Content and diffusion of water and gases in MgAl2O4 spinel crystals synthesized to produce ceramics

Content, evolution and diffusion characteristics of water and gases in fine-crystalline spinel MgAl₂O₄ were studied by kinetic thermodesoption mass spectrometry. Water is the main volatile component by quantity in the spinel structure. From the spinel crystals with an average size of 0.52 μm, water is released in vacuum in three temperature ranges: at 100–200 °C due to desorption from micropores, at 300–600 °C due to near-surface dehydroxylation and at 500–800 °C due to diffusion of water from the crystal bulk. The content of structural water, diffusively released from the crystals, is about 3000 ppm. The coefficients and activation energy of diffusion of water from spinel crystals in the range 500–700 °C were calculated. This allows us to estimate at any temperature the degassing time of the spinel with a certain degree of dispersion and ceramics made of it, and thereby promote the production of high-quality ceramics.     

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.     

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.%.     

Effect of lanthanum content on the conduction behaviors and relaxation processes of lead lanthanum zirconate titanate antiferroelectric ceramics

(Pb_1-xLa_x) (Zr_0.92Ti_0.08)_1-x/4O₃ (PLZT x/92/8, x = 3, 5 and 7 at%) ceramics with compositions near the antiferroelectric (AFE)-ferroelectric (FE) phase boundary were fabricated by a solid-state reaction method. The effect of lanthanum content on the conduction behaviors and relaxation processes has been investigated. It was verified that the main phase with orthorhombic structure was formed in all compositions. The increase of lanthanum substitution resulted in an enhancement of diffuse phase transition. Impedance analysis suggested that the ac conductivity decreased with increasing lanthanum content. Moreover, thermally stimulated depolarization current study was utilized to establish the correlation between defect structures and relaxation processes. It showed three peaks with distinct characteristics, which originated from dipole orientation, oxygen vacancy migration and phase transition respectively. The oxygen vacancy-related defects induced by lanthanum doping were mainly responsible for the variation of conduction behaviors and relaxation processes.     

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.     

Effects of Ga substitution for Al on the fabrication and optical properties of transparent Ce:GAGG-based ceramics

Ce³⁺-doped Gd₃(Al_1-xGa_x)₅O₁₂ (Ce:GAGG) transparent ceramics were successfully prepared via a solid state reaction/oxygen sintering method. The effects of Ga substitution on the structure and optical properties of the ceramics were investigated. The highest quantum yield and relatively high scintillation light yield were achieved in the Gd₃(Al_0.6Ga_0.4)₅O₁₂. The investigated processing technique demonstrated advantages such as increased flexibility and short processing time, thus being very cost effective. The investigated approach provides a much more economical alternative to the conventional melt growth processes used to fabricate single crystals.     

The microstructures and properties of Fe2O3 and TiO2 co-doped corundum ceramics for solar thermal absorbing materials

Solar thermal absorbing materials are the key components of concentrating solar power. In this study, Fe₂O₃ and TiO₂ co-doped corundum ceramics were prepared by pressureless sintering. The effects of different Fe₂O₃/TiO₂ ratios on the phase composition, microstructure, thermal shock resistance and solar absorptance were investigated via XRD and EPMA testing. The results showed that, with the decrease of Fe₂O₃/TiO₂ ratio, the appropriate amount of FeAlTiO₅ would decompose into ferrite particles, which played a bridging role between the corundum grains making the samples have excellent thermal shock resistance. A6 (90% bauxite, 9.5% Fe₂O₃ and 0.5% TiO₂) sintered at 1460 °C had the optimum comprehensive properties, with a bending strength of 154.80 MPa and an absorptance of 89.20% in the spectral range from 0.3 to 2.5 μm. After 30 thermal shock cycles (1000 °C–25 °C, air-cooled), the bending strength of A6 was 222.05 MPa, and the absorptance was 90.40%, which were 43.44% and 1.35% higher than those before thermal shock, respectively. Therefore, it was suitable as an excellent solar thermal absorbing materials.     

