Chun Lei Wan

Electro-Mechanical and Thermal Properties of Multiwalled Carbon Nanotube Reinforced Alumina Composites

Multi-walled carbon nanotube (MWNT) reinforced alumina composites with different MWNT contents (5 and 10 vol %) were fabricated by spark plasma sintering. The room temperature dc electrical conductivity, thermal conductivity, and mechanical properties were investigated. Results showed that the electrical conductivity has improved around twelve orders of magnitude by addition of 5 vol% of MWNT. The fracture toughness changed from 3.2 to 4.4 MPa m1/2 with 39% improvement over monolithic Al2O3. The thermal conductivity decreased with increase of MWNT contents. The low values of thermal conductivity suggest that interfacial thermal barrier play an important role in determining these properties. MWNT can be used to improve concurrently electrical, mechanical properties of Al2O3 but with lower values of thermal properties.

Structure and Ionic Conductivity of Ln2Zr2O7-Type Rare Earth Zirconates

Three rare earth zirconates (Sm2Zr2O7, Gd2Zr2O7 and Er2Zr2O7) were prepared by solid state reaction. The crystal structure and ionic conductivity of these zirconates were characterized by X-ray diffraction (XRD) and complex impedance spectroscopy. The results show that Sm2Zr2O7 exhibits single-phase pyrochlore structure and Er2Zr2O7 exhibits single-phase fluorite structure, while Gd2Zr2O7 has pyrochlore and fluorite structure. Among three zirconates, the ionic conductivity of Sm2Zr2O7 is highest, while that of Er2Zr2O7 is lowest.

Effects of Processing Parameters of SPS on the Densification and Texture of LaPO4 Ceramics

Spark plasma sintering was used to fabricate the LaPO4 ceramics and the effect of SPS holding time and sintering temperature on the densification and texture of LaPO4 ceramics were studied. The results revealed that holding time had no obvious influence on the densification of LaPO4 ceramics under the present process. The density increases with the increase of sintering temperature, when it reached 1350°C, the relative density kept nearly constant of 98.6 %. The preferred orientation of LaPO4 ceramics approximately increases with the increase of sintering temperature, but contrary impact in holding time.

Low Thermal Conductivity Ceramics for Thermal Barrier Coatings

This paper summarizes the basic properties of a series of rare-earth zirconate ceramics (Gd2Zr2O7, Sm2Zr2O7, Dy2Zr2O7, Er2Zr2O7 and Yb2Zr2O7). The phases and microstructures were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Thermal properties of these materials were determined. The results indicated that Sm2Zr2O7 rare-earth zirconate ceramics have the lower thermal conductivity and the highest thermal expansion coefficient than other rare-earth zirconate ceramics. The dielectric constant decreases with the increase of atomic number.

Thermal Expansion Coefficients of Sm2Zr2O7 Doped with MgO

Thermal expansion coefficient (TEC) is one of the most concerned properties for thermal barrier coating (TBC) materials. To enhance thermal expansion coefficient, a series of rare-earth zirconates Sm2Zr2O7 with various contents of MgO were investigated. The samples were synthesized by solid state reaction at 1600°C for 10h. And the thermal expansion coefficients of the samples were measured. The results showed that all the doping samples had higher expansion coefficients than pure Sm2Zr2O7. Moreover, compared with the data reported of 8YSZ, ~15% enhancement had been achieved above 300°C.

Thermophysical Properties of Samarium-Cerium Oxide for Thermal Barrier Coatings Application

Sm0.4Ce0.6O1.8 specimen with a defective fluorite structure was synthesized and its thermophysical properties were characterized for thermal barrier coatings (TBCs) application. At high temperature, Sm0.4Ce0.6O1.8 exhibited much lower thermal conductivity than 7wt% yttria-stabilized zirconia (7YSZ)-the commonly used composition in current TBCs. Sm0.4Ce0.6O1.8 also possessed large thermal expansion coefficient, which could help reduce the thermal mismatch between the ceramic coating and bond coat.

Degassing Behavior and Solid State Reaction of Nano-Ti(CN) Base Cermets in Sintering

The chemical composition and solid state reaction of the nano-Ti(CN) base cermets in different sintering temperature were studied. The total carbon and oxygen content in compact were declined gradually with the increasing of sintering temperature, the nitrogen content in compact began to decline above 1100°C, the peak of de-gassing of N2 was formed before the emergence of liquid phase, the decomposition of N2 was arisen acutely above 1500°C. Mo2C and TaC diffused and took part in solid state reaction with Ti(CN) above 900°C, the solid state reaction was finished below 1200°C. WC diffused and took part in solid state reaction with Ti(CN) above 1100°C, it was dissolved below 1250°C, there were only two phases, Ti(CN) and Ni(Ni+Co), in the alloy.

A Promising LaSmZr2O7 Ceramic with Pyrochlore Structure for Thermal Barrier Coatings

Based on La2Zr2O7 ceramic for thermal barrier coatings, LaSmZr2O7 ceramic doped with samarium ion was successfully prepared using solid state reaction method. The pellets were sintered at 1600°C for 10 hours in air. The phase structure, thermal conductivity and thermal expansion coefficient of LaSmZr2O7 ceramic and La2Zr2O7 ceramic were measured by XRD, laser-flash device, high-temperature dilatometry, respectively. The results show that the crystal structure of LaSmZr2O7 ceramic is not affected by the doped samarium ion in the zirconium lattice. The thermophysical results show that the thermal conductivity of the LaSmZr2O7 ceramic is lower than that of La2Zr2O7 ceramic, while the thermal expansion coefficient is higher than that of La2Zr2O7 ceramic. These results indicate that LaSmZr2O7 ceramic or Ln2Zr2O7 ceramics doped with other rare earth ions could be candidate materials for future thermal barrier coatings.

A New Homogeneous Precipitation Route to Ammonium Aluminum Carbonate Hydroxide

A new precipitation route to ammonium aluminum carbonate hydroxide (AACH) was presented. There was an incubation period before AACH precipitated. Effects of synthesis parameters such as pH of ammonium hydrogen carbonate solution, precipitation temperature, concentration of aluminum citrate and type of anions on the precipitation of ammonium aluminum carbonate hydroxide were discussed. It was found that the incubation period decreased with the increasing pH of AHC solution, concentration of aluminum citrate and the decreasing reaction temperature. Compared to NO3 -, SO4 2- delayed the formation of AACH and enlarged the particle size of AACH.

Processing and Thermal Properties of Multiwall Carbon Nanotube/Bismuth Antimony Telluride Composites for Nuclear Energy Applications

In this work the effects of ball milling and carbon nanotubes incorporation on the thermal conductivities of the bulk BiSbTe composites were evaluated. The coarse BiSbTe particles were obtained by crushing BiSbTe lumps and subsequently high energy ball milling was employed in an inert environment to form the fine BiSbTe powder. Multiwall carbon nanotubes in different (0.0, 0.5 and 1.5) vol. % were uniformly mixed in the BiSbTe powder through a combination of ultra-sonication and ball milling, and then processed by rapid high frequency induction heated sintering (HFIHS) to achieve fully dense nanocomposite. Thermal diffusivity of the composites was evaluated and heat capacity was approximated using Pyrocerarm as a reference material. The effect of carbon nanotubes inclusion and BiSbTe particle size reduction on the thermal conductivity was studied from 300 to 500 K. The results show a significant reduction in the thermal conductivity due to the enhanced thermal boundary interface resistance correlated with the fine microstructure/nanostructure in the composites as compared to pristine bulk bismuth antimony telluride.