Effects of LiF on the microstructure and optical properties of hot-pressed MgAl2O4 ceramics

Transparent magnesium aluminate spinel (MgAl₂O₄) ceramics were fabricated by hot-pressing of the MgO and α-Al₂O₃ powder mixture using LiF as a sintering aid. Effects of the LiF additive on densification, microstructure and optical properties of MgAl₂O₄ ceramics were systematically investigated. It has been found that the addition of LiF can effectively remove the porosity and increase the optical transparency of MgAl₂O₄ ceramics. For the spinel ceramics HP-ed at 1550 °C for 3 h with 1 wt% LiF addition, the average grain size is about 36 µm and the in-line transmittance exceeds 60% at the wavelength of 800 nm.     

Annealing induced discoloration of transparent YAG ceramics using divalent additives in solid-state reaction sintering

Tetraethyl orthosilicate (TEOS) was commonly served as a sintering additive to promote the densification of transparent Y₃Al₅O₁₂ (YAG) ceramics. However, Si⁴⁺ that decomposed from TEOS would restrain the conversion of dopants into a higher valence state (e.g., Cr³⁺ → Cr⁴⁺). In this study, by using divalent sintering additives (CaO and MgO), the colorless and highly transparent YAG ceramics (T = 84.6%, at 1064 nm) were obtained after vacuum sintering at 1840 °C for 8 h and without subsequent annealing in air. An absorption peak centered at ∼320 nm was observed before annealing, and it extended to ∼550 nm after annealing at 1450 °C for 10 h in air. A discoloration phenomenon occurred and more scattering centers were observed with the formation of new [Mg/Ca²⁺F⁺] color centers. Air annealing did not improve the optical quality of the as-fabricated YAG ceramics with divalent dopants as sintering additives, owing to the formation of scattering centers.     

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.     

Effect of Ca2+ and Mg2+ ions on the sintering and spectroscopic properties of cr-doped yttrium aluminum garnet ceramics

In this work, we investigated the effects of Ca²⁺ and Mg²⁺ ions and annealing temperature on the spectroscopic parameters of chromium-doped yttrium aluminum garnet ceramics (Cr:YAG). Samples were obtained with either a separate or a simultaneous addition of calcium and magnesium oxides. To achieve this, aqueous suspensions were prepared using Y₂O₃, Al₂O₃, Cr₂O₃, MgO, and CaO high-purity powders as raw materials. The obtained suspensions were freeze-granulated, pressed into pellets, debinded, and subjected to reactive sintering in vacuum at 1715°C for 6 h. Each material was annealed in air with temperatures between 1300 and 1700°C. Samples were also compared to Cr:YAG ceramics with the addition of silica as a sintering aid. All the materials obtained were then exposed to 445 nm excitation, and emission spectra in the visible and infrared wavelengths were recorded. The results showed that the emission spectra of Cr:YAG ceramics varied according to the annealing conditions: as-sintered samples exhibited strong emissions of around 680 nm and, after air annealing, of around 1400 nm. This phenomenon is attributed to the Cr³⁺→Cr⁴⁺ transition. Samples doped solely with MgO exhibited the highest emission intensity in the infrared region. Thus, Mg²⁺ ions provided the best conversion efficiency of chromium ions.     

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.     

Effect of rare earth ions doping on the thermal properties of YAG transparent ceramics

Yttrium aluminium garnet doped with rare earth ions is one of the most common active media in solid state lasers. In high-power lasers, thermal management is crucial, requiring information on the thermal properties. In this work, the thermal diffusivity and conductivity of polycrystalline YAG ceramics doped with Yb and Er were measured by laser flash method at various temperatures ranging from room temperature to 900 °C. Transparent ceramic YAG samples were prepared by solid state reactive sintering of oxide powders under vacuum. Thermal diffusivity and conductivity showed similar trends, decreasing with increasing temperature as well as with the increase of dopant content from 0 to 20 at.%. The measured values were compared with literature data and empirical relations. Similar values were obtained both for Yb and Er doping. We thus suggest that the data of thermal diffusivity and conductivity of Yb:YAG may be used as a first approximation for Er:YAG.     

