Jiaoqun Zhu

Synthesis of single-phase polycrystalline Ti3SiC2 and Ti3AlC2 by hot pressing with the assistance of metallic Al or Si

Abstract In this paper, aluminum and silicon were used as additives for the synthesis of Ti 3 SiC 2 and Ti 3 AlC 2 , respectively. An appropriate amount of these additives accelerated the synthesis reaction of Ti 3 SiC 2 and Ti 3 AlC 2 . Polycrystalline bulk samples of Ti 3 SiC 2 and Ti 3 AlC 2 were fabricated by hot pressing (HP) the starting mixtures of titanium carbide, titanium, silicon, and aluminum powders at a temperature of 1300–1400 °C under a pressure of 30 MPa. The prepared samples are predominantly single phase and fully dense. Hot pressed Ti 3 SiC 2 and Ti 3 AlC 2 grains are plane-like with average sizes of 5 and 15 μm in the longitudinal direction, respectively. The energy-dispersive spectroscope (EDS) results reveal that the atomic ratio of Si to Al in the sintered product is larger than that in the starting mixture.

Effect of aluminum on the reaction synthesis of ternary carbide Ti3SiC2

Abstract Addition of aluminum in the starting material considerably improves the single phase content in the synthesis of Ti 3 SiC 2 . Fully dense, essentially single-phase polycrystalline samples of Ti 3 SiC 2 were fabricated by spark plasma sintering 3Ti/1Si/0.2Al/2C at 1250 °C, or by hot pressing 2TiC/1Ti/1Si/0.2Al at 1300–1400 °C.

Synthesis of high-purity Ti3SiC2 and Ti3AlC2 by hot-pressing (HP)

Recently, both Ti3SiC2 and Ti3AlC2 had gained interest due to their unique combination of properties that make them candidates for many high temperature applications. They combine the merits of both metals and ceramics. Like metals they are thermally and electrically conductive, easy to machine with conventional tools, and resistant to thermal shock; like ceramics they have high strength, high melting point and thermal stability. Moreover, Ti3SiC2 and Ti3AlC2 are also damagetolerant materials [1]. The fabrication of single phase, bulk dense samples of both Ti3SiC2 and Ti3AlC2, has been proved to be very difficult. Recently, Barsoum and his co-workers have fabricated high-purity Ti3SiC2 and Ti3AlC2 polycrystals by hot isostatically pressing (HIP) a mixture of Ti, graphite and SiC powders, and Ti graphite and Al4C3 respectively [2, 3], however the process was very complex. Most recently, our early research revealed that appropriate addition of aluminum improved the synthesis of Ti3SiC2, And polycrystalline bulk Ti3SiC2 material with high-purity could be fabricated by spark plasma sintering (SPS) an elemental powder mixture with a starting composition of Ti: Si: Al: C = 3: 1: 0.2: 2 in molar ratio [4]. The objective of this work was to fabricate high purity Ti3SiC2 and Ti3AlC2 by hot pressing. The effect of raw materials ingredients on their synthesis was especially investigated. All of the work was conducted using powder mixtures of TiC (99.2% pure, 8.4 μm), Ti (99.0% pure, 10.6μm), Si (99.5% pure, 9.5μm), and Al (99.8% pure, 12.8 μm) (all from Institute of Non-Ferrous Metals, Beijing, China). In brief, the mixture with a designed composition was first mixed in ethanol for 24 h, then placed in a graphite die, 20 mm in diameter, and finally sintered in a hot pressing system. The samples were heated at a rate of 50 ◦C/min until the requisite temperature was reached; the soaking times were 2 h, and the pressure was 30 MPa. Shown in Fig. 1 are the X-ray diffraction patterns of samples obtained from the mixture of raw materials ingredients of 2TiC + 1Ti + 1.2Si (in molar). Sintered at the temperature range of 1200–1400 ◦C, the main phases were Ti3SiC2 and TiC. At 1200 ◦C, the peaks of Ti3SiC2 were very weak in contrast with those of TiC. The strengths of peaks of Ti3SiC2 reached the maximum levels at the sintering temperature of 1300 ◦C. Sintered at 1400 ◦C, peaks of Ti3SiC2 became weakened, while those of TiC became strong. The results indicated that it is difficult to synthesize high purity Ti3SiC2 from the mixture with the above materials ingredients. Fig. 2 shows the X-ray diffraction patterns of samples obtained from the mixture of raw materials ingredients of 2TiC + 1Ti + 1Si + 0.2Al (in molar), which means 0.2 molar of Si was substituted by the same amount of Al. Judging from the X-ray diffraction patterns, for sample sintered at 1200 ◦C, the product reached a high purity, only a very weak peak of TiC (2θ = 41.82 ◦) was identified by X-ray diffraction. For samples sintered at both 1300 ◦C and 1400 ◦C. The products were of pure Ti3SiC2, no phase but Ti3SiC2 was identified by X-ray diffraction. Shown in Fig. 3 are the X-ray diffraction patterns of samples obtained from the mixture of raw materials ingredients of 2TiC + 1Ti + 1Al + 0.2Si (in molar). The results revealed that high purity Ti3AlC2 could be synthesized by hot pressing at the temperature range of 1300–1500 ◦C from the mixture of the mentioned above raw materials ingredients. Fig. 4 shows the scanning electron micrographs of the fracture surfaces of Ti3SiC2 and Ti3AlC2 materials synthesized at 1400 ◦C. The grains of both Ti3SiC2 and Ti3AlC2 were plate-like. Ti3SiC2 grains have a size of 4–10 μm in elongated dimension, while Ti3AlC2 developed to larger grains with a size of 10–25 μm in the same direction. The densities of Ti3SiC2 and Ti3AlC2 materials prepared at 1400 ◦C were measured to be 4.43 and 4.19 g/cm3, which reached 97.8%, and 98.6% of their theory densities, respectively. It is concluded that polycrystalline bulk Ti3SiC2 and Ti3AlC2 materials with high purity and density

