Guo-Dong Zhan

Electrical properties of nanoceramics reinforced with ropes of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWCNTs) were used to convert insulating nanoceramics to metallically conductive composites. Dense SWCNT/Al2O3 nanocomposites with CNT contents ranging from 5.7 to 15 vol % and with nanocrystalline alumina matrices have been fabricated by spark-plasma-sintering that retains the integrity of SWCNT in the matrix. The conductivity of these composites increases with increasing content of CNTs. The conductivity has been increased to 3345 S/m in the 15 vol % SWCNT/Al2O3 nanocomposite at room temperature. This is an increase of 13 orders of magnitude over pure alumina and of more than 735% over previously reported results in CNT–ceramic composites.

Alumina-based nanocomposites consolidated by spark plasma sintering

Abstract Spark plasma sintering is a new process by which ceramics and composites can be consolidated very rapidly to full density. In the present study, piezoelectric Nd 2 Ti 2 O 7 second phase toughening nanocrystalline alumina composites with higher toughness were successfully developed at relatively low temperatures through this technique.

Mechanism of grain growth in liquid-phase-sintered β–SiC

The mechanism for grain growth of β–SiC was investigated by annealing hot-pressed β–SiC–oxynitride glass (Y–Mg–Si–Al–O–N) ceramics at 1800 °C. An observed decrease in grain growth with increasing weight fraction of liquid confirms a diffusion-controlled growth mechanism in the system. The growth of nearly spherical β–SiC grains in the annealed specimen also supports the above conclusion.

Anisotropic thermal properties of single-wall-carbon- nanotube-reinforced nanoceramics

Dense single-wall-carbon-nanotube-(SWCNT)-reinforced alumina nanocomposites have been fabricated by a novel spark-plasma-sintering technique. Anisotropic thermal properties have been found in carbon nanotube composites. The introduction of ropes of SWCNTs gives rise to a decrease in the transverse thermal diffusivity with increasing carbon nanotube content while it does not change the in-plane thermal diffusivity. This is scientifically interesting and technologically important for the development of materials for novel thermal barrier coatings.

Superplastic behavior of liquid-phase sintered β-SiC prepared with oxynitride glasses in an N2 atmosphere

Abstract The superplastic behavior of liquid-phase sintered β-SiC with oxynitride glasses was compared in compression and tension tests at 1973–2048 K in an N2 atmosphere. Strain hardening was observed under all experimental conditions. Stress exponents in the compression tests of SiC with different additives were about 2 in the temperature ranging from 1973 to 2023 K. Amorphous phases were observed at the grain boundaries and at multi-grain junctions in the as-sintered materials. Superplastic behavior was greatly influenced by grain growth, vaporization of grain-boundary phase and formation of crystalline phase during tensile deformation.

Effects of heat treatment and sintering additives on thermal conductivity and electrical resistivity in fine-grained SiC ceramics

The effects of heat treatment and sintering additives on the thermal conductivity and electrical resistivity of fine-grained SiC materials were investigated. The thermal conductivity and the electrical resistivity of dense SiC materials were measured at room temperature by a laser flash technique and a current-voltage method, respectively. The results indicated that the thermal conductivity and electrical resistivity of the SiC materials were dependent on the sintering additives and the resultant microstructure. Annealed materials with oxide additives developed microstructures consisting of elongated grains of various α/β-SiC polytypes. In contrast, annealed materials with oxynitride additives had microstructures consisting of fine equiaxed grains entirely of β-SiC phase. For the annealed materials with oxide additives the observed thermal conductivity was over 110 W/mK. For the annealed materials with oxynitride additives the observed value was 47 W/mK. The electrical resistivity of a hot-pressed material with oxide sintering additives decreased after annealing. For annealed materials with oxynitride additives, the electrical resistivity was even lower. High-resolution electron microscopy revealed a thin amorphous phase along the grain boundaries. Energy dispersive x-ray spectroscopy results showed that there was segregation of both Al and O atoms and a very small amount of Y atoms at grain boundaries. The results indicated that the chemistry and structure of the grain boundary has significant influence on thermal and electrical properties in SiC.

Tensile Ductility of Liquid-Phase Sintered β-Silicon Carbide at Elevated Temperature

Nano-grain sized β-SiC was prepared with various additives by hot-pressing in Ar. The relative bulk densities were more than 95 %. As-sintered bodies were composed of equiaxed grains. Amorphous phase existed at grain boundaries in each as-sintered material. Tension tests were performed at the initial strain rates from 3 X 10 -4 s -1 to 3 X 10 -5 S * at 1973 - 2048 K. Tensile elongation more than 60 % was obtained. In deformed specimens, grain growth was observed. However, anisotropy of grain growth was not observed. The grain-boundary phase was vaporized during tension tests. The β phase of SiC was stable even after tension test. Therefore, critical deformation mechanism was thought to be grain-boundary sliding. Deformation behavior was influenced by the grain-boundary phase.

Effect of atmosphere on superplastic deformation behavior in nanocrystalline liquid-phase-sintered silicon carbide with Al2O3-Y2O3 additions

Nanocrystalline β-SiC with additions of 5.135 wt% Al2O3 and 3.867 wt% Y2O3 was subjected to tensile deformation in order to study its microstructural behavior under the dynamic process. The liquid-phase-sintered body had a relative density of >95% and an average grain size of 190 nm. Tension tests were conducted at initial strain rates range from 3 × 10−4 to 2 × 10−5 s−1, in the temperature range 1873–2048 K, in both argon and N2 atmospheres. Although grain-boundary liquids formed by the additions vaporized concurrently with the decomposition of SiC and grain growth, the maximum tensile elongation of 60% was achieved in argon. The grain-boundary amorphous phase formed a crystalline phase during testing in an N2 atmosphere and fracture occurred at <8% elongation. Grain-boundary sliding was still the dominant mechanism for deformation.

Preferred orientation of beta-phase and its mechanisms in a fine-grained silicon-nitride-based ceramic

A quantitative texture analysis, including calculations of the orientation distribution function, is applied to investigate the preferred orientation of β–Si 3 N 4 in a fine-grained material containing almost equiaxed grains that has been hot-pressed, annealed, and plane-strain compressed. The results show that (i) plane strain compression can produce relatively strong textures that were dependent on the compressive strain; (ii) the basal plane of hexagonal β–Si 3 N 4 was normal to the hot-pressing direction for the hot-pressed and annealed samples, whereas it was parallel to the stress axis for deformed samples; and (iii) the mechanisms for texture development were preferred grain growth for the annealed sample and grain rotation for the hot-pressed and deformed samples, respectively.

Improvement of Mechanical Properties after Superplastic Deformation of Silicon Nitride

Fine and uniform powder of β-silicon nitride was liquid phase sintered by hot-pressing at 1700 °C. Deformation behavior was investigated under compressive stress at 1500 - 1600 C. The material deformed at a strain rate, 1-7 x 10 -4 /s, under a pressure of 10-100 MPa at 1550 °C. Strain hardening was not observed even at slow deformation rate because of the absence of appreciable dy namic grain growth. The strength and fracture toughness of as-hot-pressed materials was 793 MPa and 2. 8 MPa.m 1/2 , respectively. The properties were improved by the deformation to 912 MPa and 3.5 MPa.m 1/2 . respectively. Further improvement was achieved by the annealing at 1700 C for 30 min. The bending strength and fracture toughness increased to 982 MPa and 3.7 MPa.m 1/2 , respectively, by slight grain growth. Present work provides a process to produce complex-shaped components by superplastic deformation at low temperatures and to improve the mechanical properties by further heat treatment.