Jiaqi Hong

Bending strength and microstructure of Al2O3 ceramics densified by spark plasma sintering

Al2O3 ceramics were superfast densified using spark plasma sintering (SPS) by heating to a sintering temperature between 1350 and 1700°C at a heating rate of 600°C/min, without holding time, and then fast cooling to 600°C within 3 min. High-density Al2O3 ceramics could be achieved at lower sintering temperatures by SPS, as compared with that by conventional pressureless sintering (PLS). The bending strength of Al2O3 superfast densified by SPS in the range of sintering temperature between 1400 and 1550°C reached values as high as 800 MPa, almost twice that obtained by the PLS. SEM observations indicated that intragranular fracture was the preponderant fracture mode in these samples, resulting in these excellent bending strength values.

SiC-ZrO2(3Y)-Al2O3 nanocomposites superfast densified by spark plasma sintering

Abstract Heterogeneous precipitation method has been used to produce 5 wt% SiC-15 wt% ZrO2-Al2O3 powder, from aqueous suspension of nano-SiC, aqueous solutions of zirconium oxychloride, yttrium nitrate and aluminium chloride and ammonia. The resulting gel was calcined at 1100 °C. Nano-SiC-ZrO2(3Y)-Al2O3 composites were superfast densified using spark plasma sintering (SPS) process by heating to a sintering temperature in the range of 1350 to 1600 °C, at a heating rate of 600 °C/min, with no holding time, and then fast cooled to 600 °C within 2–3 minutes. High density composites could be achieved at lower sintering temperatures by SPS, as compared with that by hot-press sintering process. Bending strength of 5 wt% SiC-15 wt% ZrO2(3Y)-Al2O3 superfast densified by SPS at 1450 °C reached as high as 1.2 GPa. Microstructure studies found that the nano-SiC particles were mainly located within the Al2O3 grains and the fracture mode of the nanocomposites was transgranular fracture.

Phase transformation in the Al2O3–ZrO2 system

Co-precipitation methods have been used to produce 20 mol% Al2O3–80 mol% ZrO2 mixed oxides, from aqueous solutions of zirconium oxychloride and aluminium chloride, followed by precipitation with ammonia. The resulting gel was calcined at increasing temperatures, and X-ray diffraction confirmed that the structure remained amorphous up to 750°C and then crystallized as a single-phase cubic zirconia solid solution, but with a reduced unit-cell dimension. At higher temperatures, the unit-cell dimension increased and, above 950°C, this phase started to transform to a tetragonal zirconia (t-ZrO2) phase, again of reduced cell dimensions compared with t-ZrO2, with simultaneous appearance of small amounts of θ-Al2O3. Above 1100°C, the tetragonal phase transformed to monoclinic zirconia on cooling, and the amount of θ-Al2O3 increased. Above 1200°C, the θ-Al2O3 transformed to the stable α-Al2O3. These results confirm that aluminium acts as a stabilizing cation for zirconia up to temperatures of about 1100°C.

Crystallization of Al2O3/ZrO2 solid solution powders prepared by coprecipitation

Abstract A co-precipitation method has been used to prepare Al 2 O 3 /20 mol% ZrO 2 powders from aqueous solutions of aluminum chloride and zirconium oxychloride followed by coprecipitation with an ammonia solution. X-ray diffraction analysis shows that the starting powder has the bayerite (Al(OH) 3 ) crystalline structure, transforming to single-phase γ -Al 2 O 3 /ZrO 2 solid solution (ss) after dehydration. As the temperature is increased, the solid solution experiences a series of partitioning and transformation processes as presented in this paper.

Intense 2.8 μm emission of Ho 3+ doped PbF 2 single crystal

A Ho³⁺-doped PbF₂ mid-IR laser crystal was successfully grown using the vertical Bridgman method. An intense 2.8 μm emission in Ho:PbF₂ crystal was observed for the first time. By analyzing the absorption and emission measurements of the Ho:PbF₂ crystal with the Judd-Ofelt theory, the intensity parameters Ω(2,4,6), exited state lifetimes, branching ratios, and emission cross-sections were calculated. It is found that the Ho:PbF₂ crystal has high fluorescence branching ratio (20.99%), large emission cross section (1.44×10⁻²⁰ cm²), long fluorescence lifetime (5.4 ms), and high quantum efficiency (88.4%) corresponding to the stimulated emission of Ho³⁺: ⁵I₆→⁵I₇ transition. The structure of Ho:PbF₂ crystal was also analyzed by the Raman spectrum, and it was found that the Ho:PbF₂ crystal possesses low phonon energy of 257 cm⁻¹. We propose that the Ho:PbF₂ crystal may be a promising material for 2.8 μm laser applications.

