Jing Kun Guo

Fabrication and Dielectric Properties of CNTs/SiO2 Composites

Stable and disperse carbon nanotubes (CNTs) aqueous solution was prepared by using cetyltrimethylammonium bromide (C16TMAB) as dispersing agent and zeta potential of CNTs solution were measured to quantitatively characterize the stability of CNTs in C16TMAB aqueous solution. The CNTs /SiO2 composites were fabricated with composite powders synthesized by rapid Sol-gel method and sintered by hot-pressing technique. The electric and dielectric properties of the composites were measured and the microstructure of CNTs and composites were investigated by Field emission scanning electron microscope (FSEM) and transmission electron microscopy (TEM). The mechanisms had been investigated to explain the effect of the CNT compositions on the electric and dielectric properties of CNTs /SiO2 composites in this paper.

Low-Temperature Synthesis of Mono-Phase Ultrafine Yttrium Aluminum Garnet by Sol-Gel Combustion Process

Ultrafine yttrium aluminum garnet (Y3Al5O12, YAG) powders were synthesized by sol-gel combustion process from a mixed solution containing aluminum and yttrium nitrates. The precursor and powders were characterized using X-ray diffraction, thermogravimetry-differential scanning calorimetry-mass spectrometry, and transmission electronic microscope. Mono-phase cubic powders were obtained at 900oC by directly crystallizing from amorphous materials and no intermediate phase was observed. The powders processed at 900oC were uniform and in good dispersity with average particle size of about 20 nm.

Preparation of Al2O3-ZrO2-Ni Nano-Composites by Spark Plasma Sintering

In the present study, a process to prepare Al2O3-ZrO2-Ni nano-composite is explored. The nano-sized nickel particles, around 1-10 nm, can be successfully prepared by using a solution and coating technique. These nano-sized Ni particles disperse uniformly onto the surface of micro-sized Al2O3 and ZrO2 particles after the coating treatment. The powder mixtures were then consolidated by employing a spark plasma sintering (SPS) technique at 1350C for 1 to 5 minutes. Dense composite is resulted after SPS, though a slight density variation within the specimen is also noted.

A Method of Preparation of TiB2 Nano-Powders and its Mechanism

A new kind of method was investigated to prepare TiB 2 nano-powder. By mechanical alloying and acid treatment, homogenous TiB 2 nano-powders of 50nm in diameter are prepared. The mechanism by such treatment was also investigated. Two fact rs were believed to predominantly influence the formation of TiB 2 nano-powder, one is the hydrogen adsorption of TiB2 and the other is its layer structure. Introduction Mechanical alloying, or ball milling, is a kind of method to get fine grain powders [1-2]. The milled powders are mixed with balls, whose materials are WC-Co or iron. High energy can be transferred to milled powders by hitting and cracking between balls. The powder s sizes are decreased gradually with increasing milling time. Many materials systems get f ine powders by this way [3-4]. But generally speaking, it can not get homogeneous powders. Long time milling will cause another problem, e.g.,the second phase or un-wanted elements contamination appears in milled powders because of some balls wasti ng way. In order to diminish the influence of contaminating of WC, acid treatment is needed in indus try [5]. But when we treat TiB2 powders with ball milling and acid treatment to get fine grain powders, the homogeneous TiB 2 nano-powders are got and is beyond our predication. So the preparation met hod and its mechanism are discussed in this paper. Experimental TiB2 powders were milled by WC-Co ball for 40 hours, the ratio of ball to TiB2 powders is 20 to 1. The milled powders were treated by acid mixture (HNO 3: H3PO4: H2O=1:1:5). Fine TiB2 powders were got after washing and baking the acid treated powders. The powder phases and morphology were analyzed by XRD and TEM. C and H contents in TiB 2 powders were measured by combustion method with equipment of Carlo Erba 1106 Element Analyzer. In order to diminish the influence of absorbed moisture, the powders had been treated at 150 C in vacuum for 2h. Results and Discussion Structure and morphology of TiB 2 powder with and without acid treating. TEM results (as shown in Fig.1) indicate that the sizes of TiB 2 powders without acid treating are 0.1~0.3um, but they are decreased to about 50nm with acid treating, and distribute homogeneously. Fig.2 is XRD analysis of TiB 2 powders with and without acid treating. The result indicates that WC phase exists in TiB 2 powders without acid treating. This was introduced by milling with WC ball. The aim of acid treatment is to decompose WC, with reaction as [5]: 3WC+10HNO3 3H2WO4 +10NO +3CO2+2H2O 12 H2WO4+H3PO4 H3P(W3O10)4+12H2O WC phase disappeared in TiB 2 powder with acid treating, as shown in Fig.2. But the result that TiB2 powders with acid treating decrease homogeneously into nano-powders is beyond our predication. Further effort was needed to investigate why TiB 2 nano-powders were formed by such Key Engineering Materials Online: 2003-09-15 ISSN: 1662-9795, Vol. 249, pp 119-122 doi:10.4028/www.scientific.net/KEM.249.119