ZrN+AlN+Y2O3复合助烧结剂对Si3 N4陶瓷烧结性能的影响

对比了ZrN+AlN助烧结剂与ZrN+AlN+Y2O3助烧结剂对1800℃、25 MPa下热压烧成Si3N4陶瓷显微结构和力学性能的影响,并着重对ZrN+AlN+Y2O3复合助烧结剂促进Si3N4陶瓷烧结的机理进行了探讨.结果表明加ZrN+AlN+Y2O3助烧结剂能明显促进Si3N4陶瓷的烧结,提高陶瓷强度,其相对密度可达97.84%,常温弯曲强度为601.21 MPa,断裂韧性达8.9 MPa·m1/2;而加ZrN+AlN助烧结剂的Si3N4陶瓷未致密化.     

Transparent ceramics green-microstructure optimization by pressure slip-casting: Cases of YAG and MgAl2O4

This paper presents experimental data and its interpretation regarding the forming of YAG and spinel green-bodies, intended for transparent parts fabrication, by the pressure slip casting (PSC) method. Conditions for an optimal operation are established based on the modeling of the filtration kinetics. It emerges that the method is able to provide highly sinterable green parts by ensuring that the cakes porosity exhibits low average size and narrow size distribution. Results were compared with other popular forming approaches like slip casting (SC) and cold isostatic pressing (CIP). PSC was found as superior, to the other approaches, as far as obtainment of high sinterability green- bodies is concerned. In the case of YAG, it was shown that PSC method even allows the replacement of the traditional long vacuum firings by a two stage densification operation in which an initial air-firing is completed by a hot isostatic pressing step.     

Influence of the ratio of sintering aids on the properties of porous Si3N4 ceramics fabricated by digital light processing

The sintering aids play an important role in affecting the properties of porous Si₃N₄ ceramics. However, there are few researches on the properties of porous Si₃N₄ ceramics fabricated by digital light processing (DLP) with different ratios of sintering aids. In this paper, porous Si₃N₄ ceramics with different ratios of sintering aids (Y₂O₃-Al₂O₃) were formed by DLP technology. The influence of Y₂O₃-Al₂O₃ ratios on the properties of Si₃N₄ slurry and porous ceramic was studied systematically. The ratio of Y₂O₃-Al₂O₃ had little effect on the rheology and cure depth of Si₃N₄ slurry due to the low addition of sintering aids. The increase of Y₂O₃-Al₂O₃ ratio promoted the anisotropic growth of β-Si₃N₄. When the ratio of Y₂O₃-Al₂O₃ was 9:1, the aspect ratio of β grains reached the maximum. As the ratio of Y₂O₃-Al₂O₃ powders increased, the linear shrinkage of porous Si₃N₄ ceramics showed an increasing and then decreasing trend in three directions. When the Y₂O₃-Al₂O₃ ratio was 3:7, the shrinkage rate in the length, width and height direction reached the maximum (27.03%, 30.27% and 40.02%, respectively). The bulk density and flexural strength exhibited an initial increase followed by a subsequent decrease, while the porosity showed the opposite trend. When the Y₂O₃-Al₂O₃ ratio was 9:1, the porosity reached a maximum of 28.1%. And the bulk density and flexural strength were 2.42 g/cm³ and 421.58 MPa, respectively. This study is of great significance as it lays the experimental foundation in the performance control of porous Si₃N₄ ceramics fabricated by DLP.     

