomposites are good high-temperature structural materials and that spark plasma sintering is appropriate for fabricating high-performance YAG–5 vol.% SiC nanocomposites with a fine microstructure because the method involves a low sintering temperature.

Ni-coated Al 2O 3 powders

A heterogeneous precipitation method was employed to prepare Ni-coated-Al 2O 3 powders using Al 2O 3, Ni(NO 3) 2·6H 2O and NH 4HCO 3 as the starting materials. The coated powders were characterized by TEM, AES, ζ-pH. Results showed that a continuous amorphous NiCO 3·2Ni(OH) 2·2H 2O film uniformly coated the Al 2O 3 particles surface. After calcining at 400 °C for 2 h in air and reducing at 700 °C for 4 h in hydrogen atmosphere, NiCO 3·2Ni(OH) 2·2H 2O was converted to Ni with size of about 20 nm, meanwhile, the continuous amorphous film become discontinuous.

Synthesis of nickel nanoparticles dispersed in γ-alumina by heterogeneous precipitation

A heterogeneous precipitation method is described for obtaining nanocomposite powders consisting of Ni nanoparticles homogeneously dispersed within γ-Al 2O 3. The amorphous Al(OH) 3 was nucleated on the surfaces of NiO nanoparticles and crystallized to γ-Al 2O 3 at 900 °C. The porosity of γ-Al 2O 3 allowed the growth of NiO nanoparticles during calcinations. After the calcined nanocomposite powders were selectively reduced at 700 °C in a hydrogen atmosphere, NiO nanoparticles were converted to Ni with the size of 25–35 nm. TEM observation and AES analysis showed Ni nanoparticles were uniformly dispersed in the γ-Al 2O 3 matrix.

Preparation, thermal stability and sintering behavior of Ni/Al2O3 coated powders

Alumina-coated Ni powders were prepared by a heterogeneous precipitation method. The thermal stability of the coated powders was investigated by differential thermal and thermogravimetric analysis in air and argon, respectively. It was found that the alumina coating layers could effectively hinder the oxidation process of the Ni particles. The sintering behavior of the powders by hot pressing indicated that the coated powders exhibited a higher sinterability than ball-milled powders, and could be fully sintered at as low a temperature as 1350°C. The microstructure of Al2O3/Ni composites from coated powders was different from that of convenient-processed composites.

Characteristics of silicon nitride powder - effects on composites with alumina and yttria

A coated silicon nitride (Si3N4) powder with yttria and alumina precursors as sintering additives was prepared by a heterogeneous precipitation method. The rheological and electrophoretic properties of the suspensions obtained from the coated (CO) powder were investigated and compared with those of pure Si3N4powder and of the mechanically mixed (MM) powders of Al2O3, Si3N4, and Y2O3. The results showed that the CO powder calcined at 500°C exhibited improved dispersion properties compared with the pure Si3N4powders. The CO powder possessed the surface character of Al2O3and Y2O3particles, that made it easier to process in aqueous media, yielding a higher solid loading than the pure Si3N4powder. These improvements were attributed to a change in the resultant interaction forces between particles from attractive (pure Si3N4, and MM powders) to repulsive in the case of the CO powder. A homogeneous distribution of sintering additives in the Si3N4matrix was obtained.

Fabrication of Ni-coated Al 2O 3 powders by the heterogeneous precipitation method

Ni-coated Al 2O 3 powders were prepared by the heterogeneous precipitation method using Al 2O 3, Ni(NO 3) 2·6H 2O and NH 4HCO 3 as the starting materials. The amorphous NiCO 3·2Ni(OH) 2·2H 2O was uniformly coated on the surface of Al 2O 3 particles with the thickness of 20 nm. The amorphous coating layer was crystallized to NiO with the size of about 15 nm at 500°C for 2 hours in air, meantime, the dispersant was eliminated. NiO was completely reduced to Ni at 700°C for 4 hours in hydrogen atmosphere. Ni with the size of about 20 nm in the coating layer was spherical and weakly agglomerated. The continuous coating layer became discontinuous during heating treatment.

Effect of nickel content on the sintering behavior, mechanical and dielectric properties of Al 2O 3/Ni composites from coated powders

Al 2O 3/Ni composites were fabricated by hot pressing coated powders, in which nanosized Ni particles were coated with layers of alumina precursor by the heterogeneous precipitation method. The calcined powders could be sintered to above 96% of the theoretical density at as low temperature as 1350°C. The average alumina grain size in the composites decreased with increasing Ni content. Compared with that of monolithic alumina, the fracture toughness and the fracture strength of alumina could be increased by 28 and 43% by adding 2 and 5.5 vol.% Ni, respectively. The enhancement in mechanical properties can be explained from the unique microstructure of sintered bodies. The dielectric constant of Al 2O 3/Ni composites was found to increase with Ni content, which should be primarily attributed to the size effect of alumina grains.

Preparation of Cr-doped Y 3Al 5O 12 phosphors by heterogeneous precipitation methods and their luminescent properties

Chromium-doped Y 3Al 5O 12 (YAG-Cr) phosphor powders were prepared by a heterogeneous precipitation method. Hydroxide precursor powders were precipitated from their respective metal sulfate solutions with urea. The formation of the YAG phosphors was investigated by means of XRD. Phase pure YAG-Cr was formed by heating the precursors at 1300°C, which is more than 300°C lower than that required for the conventional solid-state reaction method. The emission intensity of YAG-Cr phosphors increased with increasing firing temperature, and the intensities observed for phosphor powders were brighter than that of the commercial YAG-Cr phosphors.