Room and Elevated Temperature Mechanical Properties of Hot-Pressed Uni-Cf/SiO2 Composite

Unidirectional carbon fiber reinforced fused silica (uni-Cf/SiO2) composite was prepared by slurry infiltration and hot-pressing. The room and elevated temperatures flexural properties were investigated and the fracture features of the composite were observed. This composite exhibited non-catastrophic failure at room and elevated temperatures. The oxidation of carbon fiber at elevated temperatures was the main reason for the degradation of flexural strength and elastic modulus. The flexural strength tested at 1200 was 376MPa and exhibited anomalously higher than that at 1000 (277MPa), which was attributed to the viscous flow of fused silica matrix and therefore the occurrence of microcracking in the matrix was deferred. And it was inferred that the brittle to plastic transition temperature (Tb-p) of uni-Cf/SiO2 composite corresponded to a certain temperature around 1200°C.

Strengthening Zirconia by Adding Both Nickel and Alumina Particles

In the present study, the processing-properties relationships of the ZrO2/(Ni+Al2O3) composites are examined. Dense composites were prepared either by pulse electrical current sintering (PECS) at 1350C for 5 minutes or by pressureless sintering (PLS) at 1600C for 1 h. The size of Ni particles is as small as 30 nm to 50 nm. Though the size of ZrO2 grains in the matrix increases as alumina and nickel particles are added, the strength of the ZrO2/(Ni+Al2O3) composites is significantly higher than that of monolithic ZrO2.

The Microstructure, Mechanical and Dielectric Properties of CNTs/Mullite Ceramics Composites

Carbon nanotube-mullite (Al2O3/SiO2=3/2) composites have been prepared by hot-pressing the corresponding composite powders, in which the multi-walled carbon nanotubes(1~10 vol%) are homogeneously dispersed between the mullite grains. The microstructure of the specimens has been studied and discussed in relation to the properties of the matrix, the bending strength and the fracture toughness, the dielectric constant and the dissipative factor. Carbon nanotube-mullite composites are potential electromagnetic wave absorbers owing to the percolation of the carbon nanotubes.

Spheroid Growth during Sintering of Copper Coated Silicon Carbide Particles in the Fabrication of Nanocomposite

Copper coated silicon carbide clusters were used to fabricate nanocomposite. Compacts were isostatically pressed and heated in nitrogen atmosphere. Microstructure observations were carried out to show the spheroid growth of the coated clusters. Spheroid growth was found to proceed through coalescence of smaller spheroids of the coated Cu/SiC composite particles. The densification process contains steps of agglomeration – kernel-shell formation – slumping movement. Grain growth of the adherent Cu particles is suppressed due to the constraint of rigid SiC particles. This is the deterministic characteristic of the coated composite particles.

Nanocomposite YSZ-Ni Based Cermet Powders Synthesized by Complexes-Gel-Conversion Process

Nanocomposite YSZ-Ni based cermet powders were fabricated by E DTA complexes-gelreduction process (CGRP). The sample with molar ratio of Zr : Y : Ni = 100 : 4 : 30 was with reduction temperature of 900 C. Samples with molar ratio of Zr : Y : Ni = 100 : 4 : 10 were with reduction temperature of 900 C and 850C. And samples with molar ratio of Zr : Y : Ni : Me = 100 : 4 : 30 : 30 were with reduction temperature of 850 C. The identification of phases was carried out using an XRD and crystallite sizes of powders were calcu l ted from the line-broadening of 2 ) 111 ( ZrO peak by Scherrer-formula.

Improvement in thermal expansion of Ca-PSZ refractories

Abstract Ca–PSZ refractories were prepared from fused CaO–ZrO2 powder and characterised with respect to thermal expansion and phase composition. By modifying the stabilisation of ZrO2 , the thermal expansion properties of the material were shown to have improved by the formation of a ‘plateau’ in the thermal expansion curve on cooling over the 500–800°C temperature range. Consequently, a Ca–PSZ refractory with better thermal shock resistance was obtained without adverse effect on the material density. Corrosion resistance should, therefore, also be maintained.