Chunfeng Hu

Toughened and machinable glass matrix composites reinforced with graphene and graphene-oxide nano platelets

Abstract The processing conditions for preparing well dispersed silica–graphene nanoplatelets and silica–graphene oxide nanoplatelets (GONP) composites were optimized using powder and colloidal processing routes. Fully dense silica–GONP composites with up to 2.5 vol% loading were consolidated using spark plasma sintering. The GONP aligned perpendicularly to the applied pressure during sintering. The fracture toughness of the composites increased linearly with increasing concentration of GONP and reached a value of ∼0.9 MPa m1/2 for 2.5 vol% loading. Various toughening mechanisms including GONP necking, GONP pull-out, crack bridging, crack deflection and crack branching were observed. GONP decreased the hardness and brittleness index (BI) of the composites by ∼30 and ∼50% respectively. The decrease in BI makes silica–GONP composites machinable compared to pure silica. When compared to silica–Carbon nanotube composites, silica–GONP composites show better process-ability and enhanced mechanical properties.

Physical and mechanical properties of highly textured polycrystalline Nb4AlC3 ceramic

Abstract Highly textured polycrystalline Nb4AlC3 ceramic was fabricated by slip casting in a strong magnetic field followed by spark plasma sintering. Its Lotgering orientation factor was determined on the textured top and side surfaces as f(00l) ∼1.0 and f(hk0)=0.36, respectively. This ceramic showed layered microstructure at the scales ranging from nanometers to millimeters. The as-prepared ceramic had excellent anisotropic physical properties. Along the c-axis direction, it showed higher hardness, bending strength, and fracture toughness of 7.0 GPa, 881 MPa and 14.1 MPa m1/2, respectively, whereas higher values of electrical conductivity (0.81×106 Ω−1 m−1), thermal conductivity (21.20 W m−1 K−1) and Young’s modulus (365 GPa) were obtained along the a- or b-axis direction.

Enhanced thermoelectric performance of porous magnesium tin silicide prepared using pressure-less spark plasma sintering

Magnesium tin silicide based thermoelectrics contain earth abundant and non-toxic elements, and have the potential to replace established commercial thermoelectrics for energy conversion applications. In this work, porosity was used as a means to improve their thermoelectric properties. Compared to dense samples of Sb doped Mg2Si0.5Sn0.5 with a maximum zT of 1.39 at 663 K, porous samples (37% porosity) prepared by a pressure-less spark plasma sintering technique showed significantly lower thermal conductivity and higher Seebeck coefficient, resulting in an increased maximum zT of 1.63 at 615 K. The possible origins of the enhanced Seebeck coefficient can be attributed to a change of carrier concentration and modification of the band structure, produced by microstructural engineering of the surface composition and particle–particle contacts.

In-situ reaction synthesis and decomposition of Ta2AlC

Abstract Dense bulk Ta2AlC ceramic was fabricated by in-situ reaction/hot pressing of Ta, Al and C powders. The reaction path and effects of initial composition on the purity were investigated. It was found that Ta2AlC formed through the reactions between AlTa2 and graphite, or between Ta5Al3C, TaC and graphite at 1500–1550°C. By modifying the molar ratio of the initial Ta, Al, and C powders, single-phase Ta2AlC was prepared at 1550°C under an Ar atmosphere with an optimized composition of Ta: Al: C = 2: 1.2: 0.9. The lattice parameter and a new set of X-ray diffraction data of Ta2AlC were obtained. In addition, Ta2AlC was reported unstable above 1600°C and decomposed to Ta4AlC3, and then to TaCx.

