Bingchu Mei

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.

Synthesis of Ti2AlC by hot pressing and its mechanical and electrical properties

Abstract Ti2AlC bulk material was synthesized by hot pressing of mixture powders of TiC, Ti, Al and active carbon. The phase compositions of resultant product at different temperature were detected by X-ray diffractometer. The microstructures of the samples were observed by SEM. Finally, the mechanical properties and thermal properties of the sample at 1400 °C were measured. The results show that high purity Ti2AlC material with little Ti3AlC2 can be synthesized by hot pressing 0.5TiC/1.5Ti/1.0A1/0.5C at 1400 °C. Ti2AlC exhibits high mechanical properties and metallic electrical properties. Its fracture toughness is 7.0 MPa·m½, its flexural strength is 384 MPa at room temperature, and its electrical conductivity is 2.56 × 106 Ω−1·m−1 at room temperature.

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.

Dual-wavelength continuous-wave and passively Q-switched Nd,Y:SrF2 ceramic laser

Abstract. A polycrystalline ceramic based on an Nd,Y:SrF2 single crystal was successfully fabricated and its laser performance was experimentally investigated. We obtained dual-wavelength continuous-wave operation at the wavelengths of 1050.5 and 1058.0 nm. The maximum output power and slope efficiency were 750 mW and 31.5%, respectively. In the passively Q-switched operation, the shortest pulse with a 169-ns duration was also obtained, and the corresponding maximum repetition rate and single pulse energy were 7.3 kHz and 19.2  μJ, respectively.

Fabrication of highly-transparent Er:CaF2 ceramics by hot-pressing technique

Highly-transparent trivalent erbium ion doped calcium fluoride (5 mol % Er:CaF2) ceramics were fabricated by a hotpressing (HP) method using high-purity Er:CaF2 nanoparticles, which were synthesized by co-precipitation method. The mean grain size of the nanoparticles was about 24.7 nm. The nanoparticles were sintered at 600 °C, 700 °C, 800 °C and 900 °C, respectively, for 30 min under a uniaxial pressure of 30 MPa and vacuum of 10−3 Pa with 1 mol % lithium fluoride (LiF) as sintering additive. The 5 mol % Er:CaF2 ceramics sintered at 800 °C exhibits high density and pore-free microstructure with an average grain size of about 8 μm. The optical transmittance of the transparent ceramics is close to 85 % at visible and nearinfrared wavelengths. The strong and broad absorptions peaks corresponding to characteristic absorption of trivalent erbium ions make the ceramics a potential candidate for infrared and upconversion laser operating.

Synthesis and Characterization of CaF2 Nanoparticles with Different Doping Concentrations of Er3

The calcium fluoride nanoparticles with a variety of doping amounts of erbium ions were prepared by CTAB/C4H9OH/C7H16/H2O reverse micro-emulsion method. The nanoparticles were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR), absorption and fluorescence spectra. The XRD patterns indicated a typical cubic fluorite structure and no other impurities. The TEM results showed one kind of particles with uniform grain size and without agglomeration were synthesized. The FTIR spectra revealed that there were some amounts of -OH, NO3− and organic groups absorbed on the surfaces of the particles before being annealed. The absorption spectra presented many absorption peaks and bands corresponding to the rich energy levels of erbium ion. The Red-Shift of absorption bands and Blue-Shift of fluorescence peak could attribute to the weakened energy level split as a result of the decrease in crystal field strength.

Preparation of nanometer Nd3+, Y3+ co-doped CaF2 powder by coprecipitation-azeotropic distillation technique

The synthesis of Nd3+, Y3+:CaF2 nanopowder was conducted by azeotropic distillation method, which effectively dehydrated hydrous CaF2 and prevented forming hard agglomerates. X-ray diffraction (XRD), scanning electron microscopy (SEM), scanning calorimetries-thermalgravimetry (DSC-TG), Fourier transform infrared spectroscopy (FT-IR) and absorption spectroscopy were performed to characterize the powder properties. The experimental results showed that products obtained by azeotropic distillation were single phased, rather monodispersed, successfully prevented the hard agglomerate formation and effectively removed the residual water inside the as-prepared precipitate than that of the direct drying. The absorption spectra showed a wider and stronger absorption bands around 792 nm, which should be profitable for LD pumping.

Effects of Gd3+ on the photoluminescence properties of Nd3+-doped SrF2 crystal

The effect of doping concentration of Nd3+ and co-doping Y3+ on the spectroscopic properties are investigated systematically. Due to the particular clustering effect, the quench effect was demonstrated in lightly doped NdxGd0.03Sr0.97-xF2.03+x (x=0.0005,0.0015,0.0065,0.01) crystals. For a 3% Gd:SrF2 crystal, the fluorescence lifetime at 1054 nm decrease from 380.9 to 159.8 μs by doping Nd3+ from 0.15 at.% to 1 %at.%, while the emission cross section decreases to 4.12 × 10−20cm2 at 1054 nm. However, the absorb cross section were increased when the concentration of Nd3+ increase from 0.5 % to 0.65 %. Thus, there is an optimum doping concentration of Nd3+. According to the research, the optimum doping concentration of Nd3+ is 0.15 % in 3% Gd:SrF2 crystals.