CaO含量对刚玉质浇注料性能与显微结构的影响

通过控制铝酸钙水泥加入量,研究了CaO含量分别为0、0.5%、1.3%、1.7%、2.5%、3%、3.5%、4.9%、5.6%和7.5%时对刚玉质浇注料烧结性能、抗热震性和显微结构的影响,同时借助XRD、SEM、EDS和高温热膨胀仪等手段对浇注料的物相变化、热膨胀系数和显微结构进行了分析和观察。结果表明:刚玉质浇注料中CaO含量显著影响着浇注料的烧结性能、抗热震性及其显微结构,当CaO含量低于2.5%时,随着CaO含量的增加,浇注料中片状CA6逐渐增多,并伴随着大量的体积膨胀,导致浇注料体积密度下降,显气孔率上升;当CaO含量超过2.5%时,随着CaO含量增加,浇注料中CA2相含量增加,CA6相含量逐渐减少,浇注料体积密度开始增加,显气孔率下降,线膨胀率减小,CaO含量高于5.6%的浇注料出现了收缩;刚玉质浇注料的抗热震性与显微结构密切相关,CaO含量为4.9%～7.5%的刚玉质浇注料具有很好的抗热震性,这是由于浇注料基质中粒状CA2与片状CA6相互穿插,刚玉骨料边缘片状CA6向基质中生长,强化了基质与骨料的结合,同时CA2相具有低膨胀性,导致浇注料具有较高的强度保持率和抗热震性。     

Fabrication and optical properties of divalent Cu2+ ions incorporated Ce:YAG transparent ceramics for white LEDs

Transparent Ce:YAG ceramics via Cu²⁺ incorporating annealed at 1450?°C were successfully fabricated by the solid-state method to probe their potential applications in white light-emitting diodes (LEDs). The influence of Cu²⁺ concentration on the microstructure and optical properties of the Ce:YAG transparent ceramics were systematically investigated. The as-prepared ceramics possessed clean grain boundaries and homogeneous grain size distribution ranging from 3.7 to 6.5?µm. With the addition amount of Cu²⁺ increased, the red component of ceramics gradually increased and then decreased, it reached a maximum of 13.0% at 1.5?at% Cu²⁺ incorporation. By combining with commercially blue LED chips (465?nm) directly, the obtained optimal chromaticity coordinates (CIE) and correlated color temperature (CCT) of ceramics were (x?=?0.3335, y?=?0.3412) and 5450?K, respectively, while its color render index (CRI) was nearly 70 at the thickness of 1.0?mm. Therefore, this study provided an efficient approach to tailor the luminescence property of Ce:YAG ceramic for white LEDs.     

Effect of complex Si4++Mg2+ additive on sintering and properties of undoped YAG ceramics

The paper is devoted to studying of Si⁴⁺+Mg²⁺ complex additive for obtaining transparent YAG ceramics for laser applications. Ceramics were fabricated by reactive vacuum sintering of commercial Y₂O₃, Al₂O₃ powders taken in a stoichiometric mixture with TEOS and MgO as sintering aids. Microstructure and optical properties of YAG:Si⁴⁺,Mg²⁺ ceramics were investigated as a function of the Si⁴⁺/Mg²⁺ ratio. It was found that the influence of complex additive does not correspond to the direct superposition of known Si⁴⁺- and Mg²⁺-induced sintering mechanisms and involves interaction between Si⁴⁺ and Mg²⁺ ions during sintering. It was shown that C_Si/C_Mg> 1 provides more effective pore elimination and uniform microstructure when C_Si/C_Mg< 1 gives more intense inhibition of grain grown which may be important for scaling the size of ceramics.     

Effect of NaF doping on the transparency,microstructure and spectral properties of Yb3+: CaF2 transparent ceramics

Yb³⁺:CaF₂ transparent ceramics are promising laser gain media with outstanding performance. However, low transmittance in the visible range is the main challenge that restricts the application of Yb³⁺:CaF₂ ceramics in the laser system. In this paper, a new scheme to eliminate the residual pores in the Yb³⁺:CaF₂ transparent ceramics based on doping of NaF as a sintering aid is proposed. Microstructural characterization indicated that NaF could inhibit the grain growth and increase the transmittance in the visible range significantly. The corresponding transmittance was measured to be 85% at the wavelength of 400 nm. The spectra results showed that co-doped with Na⁺ ions could break the clusters of Yb³⁺ ions and modulate the spectroscopy properties of Yb³⁺: CaF₂ lattice efficiently. This paper proved that doping with NaF is an efficient strategy to improve the transmittance and fluorescence quantum efficiency of Yb³⁺:CaF₂ transparent ceramics.     