Preparation of nanocrystalline-coated carbon nanotube/Ni0.5Zn0.5Fe2O4 composite with excellent electromagnetic property as microwave absorber

A combined precipitation-hydrothermal method was used to fabricate carbon nanotube/Ni0.5Zn0.5Fe2O4 ferrite composite powders. The phase, microstructure and electromagnetic properties of CNT/NiNi0.5Zn0.5Fe2O4 composites were investigated. After surface modification, The zeta potential value of CNTs could maintain at about -50mV when pH is higher than 8, which affords a suitable surface environment for in situ coating of NiNi0.5Zn0.5Fe2O4 nanocrystallines. With increasing CNTs content, the saturation magnetization of the composites is gradually reduced, while the complex magnetic permeability changes little. The complex dielectric constant of the composites is significantly increased when the concentration of CNTs approaches the percolation threshold value of 2 wt%. When CNTs content is 5 wt%, the reflection ratios are less than -10 dB within the frequency range 2-9 GHz, and the reflection ratios reach a minimum -32.5 dB at a frequency of about 3.9 GHz.

Developments in hot pressing (HP) and hot isostatic pressing (HIP) of ceramic matrix composites

Abstract: This chapter introduces the hot pressing (HP) and hot isostatic pressing (HIP) methods and their application to produce advanced ceramics. Typical oxides, carbides, borides and nitrides are selected to describe the synthesis process. The conclusion is that HP and HIP are good technologies for fabricating dense or single-phase ceramics. Their advantages, disadvantages and future improvements are discussed.

Preparation of Graphene Nanosheets/Copper Composite by Spark Plasma Sintering

Graphene nanosheets (GNS)/copper composite has been prepared by spark plasma sintering (SPS). Microstructure of the sintered composite was characterized using scanning electron microscopy (SEM). Thermal conductivity and electrical resistivity properties are evaluated through laser flash apparatus and physical property measurement system. The obtained GNS/copper composite shows a layered structure. The GNS/copper composite prepared by SPS have an anisotropy of thermal conductivity and electrical resistivity.

Synthesis and Physical Properties of Graphene Nanosheets Reinforced Copper Composites

Cu/graphene nanosheets composites were fabricated at 800°C by the hot-pressing method using Cu and graphene as initial materials. Graphene content was 1 wt. %-5 wt. %. The fracture morphology and physical properties of the composites were investigated. It was found that the relative density increased with the increment of graphene content from 1 wt% to 5 wt. % with reaching its highest level (96.68%) at 5wt. %. The composites have the anisotropic property which is vertical to the direction of pressure is higher than parallel to the direction of pressure. With the increasing of graphene content, the thermal conductivity property and the electronic conductivity decrease first and then increase with the minimum thermal conductivity and electric conductivity at 3wt%~4wt%.

Interfacial Reactions between Polymer Derived SiC Fiber and Ti3Si(Al)C2

In this paper, interfacial reactions between polymer derived SiC fiber and Ti3Si(Al)C2 were analyzed. Strong interfacial reactions occurred in 20 vol.% SiCf/Ti3Si(Al)C2 composite during the vacuum spark plasma sintering at 1350 °C. At the begining, the Ti3Si(Al)C2 suffered spontaneous decomposition and resulted in the sublimation of gaseous Ti, Si and Al, and de-intercalation of TiC from the crystal structure, when it was combined with SiC fiber during sintering. Then, Ti reacted with the SiC phase and turbostratic graphite in the fiber and TiC was formed, and also Al was oxidized into Al2O3. Meanwhile, most of gaseous Si released from Ti3Si(Al)C2 escaped from the outer surfaces of the composite under the compressive stress of 30 MPa and the vacuum environment, the rest Si was cooled into solid state following gas-solid reaction mechanism.

Spark plasma sintering of damage tolerant and machinable YAM ceramics

Single-phase Y4Al2O9 (YAM) powders were synthesized via solid-state reaction starting from nano-sized Al2O3 and Y2O3. Fully dense (99.5%) bulk YAM ceramics were consolidated by spark plasma sintering (SPS) at 1800 °C. We demonstrated the excellent damage tolerance and good machinability of YAM ceramics. Such properties are attributed to the easy slipping along the weakly bonded crystallographic planes, resulting in multiple energy dissipation mechanisms such as transgranular fracture, shear slipping and localized grain crushing.