Fabrication and laser behaviors of Nd:YAG ceramic microchips

Transparent Nd:YAG ceramic microchips were fabricated through the slip casting shaping directly from the slurry formed by the commercial Al₂O₃/Y₂O₃/Nd₂O₃ powders, and followed by the vacuum sintering procedure. Viscosity of the slurries, the phase evolution and the densification behavior were investigated. For the Al₂O₃/Y₂O₃/Nd₂O₃ compound slurries system, the optimal condition is 0.5 wt.% NH₄PAA dispersant and 30 wt.% solid loading at pH ≥ 8. The YAG phase started to form at 1250° C and pure YAG phase could be obtained at 1400° C. The optical in-line transmittance of the Nd:YAG ceramics with thickness of 2 mm was about 83.8% at 1064 nm and 82.5% at 400 nm, which hit the upper limit of the theoretically calculated value. For the 1.0 at.% Nd:YAG ceramic microchip, the slope efficiency was 43% for 1.0 at.% Nd:YAG ceramic pumped by 920 mW cw Ti:sapphire tunable laser, and the maximum laser output power 246 W was obtained for 2.0 at.% Nd:YAG ceramics pumped by 925 W LD.     

Densification and microstructure evolution of Cr,Nd:YAG transparent ceramics for self-Q-switched laser

Transparent 0.1 at.% Cr, 1.0 at.% Nd:YAG ceramics were fabricated by solid-state reaction and vacuum sintering using commercial Y₂O₃, α-Al₂O₃, Cr₂O₃ and Nd₂O₃ as raw materials. CaO and tetraethoxysilane (TEOS) were used as charge compensator and sintering aid, respectively. The powders were mixed in ethanol and doped with TEOS, dried and pressed. Pressed samples were sintered from 1450 to 1800 °C for 10 h. The relative density increased from 68.8% to 99.4% at the sintering temperature from 1450 to 1700 °C. Grain size increased with increase of sintering temperature and obvious grain growth occurred between 1650 and1700 °C. For the Cr,Nd:YAG ceramics sintered at 1750 and 1800 °C for 10 h, nearly pore-free microstructures with average particle size of ∼10 μm were obtained. The optical transmittance of the 1800 °C sintered sample was ∼70% in the infrared wavelength.     

Structural and optical properties of Tm:Y2O3 transparent ceramic with La2O3, ZrO2 as composite sintering aid

The paper reports the use of La₂O₃ and ZrO₂ co-doping as a composite sintering aid for the fabrication of Tm:Y₂O₃ transparent ceramics. Two groups of experiments were conducted for investigating the influences of composite sintering aids on the microstructures and the optical properties of Tm:Y₂O₃ transparent ceramics in contrast to single La³⁺ and single Zr⁴⁺ doped Tm:Y₂O₃. Samples with composite sintering aids could realize fine microstructures and good optical properties at relatively low sintering temperatures. Grain sizes around 10 μm and transmittances close to theoretical value at wavelength of 2 μm were achieved for the 9 at.% La³⁺, 3 at.% Zr⁴⁺ co-doped samples sintered at 1500-1600 °C. The influences of the composite sintering aids on the emission intensities and the phonon energies of Tm:Y₂O₃ ceramics were also investigated.     

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y₂O₃ powders as well as commercial Al₂O₃ and Yb₂O₃ powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb³⁺ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO₂ formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb³⁺ easily entered the YAG lattice and had a high segregation coefficient.     

Preparation of spray-dried powders leading to Nd:YAG ceramics: The effect of PVB adhesive

The mixed powders were obtained with Al₂O₃, Y₂O₃ and Nd₂O₃ powders as starting materials using spray drying technology. The main purpose of introducing polyvinyl butyral (PVB) is to contribute to spray granulation of the powders and inhibition of the compositions segregation. The effect of the addition of PVB (0, 1, 2, and 3 wt%) on the morphologies and compositions of the spray-dried powders is discussed. When calcined at 1000 °C for 2 h, the powders with PVB as an adhesive show sphericity and better dispersion. No compositions segregation can be detected. It is found that the powders with 2 wt% PVB after calcinations are suitable for the fabrication of Nd:YAG transparent ceramics. The corresponding ceramics consists of a well-defined microstructure, and no pores or other defects are observed.     

