Jun Guo Li

Preparation of Coated ZrSiO4-ZrB2 Using Sol-Gel Method and its Oxidation Resistance

In this paper, coated ZrSiO4-ZrB2 powder was prepared by sol-gel method. The ZrSiO4 coating can improve the oxidation resistance of ZrB2 at high temperature. The effect of N (nZrSiO4/nZrB2), sintering temperature, holding time and catalytic hydrolysis condition of tetraethoxysilane (TEOS) on coated ZrSiO4-ZrB2 powder was discussed. The modern testing techniques such as XRD, SEM and TG-DSC were used to analyze the crystal phase compositions, microstructures and the oxidation resistance property. The results of experiments showed that the oxidation resistance of coated ZrSiO4-ZrB2 powder was excellent when N is 0.7, and TEOS is alkaline-catalyzed hydrolyzed.

Sintering Behavior of Fe-Si Composite Powder by DPR Technique

The reaction mechanism of silicon and iron composite powders was clarified during the fabrication of high silicon iron sheet with the Si-content of 6.5wt% by Direct Powder Rolling (DPR) technique. The changes of phase composition and structure evolvement were mainly studied. It is found that a local graded structure, Fe-Fe(Si)-Fe3Si(Si)-Si, forms when sintering at 950-1000oC, which plays an important role in the DPR process. Fe3Si(Si) phase keeps higher content of Si, and Fe(Si) phase remains the state with much lower Si-content, thus provides good mechanical proprieties of rolling and cutting. Then, the subsequent sintering at about 1200oC improves the density and makes the distribution of Si homogeneous in the final high silicon iron sheets.

Thermal Reaction and Phase Evolution of APP/Al(OH)3/α-SiO2

The thermal reaction and phase evolution of APP/Al (OH)3/α-SiO2 with different mass ratios during heating were studied by TG, FTIR, XRD and SEM, respectively. When the temperature is not higher than 300 oC, mass ratio of mAPP:mAl (OH)3:mSiO2 has no effect on the phase evolution of APP/Al (OH)3/α-SiO2 and the main interaction product is AlNH4HP3O10. APP/Al (OH)3/α-SiO2 with lower content of APP, appears larger weight loss rate due to the thermal decomposition of Al (OH)3. The thermal reaction of APP/Al (OH)3/α-SiO2 is significantly influenced by the APP content as temperature rises to higher than 300 oC. The decomposition products of APP can chemically interact with Al (OH)3 to generate Al2P6O18 and Al (PO3)3 during 600 oC ~900 °C. When the content of APP increases, much more APP can chemically react with Al (OH)3 and also with part of α-SiO2 to generate SiP2O7. SEM shows the relatively dense microstructure due to micro-bridges of liquid phase with phosphate content increasing.

Effect of Al and Si Additions on the Synthesis of Spark Plasma Sintered Zr2Al4C5 Creamic

The Zr2Al4C5 ceramic was successfully fabricated by the spark plasma sintering at 1800 °C for 10 min under uniaxial 20 MPa pressure in vacuum using a mixed raw materials of Zr, Al, Si and graphite powders. The X-ray diffraction analysis results showed that the unexpected Zr2Al3C5 phase rather than target compound Zr2Al4C5 formed in the sintered samples. An initial Zr:Al:C molar ratio of 2:4.2:4.8 for raw powders, and even 55 mol.% excess Al, did not lead to a phase transformation from Zr2Al3C5 to Zr2Al4C5. When 4 wt.% Si was induced in the starting powders, the major phase became Zr2Al4C5 and no obvious Zr2Al3C5 was detected in the sintered samples with an initial Zr:Al:C molar ratio of 2:6.2:4.8 (55 mol.% excess Al). The introduction of Si could suppress and even remove additional ZrC, and Si atoms would exclusively occupy the site of Al to make Zr2Al4C5 become a stable solid solution. The scanning electron microscopy observation showed that the as-synthesized Zr2Al4C5 grains had elongated, rod-like and/or plate-like shapes. The mechanical properties of the sintered Zr2Al4C5 ceramic were also investigated, and it showed a hardness of 11.06±0.34 GPa and a fracture toughness of 4.6 ± 0.4 MPa×m1/2.

