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Transparent polycrystalline ceramics with cubic crystal structure have played important roles in a wide variety of solid-state laser applications, whereas for non-cubic structures, single crystal only has been used. For further progress in optical technologies, effective materials beyond the current limitations are necessary. Here we report a new type of non-cubic ceramic laser material that overturns conventional common sense. It is hexagonal Nd-doped fluorapatite (Nd:FAP) ceramics with an optical quality comparable to single crystal while having random crystal orientation. It is composed of ultrafine grains with a loss coefficient of 0.18 cm −1 at a lasing wavelength of 1063 nm, and its laser oscillation was demonstrated. This is the first verification of lasing in randomly oriented non-cubic ceramics. Laser oscillation in the non-cubic ceramics was realized through both advanced liquid-phase nano-powder synthesis technology and highly controlled pulsed-current sintering techniques. Our findings should open new avenues for future solid-state laser and optical applications.

Highly optically transparent polycrystalline fluorapatite ceramics with hexagonal crystal structures were fabricated via a liquid-phase synthesis of fluorapatite powder, followed by spark plasma sintering (SPS). The effect of sintering temperature, as observed using a thermopile, on the optical transmittance and microstructure of the ceramics was investigated in order to determine suitable sintering conditions. As a result, high optical transmittance was obtained in the SPS temperature range of 950–1100 °C. The highest optical transmittance was obtained for the ceramic sample sintered at 1000 °C, and its average grain size was evaluated at only 134 nm. The grain size dramatically increased with temperature, and the ceramics became translucent at SPS temperatures above 1200 °C. The mechanical and thermal properties of the ceramics were measured to evaluate the thermal shock parameter, which was found to be comparable to or slightly smaller than that of single-crystal fluorapatite. This transparent polycrystalline fluorapatite ceramic material should prove useful in a wide range of applications, for example as a biomaterial or optical/laser material, in the future. Furthermore, the knowledge obtained in this study should help to promote the application of this ceramic material.

Hydrothermal reaction of Ln nitrate and Na 2 WO 4 at pH=8 and 200 °C for 24 hours, in the absence of any additive, has directly produced the scheelite-type sodium lanthanide tungstate of NaLn(WO 4 ) 2 for the larger Ln 3+ of Ln=La-Dy (including Y, Group I) and an unknown compound that can be transformed into NaLn(WO 4 ) 2 by calcination at the low temperature of 600 °C for the smaller Ln 3+ of Ln=Ho-Lu (Group II). With the successful synthesis of NaLn(WO 4 ) 2 for the full spectrum of Ln, the effects of lanthanide contraction on the structural features, crystal morphology, and IR responses of the compounds were clarified. The temperature- and time-course phase/morphology evolutions and the phase conversion upon calcination were thoroughly studied for the Group I and Group II compounds with Ln=La and Lu for example, respectively. Unknown intermediates were characterized by elemental analysis, IR absorption, thermogravimetry, and differential scanning calorimetry to better understand their chemical composition and coordination. The photoluminescence properties of NaEu(WO 4 ) 2 and NaTb(WO 4 ) 2 , including excitation, emission, fluorescence decay, and quantum efficiency of luminescence, were also comparatively studied for the as-synthesized and calcination products.

Aiming to characterize the effect of sintering temperature on transparency of zirconia, we have evaluated the optical properties and microstructure of translucent cubic zirconia prepared by high-pressure spark plasma sintering (SPS) at 1000-1200 ○ C. Color centers (oxygen vacancies with trapped electrons) and residual pores were primary defects in the samples. In SPS samples, the total forward transmittance and in-line transmittance are mainly affected by color centers with a limited contribution from residual pores; in contrast, the changes in reflectance are only related to the porosity. The amounts of color centers and residual pores increase with sintering temperature that reduces the total forward and in-line transmittance of the as-sintered zirconia. Annealing in oxidizing atmosphere improves the total forward and in-line transmittance. During the annealing, the concentration of color centers decreases but the porosity increases.

Ultra-high-density zircon (ZrSiO 4 ) ceramics were prepared using the spark plasma sintering (SPS) technique of zircon nanopowder with the addition of three different sintering agents, i.e., Bi 2 O 3 , V 2 O 5 and B 2 O 3 . The effect of each agent and the SPS parameters (temperature and pressure) on phase composition, microstructure, thermal and mechanical properties of the ceramics were evaluated. The identified crystalline phases of the sintered ceramics were zircon and monoclinic zirconia. The addition of a sintering agent affects the structure of zircon ceramics, i.e. the lattice parameter and the crystallite size. The sintered ceramics reached relative densities up to 99.9% of the theoretical one when V 2 O 5 or B 2 O 3 was added. SEM observations confirmed the densification of the zircon ceramics. We found the ceramics exhibited thermal conductivity ranging from 0.39 to 0.61 Wm −1 K −1 at 373 K while the coefficient of thermal expansion was 2.3–4.0 × 10 −6 /°C and the Vickers hardness was obtained to be 9.52–12.66 GPa. The Young’s ( E ), bulk ( B ), and shear ( G ) moduli, Poisson’s ratio ν, Pugh’s ratio B/G , and the ratio of H V 3 / E * 2 of the ceramics are in a range of 240 − 288 GPa, 207 − 267 GPa, 91 − 109 Pa, 1.95 − 2.45, and 0.011 − 0.019 respectively. We found that high-density, quasi-ductile zircon ceramics can be synthesized at a low sintering temperature and short holding time.

