Resistance of Si2N2O ceramics to oxidation and erosion of molten silica

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Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si₃N₄-TaC and Si₃N₄-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600 °C using a resistance-heated hot-pressing technique. The addition of 20–40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40 mol% TaC or ZrC at 1500 °C, 0–80 mol% TaC at 1600 °C, and 0–60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si₃N₄ was larger than that of β-SiAlON; α-Si₃N₄ transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si₃N₄-to-β-SiAlON phase transformation. Young's modulus for the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics increased with the carbide amount, and the hardness of dense Si₃N₄-ZrC and Si₃N₄-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si₃N₄ content. Dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics, with larger quantities of α-Si₃N₄ sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si₃N₄ proportion. Both the hardness and fracture toughness of the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics were strongly related to the proportion of α-Si₃N₄ in the sintered body.     

Microstructure and electrical properties of Lu2O3-doped ZnO-Bi2O3-based varistor ceramics

Lu2O3-doped ZnO-Bi2O3-based varistor ceramics samples were prepared by a conventional mixed oxide route and sintered at temperatures in the range of 900-1 000°C,and the microstructures of the varistor ceramics samples were characterized by X-ray diffractometry(XRD)and scanning electron microscopy(SEM);at the same time,the electrical properties and V-I characteristics of the varistor ceramics samples were investigated by a DC parameter instrument for varistors.The results show that the ZnO-Bi2O3-based varistor ceramics with 0.3%Lu2O3(molar fraction)sintered at 950°C exhibit comparatively ideal comprehensive electrical properties.The XRD analysis of the samples shows the presence of ZnO,Bi-rich,spinel Zn7Sb2O12 and Lu2O3-based phases.     

反烧结Si_3N_4复合陶瓷力学性能研究

以TiSi2、SiC和Mo粉为原料,通过反应烧结方法制备Si3N4基陶瓷,并测试其力学性能。结果表明:随着SiC含量的增加,复合陶瓷的硬度和抗弯曲强度先升高然后下降,当SiC含量为40wt%时,复合陶瓷的硬度和抗弯曲强度达到最大值,分别为HRA71和288MPa;随着Mo含量的增加,Si3N4-TiN-MoSi2-SiC复合陶瓷的硬度逐渐提高,而室温抗弯强度则是先增加后减小,当Mo含量为10wt%时,Si3N4-TiN-MoSi2-SiC复合陶瓷的抗弯强度达到最大值,为321MPa。此外随着热震次数的增加,两种复合陶瓷的弯曲强度均下降,在相同的循环次数下,Si3N4-TiN-SiC-MoSi2陶瓷的热震性能优于Si3N4-TiN-SiC陶瓷。     

Preparation and dielectric properties of porous silicon nitride ceramics

Porous silicon nitride ceramics with difference volume fractions of porosity from 34.1% to 59.2% were produced by adding different amount of the pore-forming agent into initial silicon nitride powder. The microwave dielectric property of these ceramics at a frequency of 9.36 GHz was studied. The crystalline phases of the samples were determined by X-ray diffraction analysis. The influence of porosity on the dielectric properties was evaluated. The results show that α-Si3N4 crystalline phase exists in all the samples while the main crystalline phase of the samples is β-Si3N4, indicating that the αβ transformation happens during the preparation of samples and the transformation is incomplete. There is a dense matrix containing large pores and cavities with needle-shaped and flaky β-Si3N4 grains distributing. The dielectric constant of the ceramics reduces with the increase of porosity.     

添加Li2O-MgO-B2O3玻璃的Li2MgTi3O8陶瓷的微波介电性能及其与Ag共烧相容性

研究添加Li2O-MgO-B2O3玻璃对Li2MgTi3O8陶瓷的烧结特性、相纯度、微观组织和微波介电性能的影响。结果表明：添加少量的玻璃能有效地将陶瓷的烧结温度从1025℃降低到875℃,且没有恶化陶瓷的微波介电性能。添加1.5%玻璃的陶瓷在875℃烧结4 h后具有优良的微波介电性能能,其介电常数εr=25.9,品质因数Q×f=45403 GHz,谐振频率温度系数τf≈0。陶瓷和Ag电极共烧几乎不发生化学反应,表现为良好的化学相容性。所制备的陶瓷可望用于低温烧结的多层微波器件。     

Effects of Cr2O3 doping on the electrical properties and the temperature stabilities of PZT binary piezoelectric ceramics