Synthesis of high-purity Ti2AlN ceramic by hot pressing

Abstract High-purity Ti 2 AlN ceramic was prepared at 1300 °C by hot pressing(HP) of Ti/Al/TiN powders in stoichiometric proportion. The sintered product was characterized using X-ray diffraction(XRD) and MDI Jade 5.0 software (Materials Data Inc, Liverpool, CA). Scanning electron microscopy(SEM) and electron probe micro-analysis(EPMA) coupled with energy-dispersive spectroscopy(EDS) were utilized to investigate the morphology characteristics. The results show that Ti 2 AlN phase is well-developed with a close and lamellar structure. The grains are plate-like with the size of 3–5 μm, thickness of 8–10 μm and elongated dimension. The density of Ti 2 AlN is measured to be 4.22 g/cm 3 , which reaches 97.9% of its theory value. The distribution of Ti 2 AlN grains is homogeneous.

Effect of phosphorus and fluorine on hydration process of tricalcium silicate and tricalcium aluminate

By means of hydration heat, XRD and SEM, effect of phosphorus and fluorine (P2O5 and F−) in phosphorous slag on hydration process of tricalcium silicate (C3S) and tricalcium aluminate (C3A) was explored. The results indicated that the early hydration exothermic rate of C3S and C3A was obviously lowered by P2O5 and F− in phosphorous slag, the second peak occurring time of C3A was delayed by 0.9 h, the exothermal output of C3S was reduced by 25.04% and the time of accelerating stage was postponed by 0.86 h. The early hydration degree of C3S and C3A was also decreased. Due to the infl uence of P2O5 and F−, more pores and thinner crystals can be observed in the microstructure of hardened paste and the chance of cracks was reduced.