Fabrication and thermal conductivity of AlN/BN ceramics by spark plasma sintering

Aluminum nitride/boron nitride (AlN/BN) ceramics with 15–30 vol.% BN as secondary phase were fabricated by spark plasma sintering (SPS), using Yttrium oxide (Y₂O₃) as sintering aid. Effects of Y₂O₃ content and the SPS temperature on the density, phase composition, microstructure and thermal conductivity of the ceramics were investigated. The results revealed that with increasing the amount of starting Y₂O₃ in AlN/BN, Yttrium-contained compounds were significantly removed after SPS process, which caused decreasing of the residual grain boundary phase in the sintered samples. As a result, thermal conductivity of AlN/BN ceramics was remarkably improved. By addition of Y₂O₃ content from 3 wt.% to 8 wt.% into AlN/15 vol.% BN ceramics, the thermal conductivity increased from 110 W/m K to 141 W/m K.     

Pressureless sintering of silicon carbide ceramics containing zirconium diboride

SiC-5 wt.% ZrB₂ composite ceramics with 10 wt.% Al₂O₃ and Y₂O₃ as sintering aids were prepared by presureless liquid-phase sintering at temperature ranging from 1850 to 1950 °C. The effect of sintering temperature on phase composition, sintering behavior, microstructure and mechanical properties of SiC/ZrB₂ ceramic was investigated. Main phases of SiC/ZrB₂ composite ceramics are all 6H-SiC, 4H-SiC, ZrB₂ and YAG. The grain size, densification and mechanical properties of the composite ceramic all increase with the increase of sintering temperatures. The values of flexural strength, hardness and fracture toughness were 565.70 MPa, 19.94 GPa and 6.68 MPa m^1/2 at 1950 °C, respectively. The addition of ZrB₂ proves to enhance the properties of SiC ceramic by crack deflection and bridging.     

The effect of Al2O3 on sintering and crystallization of MgSiO3-based glass-powder compacts

The influence of Al₂O₃ (8 wt.%) on sintering and crystallization features of glass powders based on magnesium silicate (MgSiO₃) was experimentally determined. The investigated compositions were Y_0.125Mg_0.875Si_0.875B_0.125O₃ and Y_0.125Mg_0.725Ba_0.15Si_0.875B_0.125O₃. For the experiments, glasses in bulk and frit forms were produced by melting in Pt-crucible at 1600 °C for 1.5 h. Glass-powder compacts were sintered at different temperatures between 900 °C and 1100 °C. The evolution of crystalline regime was determined by in situ recording of X-ray diffractograms of fine glass powders at elevated temperatures. The results and their discussion showed that addition of 8 wt.% Al₂O₃ in glass batches affected the thermal properties of the glasses and resulted in MgSiO₃-based glass ceramics well sintered between 900 °C and 1100 °C. In the BaO-free MgSiO₃ glass ceramics, clino- and orthoenstatite crystallize while the presence of BaO favours the formation of hexacelsian.     

Co-precipitation synthesis and two-step sintering of YAG powders for transparent ceramics

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) precursor was synthesized by the co-precipitation method with ammonium hydrogen carbonate as the precipitant. The influence of aging and calcination temperature on the precursor composition and transformation temperature of the YAG phase was investigated. On that basis, a two-step sintering (TSS) method (heating the sample up to 1800 °C followed by holding it at 1600 °C for 8 h) was used to fabricate bulk transparent YAG ceramics in vacuum (10⁻³ Pa) in this communication. A variety of techniques, such as X-ray powder diffraction, infrared spectra, scanning electron microscopy and UV–vis–NIR spectrophotometry were adopted to characterize the resulting YAG powders and ceramics. The results showed that aging had a dramatic effect on the precursor composition, which in turn influenced the transformation temperature of the YAG phase. Loosely agglomerated YAG powders with a mean particle size of 50 nm were obtained by calcinating the precursor without aging at 1000 °C. Finally, a transparent YAG ceramic specimen, achieving the in-line transmittance of 41% in the visible wavelength region and a nearly pore-free microstructure with uniform grains of about 4 μm, was produced via the TSS technique.     

