Processing YAG/α‐Al2O3 composites via reactive sintering Y2O3/Al2O3 NP mixtures. A superior alternative to bottom up processing using atomically mixed YAlOx NPs

This effort contrasts “bottom‐up” processing of YAG/α‐Al₂O₃ composites where both elements (as 40‐50 nm APSs nanopowders) are present at close to atomic mixing with reactive sintering where ball‐milled mixtures of the individual nanopowders (40‐50 nm APSs) give uniform elemental mixing at length scales closer to 100‐800 nm with correspondingly much longer diffusion distances. In contrast to expectations, densification with control of final grain sizes is best effected using reactive sintering. Thus, reactive sintering to densities ≥95% occurs at only 1500°C with final grain sizes of ≈1000 nm for all samples. In contrast “bottom up” processing to ≥95% densities is only achieved at 1600°C, and with final grain sizes of 1700 nm. The reason for this unexpected behavior is that YAG phase forms early in the bottom up approach greatly inhibiting diffusion promoted densification. In contrast, in reactive sintering, YAG is prevented from forming because of the longer diffusion distances such that densification occurs prior to full conversion of the Y₂O₃ component to YAG. The found hardness values are statistically superior to literature values for composites near the known eutectic composition. In an accompanying paper, the addition of a third component reverses this behavior.     

Sintering parameter optimization of Tb3Al5O12 magneto-optical ceramics by vacuum sintering and HIP post-treatment

Transparent terbium aluminum garnet (TAG) ceramics were achieved by the vacuum sintering plus HIP post-treating from the coprecipitated TAG nanoparticles. The influences of vacuum sintering temperature and sintering aid TEOS on the optical quality of the TAG ceramics were studied. The results show that with the increase of sintering temperature, the optical quality of TAG ceramics is improved gradually, and the in-line transmittance of the TAG ceramics treated at 1720°C for 20 hours under vacuum and then HIP post-treated at 1700°C for 3 hours under 200 MPa argon gas is 81.6% at 1064 nm. The sintering additive TEOS can improve the optical quality of TAG ceramics and inhibit the valence state change of Tb³⁺ ions to Tb⁴⁺ during the annealing process. The Verdet constant of the TAG ceramics at 632.8 nm is about −178 rad·T⁻¹·m⁻¹ at room temperature, which is 1.3 times that of the commercial TGG crystals (−134 rad·T⁻¹·m⁻¹).     

烧结助剂添加量对微波烧结3Y-TZP/Al2O3复相陶瓷性能的影响

本文分别以TiO2和MgO纳米粉体为烧结助剂,采用微波烧结技术制备了3Y-TZP/Al2O3复相陶瓷.研究了烧结助剂含量对材料相组成、致密化及力学性能的影响,通过XRD分析了复相陶瓷中t-ZrO2相的相对量变化,并采用SEM观察了弯曲断裂断口形貌.结果表明:随烧结助剂添加量的增加,微波烧结复相陶瓷的致密度、硬度和弯曲强度均有所增加,均优于传统烧结性能,陶瓷颗粒更细.烧结助剂添加量为0.2wt％ MgO、0.4wt％ TiO2,在1300℃微波烧结30 min时试样的致密度为98.1％,显微硬度和抗弯强度分别达18.9 GPa和626 MPa.     

High Reactive MgO‐Y2O3 Nanopowders via Microwave Combustion Method and Sintering Behavior

To decrease the sintering temperature of MgO‐Y₂O₃ composites to avoid undesired grain coarsening, high reactive MgO‐Y₂O₃ nanopowders were synthesized via microwave combustion method. The degree of combustion was enhanced effectively by adding an extra oxidant ammonium nitrate. The as‐synthesized MgO‐Y₂O₃ nanopowders, ~18 nm in size, showed high specific surface area of 64.55 m²/g and low agglomeration. Relative density of 98% was obtained when sintered at a low sintering temperature of 1350°C. The high reactivity can be attributed to the lower activation energy Q (131.13 kJ/mol), compared with samples without extra oxidant (192.97 kJ/mol).     