Optimizing liquid phase promotion effect by modifying powder specific surface area in Si4+/Mg2+ co-doped transparent Yb:YAG ceramics

Utilizing the Si⁴⁺/Mg²⁺ co-doping has been considered an effective approach to fabricate highly transparent ceramics. However, the optimum doping concentration has been reported with considerable uncertainties. In this work, highly transparent Yb:YAG ceramics were obtained via the solid-state method and the sintering behavior is discovered to be closely related to both the doping concentration of Si⁴⁺/Mg²⁺ and the specific surface area (S_BET) of powders. S_BET is effectively modified by setting the ball-milling time, where the maximum S_BET (30.914 m²/g) is achieved with 24 h ball-milling time. With increasing S_BET, less Mg²⁺ is required for better optical properties. When S_BET equals 30.914 m²/g, the highest in line transmittance @ 1100 nm of 84.85% is obtained with Si⁴⁺/Mg²⁺ doping concentrations of 0.50 wt% and 0.05 wt%, respectively. The relation between S_BET and optimum doping concentration is explained by the different magnitudes of liquid phase promotion required for different contact areas between powder particles.     

Microwave dielectric properties of YAG ceramics prepared by sintering pyrolysised nano-sized powders

YAG ceramics with good dielectric properties were prepared via a modified pyrolysis method, with yttrium nitrate as the yttrium source and combined aluminium sulphate and aluminium nitrate as aluminium sources, and subsequent sintering in a muffle furnace. The effects of the different aluminium sources on the powder characteristic and the impact of sintering temperature, sintering aids (TEOS) and additive (TiO₂) on the dielectric properties of the ceramics were studied. The results show that well-dispersed pure YAG nano-powders can be obtained after calcination at 1000 °C with an aluminium sulphate and aluminium nitrate molar ratio of 1.5:2. The relative density, permittivity (ε_r) and quality factor (Q×f) of the YAG ceramics increase with sintering temperature and TEOS addition. TiO₂ can greatly promote τ_f to near-zero but decreases Q×f. The relative density, ε_r, Q×f and τ_f of the YAG–1 wt% TEOS–1 wt% TiO₂ ceramic obtained at 1520 °C are 97.6%, 9.9, 71, 738 GHz and −30 ppm/°C, respectively.     

Binary transparent (Ho1-xDyx)2O3 ceramics: Compositional influences on particle properties,sintering kinetics and Faraday magneto-optical effects

Binary transparent magneto-optical (Ho_1-xDy_x)₂O₃ (x = 0.01–1) ceramics derived from layered rare-earth hydroxide (LRH) compounds were fabricated by vacuum sintering. They have in-line transmittances of ～67?77 % at the visible wavelength of 700 nm and ～77?84 % at the mid-infrared wavelength of 5 μm with similar maximal infrared cut-off at ～9.5 μm. The impacts of Dy³⁺ doping on particle properties, sintering kinetics and Faraday magneto-optical effects were systematically investigated. The results show that (1) The LRH precursors exhibit the nanosheet morphology with the thickness of ～6?10 nm. Dy³⁺ incorporation not only induces an expansion for the hydroxide host layer but also a contracted interlayer distance; (2) Upon calcination at 1100 °C, the LRH nanosheets collapse into sphere-like oxide particles. The addition of Dy³⁺ leads to increasing lattice constants and decreasing theoretical densities for the (Ho,Dy)₂O₃ solid solutions; (3) A smaller bandgap energy for Dy₂O₃ (～4.85 eV) was obtained relative to those of (Ho_0.9Dy_0.1)₂O₃ (～5.24 eV) and Ho₂O₃ (～5.31 eV); (4) Dy³⁺ dopant promotes grain growth and the pure Dy₂O₃ bulk has a rather smaller grain-boundary-diffusion controlled activation energy (～457 kJ/mol) than the (Ho_0.9Dy_0.1)₂O₃ counterpart (～626 kJ/mol); (5) The Verdet constants of magneto-optical (Ho_1-xDy_x)₂O₃ ceramics generally linearly increase with the rise of Dy³⁺ concentration.     