Structure and electrical properties of Nd2O3-doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 ceramics

Nd₂O₃ doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (abbreviated to BNKT) binary lead-free piezoelectric ceramics were synthesized by the conventional mixed-oxide method. The results show that the BNKT ceramics with 0–0.15 wt.% Nd₂O₃ doping possesses a single perovskite phase with rhombohedral structure. The grain size of BNKT decreased with the addition of Nd₂O₃ dopant. The temperature dependence of the dielectric constant ?_r revealed that there were two-phase transitions from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric. A diffuse character was proved by linear fitting of the modified Curie–Weiss law. At room temperature, the specimens containing 0.0125 wt.% Nd₂O₃ with homogeneous microstructure presented excellent electrical properties: the piezoelectric constant d₃₃ = 134 pC/N, the electromechanical coupling factor K_p = 0.27, and the dielectric constant ?_r = 925 (1 kHz).     

Heating parameter optimization and optical properties of Nd:YAG transparent ceramics prepared by microwave sintering

Nd-doped YAG transparent ceramics were prepared by microwave sintering. In this paper, the green bodies from high-purity commercial powders were sintered from 900 °C to 1750 °C for different lengths of time (0.5–2 h) by microwave heating. By optimizing the microwave heating parameters (the heating rate at different stages of microwave sintering, sintering temperature and holding time), the microstructures and optical properties of transparent ceramics can be effectively improved. The phase transformation, densification process and optical properties of Nd:YAG transparent ceramics were discussed. The liquid phases strongly absorb microwave radiation and affect the sintering results of samples during microwave sintering. The highest in-line transmittances of Nd:YAG transparent ceramic fabricated at 1750 °C for 2 h were 76.5% at 400 nm and 80.6% at 1064 nm. The fluorescence emission spectra and lifetime depending on different heating conditions were also discussed.     

Synthesis of Nd:YAG powders leading to transparent ceramics: The effect of MgO dopant

Neodymium doped yttrium aluminum garnet (Nd:YAG) ultrafine powders were synthesized by co-precipitation method using MgO as dopant. The addition of small amount of MgO can reduce the agglomeration and particle size of the produced Nd:YAG powders. The results show that pure phase YAG powders can be achieved by calcining of the precursors at 1000 °C for 2 h. The MgO doped Nd:YAG powders show better dispersion compared with the undoped powders. When the MgO content is 0.01 wt.%, well-dispersed Nd:YAG powders with spherical particles of 100 nm diameter were obtained. The transmission of the corresponding Nd:YAG ceramics is 82.6% at the wavelength of 1064 nm, which is comparable to Nd:YAG single crystals.     

Microstructure of composite YAG crystal/ceramics

Transparent composite YAG crystal/ceramics were synthesized by solid-state reaction method using high-purity Y₂O₃, Al₂O₃ powders as raw materials. The mixed slurry was dried, sieved, and cold-isostatically pressed with Nd:YAG crystal under a pressure of 250 MPa. The mixed powder compacts were sintered at 1780 °C for 10 h under vacuum and annealed at 1450 °C for 20 h in air. The microstructure of YAG crystal/ceramics ceramics was studied with SEM and EPMA, which showed there was an intermediate layer between Nd:YAG crystal and YAG ceramics. HRTEM image and corresponding SAED patterns studied showed that the intermediate layer was the YAG ceramics grain that grew along Nd:YAG crystal orientation and has become one part of crystal.     

The controllable microstructure of porous Al2O3 ceramics prepared via a novel freeze casting route

In traditional aqueous slurry freezing casting processing, the growth method of ice crystals is hard to control, resulting in the uncontrollable pore's morphologies of the porous ceramics. In the experimental, the pure Al₂O₃ sol was used to substitute water as a medium for preparing ceramic slurry. With Al₂O₃ sol addition, it becomes easy to control the microstructure and pore's morphologies of the porous Al₂O₃ ceramics via adjusting of the solid loading, composition of the ceramic slurries, as well as the cooling methods. The SEM micrographs showed that the sol-contained ceramic slurry combined with freeze casting processing can easily prepare the porous Al₂O₃ ceramics with different pore sizes and different morphologies. The porous Al₂O₃ ceramics prepared from 70 wt.% to 90 wt.% solid loading sol-contained Al₂O₃ slurries and sintered at 1500 °C for 2 h have open porosities from 81.7% to 64.6%.     