High strength transparent Y2O3 ceramics were fabricated from commercial powders using spark plasma sintering (SPS) technique by optimizing the heating rate. The heating rate significantly influenced the microstructures and the optical/mechanical properties of the Y2O3 ceramics. Grain growth was limited accordingly with increasing the heating rate. The ball milling process of the commercial Y2O3 powders is likely to further enhance the sinterability during the SPS processing. The dense Y2O3 ceramics, which were sintered by SPS with 100 °C/min, showed good transmittance range from visible to near infrared (IR). For a high heating rate of 100 °C/min, the in-line transmittance at a visible wavelength of 700 nm was 66%, whereas for a slow heating rate of 10 °C/min, it reduced to 46%. The hardness Hv tends to increase with increasing the heating rate and rigorously followed the Hall–Petch relationship; that is, it is enhanced with a reduction of the grain size. The toughness KIC, on the other hand, is less sensitive to both the heating rate and the grain size, and takes a similar value. This research highlighted that the high heating rate SPS processing can fabricate fully dense fine-grained Y2O3 ceramics with the excellent optical and mechanical properties.

Hexagonal (Gd 0.95 RE 0.05 )PO 4 ·nH 2 O nanowires ~300 nm in length and ~10 nm in diameter have been converted from (Gd 0.95 RE 0.05 ) 2 (OH) 5 NO 3 ·nH 2 O nanosheets (RE = Eu, Tb) in the presence of monoammonium phosphate (NH 4 H 2 PO 4 ) and ethylene diamine tetraacetic acid (EDTA). They were characterized by X-ray diffraction, thermogravimetry, electron microscopy, and Fourier transform infrared and photoluminescence spectroscopies. It is shown that EDTA played an essential role in the morphology development of the nanowires. The hydrothermal products obtained up to 180 °C are of a pure hexagonal phase, while monoclinic phosphate evolved as an impurity at 200 °C. The nanowires undergo hexagonal→monoclinic phase transformation upon calcination at ≥600 °C to yield a pure monoclinic phase at ~900 °C. The effects of calcination on morphology, excitation/emission, and fluorescence decay kinetics were investigated in detail with (Gd 0.95 Eu 0.05 )PO 4 as example. The abnormally strong 5 D 0 → 7 F 4 electric dipole Eu 3+ emission in the hexagonal phosphates was ascribed to site distortion. The process of energy migration was also discussed for the optically active Gd 3+ and Eu 3+ /Tb 3+ ions.

Aiming to characterize the effect of alumina dopant on transparency, powders of yttria stabilized tetragonal zirconia doped with alumina (TZ-3Y-E) are used as starting material to fabricate transparent tetragonal ZrO2 by high-pressure spark plasma sintering (HP-SPS). However, low transparency of the resultant TZ-3Y-E specimens does not suggest a beneficial effect of alumina dopant although nanometric grains and high density have been achieved. The mechanism is analyzed by comparing with the results of as-sintered yttria stabilized tetragonal zirconia without alumina dopant.

Tribological property of c -axis textured shell-like Ti 3 AlC 2 ceramic was investigated using reciprocating sliding balls (SUS304) under loads of 1, 5, and 9 N. It was found that the textured top surface (TTS), corresponding to the (000 l ) plane, shows the lowest mean coefficient of friction in comparison with those measured on the textured side surface (TSS), where the sliding directions are parallel (TSS-1) and perpendicular (TSS-2) to c axis, under the same load. Among all the tested orientations, the TSS-2 exhibited the lowest wear rate of 1.51×10 −3 mm 3 /(N·m) under the load of 9 N. The worn mechanisms on the TTS and TSS-1 were delamination, grain fracture, and grain spalling-off. On the TSS-2, plowing effect against balls was the dominating mechanism. This work suggests the criteria to maximize the wear resistance in the load range of 1–9 N.

Tungstate source and tartrate (Tar 2− ) additive were examined for their influences on the hydrothermal formation and characteristics of ZnWO 4 nanocrystals. It was clearly shown that quasi-equiaxed nanocrystallites of ~ 50–100 nm in diameter and nanorods of ~ 40–50 nm in diameter and up to ~ 700 nm in length can be generated with (NH 4 ) 10 W 12 O 41 ·5H 2 O and K 2 WO 4 ·2H 2 O as tungsten sources, respectively. Introducing Tar 2− into the K 2 WO 4 ·2H 2 O reaction system effectively transformed the primary crystallites of ZnWO 4 from nanorods into quasi-equiaxed nanocrystals (~ 20–50 nm) and then nanoplates (thickness of ~ 20 nm, lateral size of ~ 200 nm) and, meanwhile, aggregated the crystallites into spheroidal clusters (~ 2–3 µm in diameter) with the increasing Tar 2− /Zn 2+ molar ratio up to ~ 2. Optical spectroscopy revealed that the ZnWO 4 products exhibit broad-band photoluminescence (~ 425–700 nm) through 3 T 1u  →  1 A 1g transition of the [WO 6 ] 6− ligand under short ultraviolet excitation and the nanorods show the best luminescence among all tested samples. Calcination at 500 °C may effectively remove the adsorbed Tar 2− species and greatly improve luminescence of the samples synthesized in the presence of Tar 2− .