助烧剂和造孔剂对真空烧结SiC多孔陶瓷性能的影响

以粒度≤0.063mm的SiC为主要原料,分别加入30%(质量分数)的Al2O3-Y2O3与10%的Al2O3-高岭土复合助烧剂,并外加不同量(分别为12.8%、26.3%、30.0%和36.4%)的造孔剂羧甲基纤维素钠(CMC),制样后首先在空气炉中经过300℃2h或1100℃4h的预烧,然后在真空炉中于1550℃4h真空烧结而制备成SiC多孔陶瓷,并研究了助烧剂种类以及造孔剂CMC外加量对SiC多孔陶瓷显微组织、显气孔率及抗折强度的影响。结果显示:采用Al2O3-Y2O3作为助烧剂的SiC多孔陶瓷比Al2O3-高岭土作助烧剂的具有较高的抗折强度,显气孔率稍有减小;随着羧甲基纤维素钠量的增加,加入两种助烧剂的SiC多孔陶瓷均表现为显气孔率增加,抗折强度降低。     

Influence of TiO2 additive on sintering temperature and microwave dielectric properties of Mg0.90Ni0.1SiO3 ceramics

(1-x)Mg_0.90Ni_0.1SiO₃-xTiO₂ (x = 0, 0.01, 0.03, 0.05) ceramics were successfully formed by the conventional solid-state methods and characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), and their microstructure and microwave dielectric properties systematically investigated. It was observed that when TiO₂ content increased from 0 to 5 wt%, the Q_uf_o of the sample decreased from 118,702 GHz to 101,307 GHz and increases the τ_f value from −10 ppm/°C to +3.14 ppm/°C accompanied by a notable lowering in the sintering temperature (125 °C). A good combination of microwave dielectric properties (ε_r ∼ 8.29, Q_uf_o ∼ 101,307 GHz and τ_f ∼ −2.98 ppm/°C) were achieved for Mg_0.90Ni_0.1SiO₃ containing 3 wt% of TiO₂ sintered at 1300 °C for 9 h which make this material of possible interest for millimeter wave applications.     

Effect of air annealing on the optical properties and luminescence performance of Ce:YAG ceramics for light-emitting diodes at different Ce concentrations

(Y_1-x%Ce_x%)₃Al₅O₁₂ (x = 0.2,0.4,0.6,0.8,1.0) transparent ceramics were fabricated by vacuum sintering technology, followed by air annealing at different temperatures. Transmittance of ceramics, valence of cerium, and luminescent properties with varying annealing temperatures are studied in detail. The negative effect of Ce³⁺ oxidation induced by annealing gets increasingly evident when Ce concentration increases. Collaborating Ce:YAG ceramics with InGaN blue chips, light-emitting diodes (LEDs) with superior performance were constructed. The relationships between Ce concentration, annealing temperature, and luminous flux of LEDs are elucidated, showing that the optimized annealing temperature of Ce:YAG ceramics decreases from 1200 °C to 900 °C as Ce concentration increases from 0.2 at% to 1.0 at%. The luminous fluxes of optimized LEDs increase by ～10 % compared with that of unannealed LEDs.     

Pressure-assisted microwave sintering: A rapid process to sinter submicron sized grained MgAl2O4 transparent ceramics

This work focuses on the development of an original process based on a 2.45 GHz single-mode microwave cavity equipped with a uniaxial press, to sinter transparent spinel MgAl₂O₄ ceramic in air. The samples were conventionally pre-sintered to a density of 90% TD before microwave sintering to the final stage of densification. The influence of thermomechanical cycle on the material properties was investigated. Transmittance, grain size distribution, hardness and fracture toughness of the samples were measured and correlated to the microstructure. This new sintering process has allowed obtaining transparent samples with sub micrometric grain size and high mechanical properties, with relatively short times and low temperature. These first results can be compared to some obtained by SPS or HIP. The technical input of this method is that all the process is here conducted in air atmosphere.     