Combustion synthesis and characterization of Sr3Al2O6

In this paper, the synthesis by solution combustion (SCS) of tri-strontium aluminates (Sr₃Al₂O₆), using different fuel amounts, has been studied. The main objective was to understand the effect of the amount of fuel on phase formation, yield and purity of the product using urea as fuel. The combustion temperatures of each reaction were followed by digital pyrometry. Phase compositions were determined by X-ray diffraction (XRD). The microstructural characteristics during the SCS process were studied by field emission scanning electron microscopy (FESEM). The specific surface area was calculated with the BET model from adsorption data. The results showed that the Sr₃Al₂O₆ was the main phase obtained in all syntheses. This aluminate with high content of Sr is considered attractive for use in applications requiring radiopacity as biomaterials for dental and bone repair. The maximum measured temperature during synthesis, with excess fuel, was approximately 987°C. It was higher than that obtained in the case of using stoichiometric amounts of fuel which was 762°C. The product obtained in this second case had a submicrometer structure, with highly crystalline particles and a specific surface of 0.21 m²/g. It was higher than that obtained with excess of fuel (0.07 m²/g).     

Gd3Al5O12：Eu^3＋柠檬酸盐硝酸盐燃烧法合成及其发光性能研究

采用柠檬酸盐硝酸盐燃烧法，在较低的温度（900℃）下成功地合成单一晶相Gd3Al5O12：Eu^3＋发光粉体，紫外激发荧光光谱分析表明，粉体615m和593m荧光发射源于Eu^3＋的^5D0-^7F2和^5D0-^7F1跃迁．该方法中各工艺条件（如pH值、柠檬酸／金属离子比、煅烧温度）对Gd3Al5O12：Eu^3＋发光性能均有影响，通过试验得出了获得最佳发光性能荧光粉体的工艺参数．     

Sol–Gel Kinetics Followed by Cylindrical Attenuated Total Reflectance Infrared Spectroscopy

This paper describes a unique solution infrared technique, a cylindrical attenuated total reflectance cell, to monitor in situ kinetics of the sol-gel process, especially at short times after mixing. The sol-gel kinetics of acidic ethanol : TEOS : Al(III) solutions have been followed as a function of temperature and aluminum concentration. The hydrolysis rate and the condensation rate increase with temperature and Al/Si ratio. The Al(III) hydrolysis and concomitant increase in acidity increase the reaction rate. By controlling the initial pH, it is seen that a higher Al(III) concentration itself increases the reaction rate. The properties of a sol-gel glass depend upon the kinetics, which are controlled by the processing conditions, including cation content.     

低温烧结SiC-Al2O3-Y2O3复相陶瓷的研究

以SiC微粉为主要原料，添加适量氧化物复合烧结助剂在相对低的温度下烧结制成性能优良的精细SiC复相陶瓷材料，能满足耐磨陶瓷部件、机械密封件的使用要求，生产成本低，具有较强的实用价值。     

基于FTIR分析的稻草热解机理

The pyrolysis mechanism of rice straw (RS) was investigated using a tube reactor with Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analyzer. The results show that the maximum pyrolysis rate increases with increasing heating rate and the corresponding temperature also increases. The three-pseudocomponent model could describe the pyrolysis behavior of rice straw accurately. The main pyrolysis gas products are H2O, CO2, CO, CH4, HCHO (formaldehyde), HCOOH (formic acid), CH3OH (methanol), C6H5OH (phenol), etc. The releasing of H2O, CO2, CO and CH4 mainly focuses at 220-400°C. The H2O formation process is separated into two stages corresponding to the evaporation of free water and the formation of primary volatiles. The release of CO2 first increases with increasing temperature and gets the maximum at 309°C. The releasing behavior of CO is similar to H2O and CO2 between 200 and 400°C. The production of CH4 happens, compared to CO2 and CO, at higher temperatures of 275-400°C with the maximum at 309°C. When the temperature exceeds 200°C, hydroxyl and aliphatic C H groups decrease significantly, while C O, olefinic C C bonds and ether structures increase first in the chars and then the aromatic structure develops with rising temperature. Above 500°C, the material becomes increasingly more aromatic and the ether groups decreases with an increase of temperature. The aromatization process starts at ≈350°C and continues to higher temperatures.     