Preparation and characterization of transparent Tm:YAG ceramics

Polycrystalline (Y_3−xTm_x)Al₅O₁₂ (x = 0, 0.18, 0.9, 1.5, 3) ceramics were fabricated by a solid-state reaction method using high-purity micrometer-sized powders. Tm-doped ceramics with an almost perfect pore-free structure and high transparency were obtained by advanced ceramic processing. The average grain size and grain boundary width were 15 μm and 1 nm, respectively. The Y_2.82Tm_0.18Al₅O₁₂ (6 at.% Tm:YAG) ceramic slab (5 mm × 5 mm × 3.5 mm) was end-pumped by a Ti:sapphire laser at 785 nm and the maximum output power of 725 mW was obtained with a slope efficiency of 36.2% at 2012 nm.     

Dielectric properties of CuO-doped La2.98/3Ba0.01(Mg0.5Sn0.5)O3 ceramics at microwave frequency

The microwave dielectric properties of La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics prepared by the conventional solid-state method were investigated for application in mobile communication. A 100 °C reduction of the sintering temperature was obtained by using CuO as a sintering aid. A dielectric constant of 20.0, a quality factor (Q × f) of 50,100 GHz and a temperature coefficient of resonant frequency τ_f of −78.3 ppm/°C were obtained when La_2.98/3Ba_0.01(Mg_0.5Sn_0.5)O₃ ceramics with 0.25 wt.% CuO were sintered at 1500 °C for 4 h.     

Optical properties of Er,Yb co-doped YAG transparent ceramics

The transparent polycrystalline erbium and ytterbium co-doped yttrium aluminum garnet (Er,Yb:YAG) ceramics with various Yb contents from 5% to 25% were prepared by the solid-state reaction and the vacuum-sintering technique. The in-line transmittances of the mirror-polished ceramics exceed 80% from the visible band to the infrared band. The samples are very compact with few pores. The average grain size of the Er,Yb:YAG ceramic is about 15 μm. The upconversion luminescence spectra, infrared luminescence spectra and luminescence decay curves of the ceramics were observed and discussed. For 1%Er doped YAG ceramic, the best ion ratio of Yb³⁺ and Er³⁺ is around 15:1.     

Fabrication and characteristics of Ce-doped Y3Fe5O12 ceramics by hot-press sintering with oxygen/nitrogen atmosphere

Infrared transparent Ce-doped Y₃Fe₅O₁₂ (Ce_: YIG, Ce_xY_3-xFe₅O_12, x = 0, 0.12, 0.24, 0.36) ceramics were successfully produced by the solid-state reaction using a hot-press sintering process from the Y₂O₃, Fe₂O₃, and CeO₂ powders. The phase structure, microstructure, infrared transmittance, and magnetic and magneto-optical properties of the Ce-doped Y₃Fe₅O₁₂ ceramics were measured and analyzed. The in-line transmittances of the Ce-doped Y₃Fe₅O₁₂ ceramics with the x = 0, 0.12, 0.24 (L = 0.5 mm) at 1550 nm were about 72%, 66.5%, and 57.6%, respectively. In the state of saturation magnetization, the Faraday rotation angle per centimeter (θ_F) of Ce_xY_3-xFe₅O₁₂ (x = 0, 0.12, 0.24) ceramics measured by the light extinction method was 182.5, −410.4, and −958.3 deg./cm, respectively. The change of the θ_F was about −142.5 deg./cm when per 1at.% Ce was substituted in the dodecahedral site of YIG materials. The (Ce_0.24Y_2.76)Fe₅O₁₂ ceramics were determined as the optimized composition for its excellent infrared optical and magneto-optical properties.     

Fabrication, microstructure and optical properties of polycrystalline Er:Y3Al5O12 ceramics

Highly transparent polycrystalline Er³⁺:Y₃Al₅O₁₂ (Er:YAG) ceramics with different Er³⁺ ions content from 1% to 90% were prepared by the solid-state reaction and the vacuum-sintering technique. The grain boundary is clean and narrow with a width of about 1 nm. The best sintering temperature of the ceramics is about 1800 °C. The relationships between fabrication, microstructure and transparency of the ceramics were discussed. Grain size distributions in axial direction of cylinder samples were characterized by electron probe micro-analyzer (EPMA). The luminescence spectra were measured and discussed.