Effect of MgO doping on the structure and optical properties of YAG transparent ceramics

The paper studies the features of Mg²⁺ ions as sintering aid for reactive solid-state sintering of YAG transparent ceramics. Phase composition, microstructure and optical properties of YAG ceramics, doped by 0 ÷ 0.15 wt.% MgO, were investigated. Solubility limit of Mg²⁺ ions in YAG crystal lattice was found to be in the range of 0.06 ÷ 0.1 wt.% of MgO additive. Substitution mechanism of Mg²⁺ in ceramic YAG was identified by comparison of XRD data and ab initio calculation. It was shown that within the solubility limit Mg²⁺ ions most likely substitute Al³⁺ sites. Doping by MgO above solubility limit led to precipitation of spinel secondary phases. It was found that doping by Mg²⁺ ions increases concentration of oxygen vacancies in YAG lattice that effectively promote sintering. The optimal concentration range of MgO sintering aid that allow to achieve YAG transparent ceramics was defined as 0.03 ÷ 0.06 wt.%.     

Optical and luminescent properties of quasi-stoichiometric YAG: Cr3+ ceramics

The impact of minor stoichiometric variations on the microstructure, optical characteristics, and luminescent properties of YAG:Cr ceramics, synthesized from chemically precipitated ceramic powders, was assessed for the first time. Transparent ceramics with over 70% transparency was produced with a nominal yttrium excess ranging from 0.47 to −1 mol.%. The phase composition, microstructure, and luminescent properties of quasi-stoichiometric YAG:Cr ceramics were examined, and the impact of stoichiometric deviations on the crystal lattice parameter and average grain size in ceramics was outlined. An examination of the optical characteristics of the ceramics revealed a specific absorption band in the case of yttrium excess. The effect of stoichiometry deviation on the luminescent properties of YAG:Cr ceramics was investigated. A change in stoichiometry from −1–0.47 mol.% excess yttrium resulted in a broadening of the luminescence R-line and a decrease in the lifetime of the excited state of Cr³⁺ from 1.91 to 1.81 ms.     

One-step cold sintering process towards translucent BaF2 ceramics

BaF₂ ceramics were prepared using a one-step cold sintering process with an ultra-low sintering temperature of 150 °C and uniaxial pressures ranging from 450 to 900 MPa. The relative density and microstructure improved steadily with the increasing pressure, and a fully densified microstructure with a relative density of 97.2% was achieved at 900 MPa. For BaF₂ ceramics with a thickness of 1 mm, the optimum in-line transmittance in the visible light region (58.5%) was achieved at a wavelength of 720 nm, and the maximum value (65.3%) was obtained at 1864 nm. The permittivity of the ceramics increased gradually from 6.18 to 7.09 with increasing pressure, and the dielectric loss was optimized from 0.01 to 0.003. Additionally, the mechanical properties improved continuously with the increasing pressure, and the optimal compressive strength (257 MPa), hardness (2.01 GPa), and Young's modulus (54.8 GPa) were achieved when cold sintered at 900 MPa.     

Effect of TiO2 addition on sintering,microstructure and properties of anorthite-based ceramics derived from reduced copper slag

Reduced copper slag is a promising substitute constituent in anorthite-based ceramics. To reduce the sintering temperature of this type of slag-derived ceramics, the role of TiO₂ addition in the sintering, microstructure, physical and mechanical properties, and environmental and appearance performance was studied by experimental and theoretical methods. The results show that the Ti element was dissolved in the glassy phases and anorthite crystals instead of producing new phases. This behavior allows the liquid appearance at earlier temperatures and accelerates the ion diffusion, thus significantly enhancing the sintering and contributing to a dense microstructure with fine closed pores. Besides, this improvement is achieved at a relatively slight sacrifice of anorthite grains. The addition of 4 wt% TiO₂ results in a decrease in the vitrification temperature by 100 ^oC and an enhancement of flexural strength by 50%. Moreover, the slag-derived ceramics show extremely low leaching toxicity and a relatively higher whiteness, ensuring the cleanliness and quality of anorthite-based ceramics. However, the ceramics show lower bulk density and obvious surface defects with the TiO₂ content up to 6 wt%.