Hot Pressing of Infrared‐Transparent Y2O3–MgO Nanocomposites Using Sol–Gel Combustion Synthesized Powders

A sol–gel combustion method has been used to synthesize Y₂O₃–50 vol%MgO composite nanopowders. Solutions of the precursor nitrates were mixed with citric acid and ethylene glycol, heated from 200°C to a predetermined temperature gradually, giving nanocrystalline ceramic powders. The influence of the ratio of yttrium nitrate to the whole precursor mixture and the holding temperature on the properties of the composite nanopowder was investigated using a combination of thermal analysis, X‐ray diffraction, specific surface area analysis, and scanning electron microscopy techniques. When the ratio of yttrium nitrate to the whole precursor mixture reaches 22.5 mol%, the average particle size of synthesized composite nanopowder is 13 nm and the specific surface area is 45.9 m²/g. Then the synthesized Y₂O₃–MgO composite nanopowder was consolidated by the hot‐pressing technique at 1200°C with different dwell time. As a result, the nanocomposite ceramic prepared with a dwell time of 60 min got the highest transmittance of 75% at 5 μm wavelength. The cut‐off wavelength of Y₂O₃–MgO nanocomposite ceramic reaches 9.8 μm, which is superior to other mid‐IR transparent materials. In addition, the fabricated sample is more or less transparent in visible wavelengths and the transmittance at 0.8 μm is as high as 14.5%.     

傅里叶红外光谱测量人造石英晶体的红外质量指标

红外质量指标α值逐步代替Q值成为人工石英晶体的一项重要的测试项目,其测试方法广为关注。文章按照GB/T 3352-94的测试方法和技术指标[1],使用傅里叶红外光谱仪,并采用适当的数据处理方式,探索了一种人造石英晶体α值的测量方法。     

Impact of Eu3+ Dopants on Optical Spectroscopy of Ce3+: Y3Al5O12‐Embedded Transparent Glass‐Ceramics

Transparent glass‐ceramics containing Ce³⁺: Y₃Al₅O₁₂ phosphors and Eu³⁺ ions were successfully fabricated by a low‐temperature co‐sintering technique to explore their potential application in white light‐emitting diodes (WLEDs). Microstructure of the sample was studied using a scanning electron microscope equipped with an energy dispersive X‐ray spectroscopy. The impact of co‐sintering temperature, Ce³⁺: Y₃Al₅O₁₂ crystal content and Eu³⁺ doping content on optical properties of glass‐ceramics were systematically studied by emission, excitation spectra, and decay curves. Notably, the spatial separation of these two different activators in the present glass‐ceramics, where Ce³⁺ ions located in YAG crystalline phase while the Eu³⁺ ones stayed in glass matrix, is advantageous to the realization of both intense yellow emission assigned to Ce³⁺: 5d→4f transition and red luminescence originating from Eu³⁺: 4f→4f transitions. As a result, the quantum yield of the glass‐ceramic reached as high as 93%, and the constructed WLEDs exhibited an optimal luminous efficacy of 122 lm/W, correlated color temperature of 6532 K and color rendering index of 75.     

Y2O3改性Al2O3陶瓷微波介电损耗研究

本文以Al2O为模型材料研究了影响材料介电损耗的因素,发现Y2O3掺杂有利于陶瓷的致密化,并能对Al2O陶瓷的微结构与晶粒尺寸起到调控作用。由于离子半径的差别,Y^3＋离子难以进入刚玉结构的间隙或发生取代置换Al^3＋抖离子形成固溶体,而是与Al2O反应生成Al5Y3O12在Y2O3掺杂改性过程中,非本征因素成为影响Y2O3陶瓷微波介电性能的主要因素,使得Al2O陶瓷微波介电损耗从8．4×10^-5增加到2．2×10^-4。     

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.     