Effect of High-Energy Ball Milling of ZrB2 Powder on the Microstructure and Mechanical Properties of ZrB2-SiC Ceramics

The sinterability of ZrB2-20vol.% SiC ceramics by high-energy ball milling as well as introduction of Zr and Al as sintering additives. Densification process and microstructure of ZrB2-SiC ceramics were investigated. After high-energy ball milling, the average particle size decreased to about 500 nm-2 μm, and ZrB2-SiC powder can be sintered to 98.92% theoretical density at 1800 °C, but a trace of amount of oxidation (ZrO2) were detected in sintered sample. Introduction of Zr, Al and C combined with high-energy ball milling enhanced the densification of ZrB2-SiC ceramics and reduced the particle sizes, and the relative density of obtained ceramic reached up to 99.49% at 1800 °C. The additions of Zr, Al and C can remove the oxide impurities of the surface of ZrB2 particles and form a reaction between oxide impurities. The fracture toughness increased of the 40% when ZrB2 powders were milled by high-energy ball milling, and increased to 4.77±0.18 MPa•m1/2. However, the attrition-milled composites had lower hardness and Young’s modulus, which was attributed to the presence of a second phase in the grain boundaries.

Effect of Heating Rate on the Synthesis and Sintering of Ternary Carbide Zr2Al3C4 through Spark Plasma Sintering

The Nearly Full Dense Zr2Al3C4 Ceramic Was Successfully Fabricated at 1800 °C for 10 min under a Uniaxial Load of 20 MPa in Vacuum by the Spark Plasma Sintering Process, Using a Mixture of Zr, Al and Graphite Powders as Raw Materials. The Reaction Route of Synthesis as Well as the Sintering Conditions of the SPS Technique Were Discussed Based on X-Ray Diffraction Results. The Results Showed that the Heating Rate Can Largely Affect the Loss and Aggregation of Molten Al. Moreover, the Contents of Al4O4C and the Elevated Sintering Temperature Were Beneficial for the Synthesis of Zr2Al3C4 Ceramic. The Microstructures of the Samples Were Observed by Scanning Electron Microscopy, Showing that the as-Synthesized Zr2Al3C4 Has an Anisotropic Microstructure Consisting of Elongated Grains. Compared to the Hot-Pressing, the Starting Temperature for the Formation of Zr3Al3C5 and Al4O4C Phases Was Rather Low. It Indicates that the SPS Technique Can Rapidly Synthesize Zr2Al3C4 from the Zr/Al/C Powders in a Relatively Low Temperature Range. The Mechanical Properties of the Sintered Materials Were Also Investigated, Including the Hardness of 11.66±0.34 GPa, and Fracture Toughness of 4.0 ± 0.4 MPa×m1/2.

Oxidation of ZrB2/ZrO2 and ZrB2/ZrO2/SiC Ceramics

The purpose of this study was to investigate the oxidation of ZrB2/ZrO2 (ZZ) and ZrB2/ZrO2/SiC (ZZS) ceramics. The ceramics were fabricated by spark plasma sintering (SPS) at 1900°C and exposed to ten-minute oxidation cycles in stagnant air at 1200°C in a box furnace with molybdenum disilicide heating elements. Results of relative density, surface phase change and the rate of weight growth show that the addition of ZrO2 improved the sintering properties of ZrB2 ceramics. While the resistance to oxidation declined with the increase content of ZrO2. And the addition of SiC improved the resistance to oxidation of ZrB2/ZrO2 composites significantly.

Preparation and Characterization of the Coated ZrB2 @ ZrO2 Ceramic

In this work, ZrB2 powder coated with ZrO2 (ZrB2 @ ZrO2) were applied to promote the densification of ZrB2 prepared by pulsed electric current sintering (PECS). The coating process and sintering behavior were investigated. While the pH value was about 9, the ZrB2 particles were successfully coated with ZrO2 by co-precipitation methods. The thickness of ZrO2 layer was about 3-5nm. The coated powder was sintered at 1600°C ~1950°C by PECS in vacuum. For comparison, the pure ZrB2 powder was sintered at the same conditions. The relative density of the sample coated with ZrO2 was 97.8% at PECS 1950°C, which was higher than that of sample without coating. The densification process can be divided into three stages. The ZrO2 coating layer plays an important role on the densification of ZrB2.

Compressive Strength and Phase Transformation of Zirconia Porous Ceramics at Different Sintering Temperatures

A series of zirconia porous ceramics with different density are fabricated with commercial zirconia powder and zirconia hollow balls by pressureless sintering technology. The microstructure and phase transformation are characterized respectively by SEM and XRD testing methods. The result indicates that the density and compressive strength depend greatly on zirconia powder content at the same sintering temperature, and elevating sintering temperature just has a little effect on the density and compressive strength for the samples of the same zirconia powder content. The XRD diffraction patterns analysis shows that elevating sintering temperature is helpful to eliminate monoclinic zirconia and the best sintering temperature should be beyond 1700°C.