Influence of oxygen content on structure and properties of pressurelessly sintered aluminum-nitride- and zirconium-boride-doped silicon-carbide ceramics

SiC is a widely used material. Understanding how oxygen content affects the SiC structure and properties is crucial. In this paper, heat treatment was used to prepare SiC powder samples with different oxygen contents, which were doped with AlN and ZrB₂ and were densified by pressureless sintering at 2050 °C. The effect of oxygen content on the sintered SiC structure was determined by X-ray diffraction analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive spectroscopy. The results indicated that the oxygen content influenced the SiC phase composition, grain boundaries, and densification. Additionally, the interaction between oxygen defects and AlN played an important role in sintering. The nanoindentation, alternating-current impedance, and thermal conductivity of the densified SiC specimens were also evaluated to elucidate the influence of the oxygen content on the densified-SiC functional properties. The results revealed that the oxygen content affected all the measured mechanical, electrical, and thermal properties. Furthermore, surface oxygen impurities suggested that oxygen content had similar critical effects on both the densified SiC structure and properties.     

Effect of powder milling routes on the sinterability and optical properties of transparent Y2O3 ceramics

In ceramic processing, the size distribution of the starting powder to a certain degree is inevitable. It is prerequisite to control the size distribution, which influences the fabrication of a sound green body featuring both smaller pores and a narrower pore structure for full-density sintering facilitated by the easier elimination of pores. The milling process was systematically investigated here to elucidate the effect of powder characteristics on the sinterability and transmittance of Y₂O₃ ceramics. Three types of powder sets having different width of particle size distribution (W_PSD) while keeping the same median size (D₅₀) were prepared by changing the milling condition. By means of narrowing the W_PSD in this research, pore free transparent polycrystalline Y₂O₃ with average grain size of 730 nm was successfully fabricated by hot-pressing at 1500℃, which is 100℃ lower than the previously lowest known sintering temperature.     

Effect of alumina fiber content on pore structure and properties of porous ceramics

Porous Al₂O₃ ceramics with different contents of alumina fibers were prepared by gel-casting process. The effects of Al₂O₃ fiber content on pore size distribution, porosity, compressive strength, and load-displacement behavior of the ceramic materials were investigated. Initial results showed that with the increase of Al₂O₃ fiber content, the pore size and porosity of the material is increased, and the compressive strength is decreased. However, upon increasing the fiber content from 50 wt% to 67 wt%, the performance of the samples changed greatly. The compressive strength of the material increased, while the porosity remained unchanged, the pore size increased greatly, and the shape of the load displacement curve changed. It showed that when the fiber content increased from 50 wt% to 67 wt%, the loading body in the fiber-reinforced porous ceramics changed from particles to fibers.     

Effect of boron content on the microstructure and electromagnetic properties of SiBCN ceramics

Electromagnetic wave (EMW) absorbing materials have excellent potential for various applications in civil engineering and the military. In this study, siliconboron carbonitride (SiBCN) ceramics with excellent EMW absorption capability and oxidation resistance were obtained by adjusting the boron content. The results revealed that the graphite crystallite size in the SiBCN ceramics increased from 3.42 to 3.78 nm, whereas the thickness of the oxide layer decreased from 16.6 to 8.2 μm. The highest electrical conductivity and permittivity for the SiBCN ceramics were obtained when the boron content was 5%. The minimum reflection loss was ?35.25 dB at 10.57 GHz and a ceramic thickness of 2.0 mm. At a temperature of 600 °C, the SiBCN ceramic exhibited excellent EMW attenuation ability; particularly, the minimum reflection loss reached ?29.18 dB at 9.65 GHz and a ceramic thickness of 2.5 mm. The superior EMW absorption properties of the SiBCN ceramics at high temperatures can be ascribed to the synergistic effect of relaxation and conductivity. The results suggest that boron could enhance the transformation of amorphous carbon into crystalline graphite and increase the number of heterointerfaces and conductive paths. This work provides a method for obtaining SiBCN ceramics with excellent EMW absorption properties.     