Numerical simulation on heat transfer enhancement of phase change thermal storage devices for low-middle temperature

Using Ba(OH)2·8H2O as phase change material (PCM) and water as heat transfer fluid (HTF), we numerically simulated annular finned-tube heat exchangers. In order to measure and analyze the impact of parameters in the heating/cooling process, temperature changes of different monitoring points, fin widths, and fin pitches as key parameters were considered and applied. The experimental results show that the heat exchange process can be divided into three stages within a certain time. The faster heat transfer rate is associated with the greater temperature difference between PCM and HTF. Furthermore, fins width and pitch affect dramatically the heat charging/discharging rate. The large fins width or small fins pitch is beneficial for extending the heat exchange surface, leading to improve heat transfer efficiency.

Microstructure and Characterization of Capric-stearic Acid/Modified Expanded Vermiculite Thermal Storage Composites

In order to improve the thermal storage capacity of expanded vermiculite (EV) based form-stable composite PCM (FS-PCM) via organic modification of EV, first, EV was modified with a sodium stearate (NaSt) as surface modifier, and organic EV (OEV) with hydrophobicity and higher adsorption capacity for fatty acid was obtained. A novel capric-stearic acid eutectic (CA-SA)/OEV FS-PCM with high thermal storage capacity was then developed. OEV and CA-SA/OEV were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermal gravimetry (TG), and thermal cycling test. Results showed that OEV has obvious hydrophobicity and a higher adsorption capacity for fatty acid. Its adsorption ratio has increased by 48.71% compared with that of EV. CA-SA/OEV possesses high thermal storage density (112.52 J/g), suitable melting temperature (20.49 °C), good chemical compatibility, excellent thermal stability and reliability, indicating great application potential for building energy efficiency. Moreover, organic modification of inorganic matrix may offer novel options for improving its adsorption capacity for organic PCMs and increasing heat storage capacity of corresponding FS-PCMs.

In situ synthesis, mechanical and cyclic oxidation properties of Ti3AlC2/Al2O3 composites

ABSTRACT Ti3AlC2/Al2O3 composite materials were successfully fabricated from TiO2/TiC/Ti/Al powders by the in situ reactive hot pressed technique. The microstructure, mechanical and oxidation properties of the composites were investigated in the paper. Vickers hardness increased with the Al2O3 content. The relative density of Ti3AlC2/Al2O3 composites exhibits a declining tendency with Al2O3 content especially exceeds 10 vol. %. The Ti3AlC2/Al2O3 composites show excellent electrical conductivity. The flexural strength and fracture toughness of Ti3AlC2/10 vol. % Al2O3 are 461 ± 20 MPa and 6.2 ± 0.2 MPa m1/2, respectively. The cyclic oxidation behaviour of resistance of Ti3AlC2/10 vol. % Al2O3 composites at 800–1000°C generally obeys a parabolic law. The oxide scale of sample consists of a mass of α-Al2O3 and TiO2, forming a dense and adhesive protect layer. The result indicates that the Al2O3 can greatly improve the oxidation resistance of Ti3AlC2.

Fabrication of Al2O3-NaCl composite heat storage materials by one-step synthesis method

Thermal energy storage is an attractive option for effectiveness since it gives flexibility and reduces energy consumption and costs. New composite materials for storage and transformation of heat of NaCl-Al2O3 composite materials were synthesized by one-step synthesis method. The chemical composition, morphology, structure, and thermal properties were investigated by XRD, EDS, SEM, and DSC. The results show that NaCl can be absorbed by Al2O3 particle from 800 to 900 °C for Al2O3 particle surface is rich active structure. The results also indicate that the leakage of NaCl when the phase change can be prevented by Al2O3 particles and the enthalpy of phase change of NaCl-Al2O3 material is 362 J/g. The composites have an excellent heat storage capacity. Therefore, this study contributes to one new thought and method to prepare high temperature heat storage material and this material can be applied in future thermal engineering.