傅里叶变换红外光谱法检验塑料吸管的研究

利用傅里叶变换红外光谱仪,对塑料吸管样本分析检验。依据红外光谱图中吸收峰,可以将塑料吸管样本加以区分。该检验方法不破坏检材,重现性好,可用于鉴定塑料吸管物证。     

Selective Corrosion Preparation and Sintering of Disperse α‐Al2O3 Nanoparticles

Disperse fine equiaxed α‐Al₂O₃ nanoparticles with a mean particle size of 9 nm and a narrow size distribution of 2–27 nm were synthesized using α‐Fe₂O₃ as seeds and isolation via homogeneous precipitation‐calcination‐selective corrosion processing. The presence of α‐Fe₂O₃ acting as seeds and isolation phase can reduce the formation temperature to 700°C and prevent agglomeration and growth of α‐Al₂O₃ nanoparticles, resulting in disperse fine equiaxed α‐Al₂O₃ nanoparticles. These α‐Al₂O₃ nanoparticles were pressed into green compacts at 500 MPa and sintered first by normal sintering to study their sintering behavior and finally by two‐step sintering (heated to 1175°C without hold and decreased to 1025°C with a 20 h hold in air) to obtain nanocrystalline α‐Al₂O₃ ceramics. The two‐step sintered bodies are nanocrystalline α‐Al₂O₃ with an average grain size of 55 nm and a relative density of 99.6%. The almost fully dense nanocrystalline α‐Al₂O₃ ceramic with finest grains achieved so far by pressureless sintering reveals that these α‐Al₂O₃ nanoparticles have an excellent sintering activity.     

Effect of hexagonal-BN additions on the sliding-wear resistance of fine-grained α-SiC densified with Y3Al5O12 liquid phase by spark-plasma sintering

The effect of flaky hexagonal (h) BN additions (1, 5, and 10 vol.%) on the lubricated sliding-wear behavior of fine-grained, liquid-phase-sintered SiC ceramics fabricated by spark-plasma sintering was investigated. It was found that resistance to the initial mild, deformation-controlled wear decreases with increasing h-BN content in the composite, which progressively exhibits a greater wear rate and a sooner transition to severe wear. This is because the softer h-BN particles reduce the hardness and do not act as internal lubricant, while promoting poorer grain cohesion due to their morphologically-favored segregation at grain boundaries. By contrast, their addition is increasingly beneficial in terms of resistance to the subsequent severe, fracture-controlled wear upon prolonged sliding contact, with a lower wear rate. This is because the flaky h-BN particles increase the fracture toughness, and also act efficiently as external lubricant when pulled-out from the microstructure. Finally, implications for the design of advanced triboceramics are discussed.     

Al2O3含量对燃烧合成铝酸钙粉体物相组成的影响

本文以CaO、Al和Al2O3为原料,在氧气气氛下,采用燃烧合成法(CS)制备铝酸钙粉体,计算了CaO-Al-Al2O3-O2体系的绝热温度,结合物质自由能函数的相关理论、X衍射法(XRD),研究了Al2O3含量对燃烧合成铝酸钙粉体物相组成的影响.热力学计算及XRD物相分析结果表明:体系绝热温度随Al2O3含量的增多而降低,但均大于1800 K,说明体系反应可自持;当Al2O3/(Al2O3+ CaO) (mole)分别为0.3,0.47和0.55时,物相组成分别为C3A和C12A7,C12A7和CA,CA和CA2;热力学数据显示C12A7-C3A、C12A7-CA和CA-CA2之间物相可实现转化,但由于燃烧合成反应速度过快及不可控性导致物相之间尚未完成转化,致使燃烧合成铝酸钙的物相生成量与理论量有较大差异.     

Infrared‐Transparent Y2O3–MgO Nanocomposites Fabricated by the Glucose Sol–Gel Combustion and Hot‐Pressing Technique

A glucose sol–gel combustion method has been developed to synthesize composite nanopowders with equal volume fractions of Y₂O₃ and MgO. The synthesis involves the generation of precursor foam containing Y³⁺ and Mg²⁺ cations via the chemical and thermal degradation of glucose molecules in aqueous solutions. Subsequent calcination of the foam gave the composite nanopowders uniform composition and surface areas of 44–62 m²/g depending on the relative amount of glucose. Then the nanopowder with an average particle size of 19 nm was consolidated by the hot‐pressing technique with different sintering temperatures. The fabricated nanocomposite is mid‐infrared transparent as the result of fine grains, narrow grain size distribution, and uniform phase domains. The transmittance increases with increase in the sintering temperature and reaches 83.5% at 3–5 μm mid‐infrared wave range once the temperature reaches 1350°C, which is close to the theoretical value of 85%. And it is noteworthy that the cutoff wavelength reaches 9.6 μm, which is superior to those of spinel, AlON, and sapphire. And the Vickers hardness of the sample reaches 10.0 ± 0.1 GPa, which is significantly higher than those of the coarse grained single‐phase MgO and Y₂O₃. The results indicate that the glucose sol–gel combustion and hot‐pressing technique is an effective method to fabricate infrared transparent Y₂O₃–MgO nanocomposites.