The ceramics with Pb1.04Zr0.52Ti0.48O3 + z wt.% Cr2O3 were prepared using the traditional technique.The effects of Cr2O3 doping on the phase structure,the microstructure,and the electrical properties of ceramics were investigated.Meanwhile,the temperature stabilities of the resonant frequency (fr) were studied.The results showed that the △fr/fr,25℃ decreased with the addition of 0.2 wt.% - 0.8 wt.% Cr2O3 as compared with the undoped samples.The minimum value (-0.182%) of △fr/fr,25℃ was obtained for z = 0.6 wt.% Cr2O3 samples that sintered at 1260℃.The values of εT33/ε0 = 1650,tanδ = 0.006,d33 = 328 pC/N,Kp= 0.63,Qm = 2300 were obtained when Cr2O3 was 0.6 wt.%,which exhibited more excellent piezoelectric properties than other compositions such as those with z = 0.2 wt.%,0.4 wt.%,and 0.8 wt.%,but had a similar value as compared with the undoped samples.When the Cr2O3 additive increased,the Curie temperature moved toward low temperature and the changes of resonant frequency changed from positive to negative with increasing temperature.     

Effects of adding yttrium nitrate on the mechanical properties of hot-pressed AlN ceramics

Aluminum nitride (AlN) ceramics were prepared by hot-pressing with Y(NO₃)₃·6H₂O as sintering additive. The mechanical properties including flexural strength, Vickers’ hardness, and fracture toughness were studied. The relative density and mechanical property of the monolithic AlN were improved by adding Y(NO₃)₃·6H₂O, which decreased the porosity. At 2 wt% Y₂O₃, the AlN ceramic exhibited the highest strength of 383 MPa, the highest hardness of 15.39 GPa, and the highest fracture toughness of 3.1 MPa m^1/2. However, doping with more additive, the strength, hardness, and toughness of AlN ceramics decreased because of the weak interfacial bonding between AlN matrix and the yttrium aluminates phase.     

Effect of Y2O3 on microstructure and mechanical properties of hypereutectic Al-20%Si alloy

The effect of Y203 on the microstructure and mechanical properties of the hypereutectic Al-20%Si（mass fraction） alloy was investigated. The results show that, with the addition of Y203 into the Al-P-Ti-TiC modifier, the average size of primary silicon in the Al-20%Si alloy modified by Al-P-Ti-TiC-Y203 modifier （approximately 15μm or less） is significantly reduced, and the morphology of eutectic silicon changes from coarse acicular and plate like to refined fibrous. The Brinell hardness （HB189） and tensile strength （301 MPa） of Al-20%Si alloy modified by the Al-P-Ti-TiC-Y2O3 increase by 11.6% and 10.7%, respectively, for the alloys aider heat treatment.     

Preparation of pure nano-grained Si2N2O ceramic

Pure Si₂N₂O ceramic is fabricated by nitridizing a powder mixture of Si and SiO₂. Results show that the prepared Si₂N₂O is composed of nano-particles with a size of about 50 nm. Analysis and discussion are focused on the formation mechanism of the nano-grained Si₂N₂O ceramic based on thermodynamics calculation, micro-morphologies and phase composition analysis. Preferential formation of Si₃N₄ on the surface of nano-SiO₂ particles at below melting point of Si and then successive in-situ transformation of SiO₂ and Si₃N₄ into Si₂N₂O at above melting point of Si are considered as the key reasons to form the pure nano-grained Si₂N₂O ceramic.     

Preparation and properties of Ba3ZnzCo2-zFe24O41／SiO2 microcrystalline glass ceramics

Ba3ZnzCo2-zFe24O41/SiO2(Z varies from 0.0 to 1.2 in step of 0.4) microcrystalline glass ceramics were prepared by citrate sol-gel process at 1200℃ in the system of BaO-Fe2O3-CoO-ZnO-SiO2. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The complex dielectric constant and complex permeability of Ba3ZnzCo2-zFe24O41/SiO2 microcrystalline glass ceramics were measured in the range of 0.1-6.0 GHz by transmission/reflection coaxial line method. The natural resonance phenomenon was observed in μ”spectra for all the Ba3ZnzCo2-zFe24O41/SiO2 microcrystalline glass ceramics, which was closely affected by the substitution of Zn^2 ion.     

Effects of B2O3-SiO2 doping on electrical properties of Ba0. 65 Sr0. 35 TiO3 ceramics

Ba0. 65 Sr0. 35 TiOa ceramics doped with B2O3-SiO2 glass composition were prepared by using conventional solid-state reaction method. The effects of glass dopant on the dielectric and ferroelectric properties were investigated. The results show that the dielectric constant decreases while the dielectric loss increases after doping. And as the glass content increases the dielectric constant decreases while the dielectric loss changes slightly. From the complex impedance analysis the resistance and the relaxation time of the grain and the grain boundary can be calculated.Comparing the P--E hysteresis loop of undoped Ba0.65 Sr0.35 TiO3 ceramics with that of B2O3-SiO2 doped Ba0.65 Sr0.35 TiO3 ceramics, it can be seen that the remanent polarization decreases when the B2O3-SiO2 content is lower than 8% (molar fraction), and the coercive field increases with the increase of B2O3-SiO2 content.     