Fabrication and characterization of cerium-doped terbium gallium garnet with high magneto-optical properties

High optical quality (Tb((1-x))Ce(x))₃Ga₅O₁₂ (TCGG) single crystal has been grown by the Czochralski method. The optical and magneto-optical properties of the TCGG are analyzed in detail and the Verdet constant (V) of TCGG is compared with that of undoped terbium gallium garnet (TGG) crystal. TCGG presents a very high transmittance, particularly in the visible-near infrared (VIS-NIR) region, and its V is obviously larger than that of TGG in the VIS-NIR region. The figure of merit and optical features point out the superior characteristics of TCGG with respect to TGG.

Enhanced emission of 2.86 μm from diode-pumped Ho 3+ /Yb 3+ -codoped PbF 2 crystal

A novel Ho(3+)Yb(3+)-codoped PbF(2) mid-IR laser crystal was successfully grown and analyzed. Enhanced emission at 2.86 μm was observed from the crystal under excitation of a common 970 nm laser diode for the first time. The effect of Yb(3+) codoping on the 2.86 μm photoluminescence of Ho(3+) was investigated. In comparison to Ho(3+)-singly doped PbF(2) crystal, the Ho(3+)/Yb(3+)-codoped PbF(2) crystal possessed comparable quantum efficiency (88.8%), and fluorescence branching ratio (20.52%) along with a larger calculated emission cross section (1.90×10(-20) cm(2)) corresponding to the laser transition (5)I(6)→(5)I(7) of Ho(3+). It was found that the introduced Yb(3+) enhanced the 2.86 μm emission by depopulating the Ho(3+):(5)I(7) level. The energy transfer (ET) efficiency from Yb(3+):(2)F(5/2) to Ho(3+):(5)I(6) is as high as 96.7%, indicating that Yb(3+) ion is an effective sensitizer for Ho(3+) ion in PbF(2) crystal. These results suggest that Ho(3+)/Yb(3+)-codoped PbF(2) crystal may become an attractive host for developing solid state lasers at around 2.86 μm under a conventional 970 nm LD pump.

Synthesis and sintered properties of mullite powder from seeded diphasic Al2O3-SiO2 gel

Abstract Diphasic Al2O3-SiO2 gel was prepared by using AlCl3 and fumed silica as the two starting materials through coprecipitation. The seeded gel was produced by incorporating a small amount of crystalline mullite particles into the diphasic Al2O3-SiO2 gel. Phase evolution as a function of heat treatment in the unseeded and seeded diphasic gels were characterized by differential thermal analysis (DTA) and X-ray diffraction (XRD). The seeding was found to promote the formation of crystals, but, in spite of that, the crystallization temperature could not be lowered. A diphasic Al2O3-SiO2 gel with 4 wt% seeds could yield a well crystallized mullite without any other residual phases after calcination at 1300 °C for 2 h, while the unseeded gel did not show complete crystallization even though the calcination temperature was raised further. The mullite powder from seeded diphasic gel is of high purity and high activity, and can be sintered at 1650 °C for 4 h to over 99.0% of the theoretical density, showing a high flexural strength of 310 MPa.

Toughening mechanisms and properties of mullite matrix composites reinforced by the addition of SiC particles and Y-TZP

Mullite matrix composites reinforced by SiC particles and Y-TZP, were fabricated by hot-pressing. The effects of adding SiC particles and Y-TZP to mullite or mullite-based materials on properties and toughening mechanisms in the composites were investigated. Crack deflection is proposed as the principal toughening mechanism, produced by the addition of SiC particles. Transformation and microcrack toughening are the two main toughening mechanisms caused by Y-TZP addition. However, the magnitude of their contribution varied with increasing Y-TZP addition. With low Y-TZP addition, the transformation toughening dominated, while at a higher Y-TZP content, the microcrack toughening was dominant. The simultaneous addition of SiC particles and Y-TZP to mullite resulted in higher increases in both flexural strength and fracture toughness, than the simple sum of those obtained by the separate processes. It appears that the two toughening processes were coupled, thereby leading to synergistic toughening and strengthening effects in the mullite composites.

Intense 2.89 μm emission from Dy³⁺/Yb³⁺-codoped PbF₂ crystal by 970 nm laser diode pumping.

A novel Dy(3+)/Yb(3+) co-doped PbF2 mid-IR laser crystal was successfully grown using the vertical Bridgman method. Efficient emission at around 3 μm from the crystal was observed under excitation of a conventional 970 nm laser diode (LD). The energy transfer efficiency from Yb(3+) to Dy(3+) in Dy(3+)/Yb(3+):PbF2 crystal is as high as (97.7±0.3)%. It is also found that the Dy(3+)/Yb(3+):PbF2 crystal possesses long fluorescence lifetime (15.4±0.2) ms, high quantum efficiency (95.0±0.3)%, and large emission cross section (1.37±0.11)×10(-20) cm2 corresponding to the stimulated emission of Dy(3+):(6)H(13/2)→(6)H(15/2) transition. Additionally, the phonon energy of the crystal was analyzed by the Raman spectrum. These results indicate that Dy(3+)/Yb(3+):PbF2 crystal may become a promising material for 3 μm solid state lasers under a conventional 970 nm LD pump.