Effect of Fe2O3 doping on colour and mechanical properties of Y-TZP ceramics

Yttria-stabilized zirconia (Y-TZP) samples with different Fe concentrations were prepared aiming to study the effects of Fe₂O₃ doping on colour and mechanical properties. Since colour is an important optical property for jewellery and watchmaking, the investigation of colour in zirconia ceramics has a great scientific and technological interest. An investigation of the mechanical and optical properties, specifically the colour, was developed starting from commercial partially yttria-stabilized zirconia (Y-TZP) powders produced by Emulsion Detonation Synthesis (EDS). Within the strategies to get colours, the use of colouring oxides such as iron oxide (Fe₂O₃) was the chosen approach. The addition of specific ions into the ZrO₂ matrix can be used to tune zirconia colour without compromising its outstanding mechanical properties. Doping with iron oxide has proved to be a suitable, reproducible and irreversible colouring mechanism, allowing the development of a chromatically beige stable material with respect to its use in different processing conditions such as different atmospheres and temperature ranges. XRD results suggested that iron ions dissolved into tetragonal zirconia phase are at interstitial positions since the unit-cell volume of the tetragonal zirconia increases with increasing iron content. The effect of dopant addition on the mechanical properties of Y-TZP ceramics was also assessed. Compared to the undoped samples, doped ones exhibit a similar Vickers hardness (>1200?MPa) and biaxial flexural strength (>1000?MPa). However, it was observed that Fe₂O₃ doping slightly decreased the fracture toughness of Y-TZP ceramics.     

Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

二次挂浆工艺对铸造用SiC基泡沫陶瓷性能的影响

采用有机前驱体浸渍法制备SiC基泡沫陶瓷过滤器。研究了一次挂浆后的坯体分别经烘干和预烧后进行二次挂浆,制备的泡沫陶瓷在质量增加和抗热震性方面的差异,以及预烧温度对质量增加和抗热震性的影响。结果表明:1)二次挂浆制备工艺制备的SiC基泡沫陶瓷的室温抗压强度明显高于一次挂浆工艺制备的,但抗热震性又明显低于一次挂浆的。2)对二次挂浆制备的泡沫陶瓷,无论是相同的浆料二次挂浆还是不同浆料同样挂浆量挂浆,烘干后直接进行二次挂浆制得的SiC基试样的热震次数要远高于经预烧后进行二次挂浆制得试样的。3)对预烧后二次挂浆制备的泡沫陶瓷,预烧温度越高,其抗热震性越差。     

Effect of CaO doping on densification and microstructure control of highly transparent La0.4Gd1.6Zr2O7 ceramics

Calcium oxide (CaO) as sintering additive was first used to fabricate La_0.4Gd_1.6Zr₂O₇ transparent ceramics by a simple solid-state reaction and one-step vacuum sintering method. The effects of CaO dopant amount on the densification, as well as sintering behaviors and microstructure evolution of the as-fabricated La_0.4Gd_1.6Zr₂O₇ ceramics, were systematically investigated. Under the different sintering temperatures, the relationships during the sintering process between grain growth and zpore elimination were analyzed as well. It was found that 0.1 wt% CaO doping can effectively control the rate of grain growth and promote densification dominated by surface diffusion. Furthermore, Ca²⁺ entered the lattice of La_0.4Gd_1.6Zr₂O₇ ceramics to accelerate ion diffusion and suppress grain boundary migration. With the introduction of 0.1 wt% CaO doping, the highly transparent La_0.4Gd_1.6Zr₂O₇ ceramics (T = 80.4% at 1100 nm) were successfully fabricated at the traditional sintering temperature (1850°C).