用天然石英粉制备氮氧化硅粉末

以天然石英粉为原料，先进行活化预处理，然后在１３５０℃左右进行碳热还原氮化，制备了氮氧化硅粉末。实验结果表明：氮化过程中ＳｉＯ２的转化率及产物中氮氧化硅（Ｓｉ２Ｎ２Ｏ）的含量与ＳｉＯ２的活化程度、氮化温度及添加剂等因素有关，其中氮化温度以１３５０℃为佳；随着ＳｉＯ２活化程度的提高，其转化率也提高；添加少量的ＳｒＯ，可以进一步提高ＳｉＯ２的转化率。此外，还分析了ＳｉＯＣＮ系统中气体分压对Ｓｉ２Ｎ２Ｏ形成的影响。     

Effect of sintering temperature and composition on microstructure and properties of PMS-PZT ceramics

The piezoelectric ceramics xPb（Mn1/3Sb2/3）O3-（1-x）Pb（Zr1/2Ti1/2）O3 （abbreviated as PMS-PZT） were synthesized by traditional ceramics process. The effect of sintering temperature and the amount of Pb（Mn1/3Sb2/3）O3 （abbreviated as PMS） on phase structure, microstructure, piezoelectric and dielectric properties of PMS-PZT ceramics was investigated. The results show that the pure perovskite phase is in all ceramics specimens, the phase structure of PMS-PZT ceramics changes from tetragonal phase to single rhombohedral phase with the increasing amount of PMS. The dielectric constant εr, Curie temperature Tc, electromechanical coupling factor kp and piezoelectric constant d33 decrease, whereas the mechanical quality factor Qm and dielectric loss tanδ increase with the increasing amount of PMS in system. The optimum sintering temperature is 1 200-1 250 ℃. It is concluded that the PMS-PZT （x=0.07） ceramics sintered at 1 250 ℃ is suitable for high-power piezoelectric transformer. These properties include εr=674.8, tanδ =0.005 25, kp=0.658, Qm= 1 520, d33=230 pC/N, Tc=275 ℃.     

热压烧结Si3N4-Y2O3-La2O3陶瓷材料的研究

采用Y2O3和La2O3为烧结助剂，利用热压烧结法制备自增韧氮化硅陶瓷，通过L9（3^4）正交实验，优化各工艺参数并确定各因素的影响序列，研究结果表明：在该体系中，晶种作用不显著；保温时间的影响最大；最佳工艺参数为1820℃、4h、21950N压力及0．5％（质量分数，下同）晶种。最终原位形成具有较大长径比的β-氮化硅柱状晶，具有较好的综合力学性能。     

电火花线切割加工氮化硅陶瓷表面粗糙度的研究

陶瓷材料特性鲜明，有着广泛的应用前景，但其应用时对表面粗糙度要求高、分析研究高速走丝电火花线切割加工Si3N4陶瓷材料时影响表面粗糙度的主要因素，并提出了改善表面粗糙度的方法。     

First-principles investigations of structural,mechanical, electronic and optical properties of U3Si2-type AlSc2Si2 under high pressure

The structural, elastic, electronic and optical properties for U₃Si₂-type AlSc₂Si₂ compound under pressure were systematically investigated by using the first-principles calculations. The values of elastic constants and elastic moduli indicate that AlSc₂Si₂ keeps mechanical stability under high pressure. The mechanical properties of AlSc₂Si₂ are compared with those of Al₃Sc. The results indicate that AlSc₂Si₂ is harder than Al₃Sc. Anisotropic constant A^U and 3D curved surface of elastic moduli predict that AlSc₂Si₂ is obviously anisotropic under pressure. The electronic structure of AlSc₂Si₂ exhibits metallic character and the metallicity decreases with the elevated pressure. In addition, optical properties as a function of pressure were calculated and analyzed. The present work provides theoretical support for further experimental work and industrial applications.     

Preparation and microwave dielectric properties of Csf/Si3N4 composites

The CJSi3N4 composites were prepared by hot-press sintering method using α-Si3N4 power, short carbon fibers and La2O3-Y2O3 sintering additives. The mechanical and microwave dielectric properties of CjSi3N4 composites were studied and discussed. The results show that the addition of the short carbon fibers can not destroy the relative density of the sintered samples, but it deteriorates the flexural strength of the sintered samples, so the flexural strength of the silicon nitride matrix is the highest among the samples. The real part （ε3 and the imaginary part （ε＇3 of the permittivity of CsfCSi3N4 composites greatly increase with increasing voltmae fraction of the short carbon fibers, achieve the maximum 73.1 and 101.5, respectively. A strong frequency dependence of the imaginary part （ε″） of the permittivity is observed.