Fabrication and microstructure of Al 2O 3–ZrO 2(3Y)–SiC nanocomposites

Al 2O 3–ZrO 2(3Y)–SiC composite powder was prepared by the heterogeneous precipitation method. Calcinating temperature of the powder was important to obtain dense sintered body. The nanocomposites were got by hot-pressing, and addition of ZrO 2 did not raise the sintering temperature. Some Al 2O 3 grain shape was elongated, and Al 2O 3 grain size was about μm. Nano SiC particles were observed uniformly distributing throughout the composites, and most of them were located within the matrix grains. Because SiC particles located within ZrO 2 grains influenced the phase transformation of ZrO 2, the sintering of nanocomposites, which controlled grain size and transformable ZrO 2 amount, become important to get high performance. The strength of 80 wt% Al 2O 3–15 wt% ZrO 2–5 wt% SiC nanocomposites was 555 MPa, and toughness was 3·8 MPa m 1/2, which were higher than those of monolithic Al 2O 3 ceramics. ©

Mechanical Properties and Microstructure of Nano-SiC–Al 2O 3 Composites Densified by Spark Plasma Sintering

Heterogeneous precipitation method has been used to produce 5 vol% SiC–Al 2O 3 powder, from aqueous suspension of nano-SiC, aqueous solution of aluminium chloride and ammonia. The resulting gel was calcined at 700°C. Nano-SiC–Al 2O 3 composites were densified using spark plasma sintering (SPS) process by heating to a sintering temperature at 1350, 1400, 1450, 1500 and 1550°C, at a heating rate of 600 °/min, with no holding time, and then fast cooling to 600°C within 2–3 min. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 vol% SiC–Al 2O 3 densified by SPS at 1450°C reached as high as 1000 MPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al 2O 3 grains and the fracture mode of the nanocomposites was mainly transgranular fracture.

SiC-ZrO 2(3Y)-Al 2O 3 nanocomposites superfast densified by spark plasma sintering

Heterogeneous precipitation method has been used to produce 5 wt% SiC-15 wt% ZrO 2-Al 2O 3 powder, from aqueous suspension of nano-SiC, aqueous solutions of zirconium oxychloride, yttrium nitrate and aluminium chloride and ammonia. The resulting gel was calcined at 1100 °C. Nano-SiC-ZrO 2(3Y)-Al 2O 3 composites were superfast densified using spark plasma sintering (SPS) process by heating to a sintering temperature in the range of 1350 to 1600 °C, at a heating rate of 600 °C/min, with no holding time, and then fast cooled to 600 °C within 2–3 minutes. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 wt% SiC-15 wt% ZrO 2(3Y)-Al 2O 3 superfast densified by SPS at 1450 °C reached as high as 1.2 GPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al 2O 3 grains and the fracture mode of the nanocomposites was transgranular fracture.

Dispersion Properties of Silicon Nitride Powder Coated with Yttrium and Aluminium Precursors

A coated silicon nitride (Si3N4) powder with yttria and alumina precursors as sintering additives was prepared by a heterogeneous precipitation method. The rheological and electrophoretic properties of the suspensions obtained from the coated (CO) powder were investigated and compared with those of pure Si3N4powder and of the mechanically mixed (MM) powders of Al2O3, Si3N4, and Y2O3. The results showed that the CO powder calcined at 500°C exhibited improved dispersion properties compared with the pure Si3N4powders. The CO powder possessed the surface character of Al2O3and Y2O3particles, that made it easier to process in aqueous media, yielding a higher solid loading than the pure Si3N4powder. These improvements were attributed to a change in the resultant interaction forces between particles from attractive (pure Si3N4, and MM powders) to repulsive in the case of the CO powder. A homogeneous distribution of sintering additives in the Si3N4matrix was obtained.

A Study on the Morphological Properties of Uranium Dioxide

UO2 powders were prepared from uranium oxalate and ammonium diuranate by reduction with hydrogen at various temperatures ranging from 600°C to 1300°C. Their morphological properties were determined by optical microscope, electron microscope, and X-ray diffraction techniques.Observation by electron microscope showed that the UO2 particles prepared, in all cases, consisted of the small unit particles which were linked with each other resulting formation of aggregates. The size determinations of unit particle were undertaken by carrying out direct measurement on the electron microphotographs and also by the X-ray method by means of line broadening of Debye-Scherrer ring. Results obtained show fairly good agreement, and the size of unit particles increased with increasing temperature of reduction. The aggregates of UO2 particles, we obtained, resembled in shape with original crystals of their mother salts and, thus, the shape of aggregates prepared from oxalate was square plate like and from ammonium diuranate which was obtained by homogeneous precipitation method with urea, was irregulare plate like. The size of these aggregates decreased with increasing temperature of reduction.The crystal of ammonium diuranate obtained by heterogeneous precipitation method with ammonia, was so fine crystal, that we could not observe the shape of mother salt maintained on the aggregate of UO2 prepared from this salt.