压电精密驱动柔性微夹钳设计

为了实现对微纳尺度下物件的精密夹持,建立了柔性微夹钳系统。并对该系统柔性夹钳设计、运动学、动力学和控制方法等进行研究。首先,利用柔性铰链设计方法设计了柔性微夹钳,利用伪刚体法建立了机械的伪刚体模型。接着,以伪刚体模型法建立了系统的运动学模型,即机械放大比和输入刚度等数学模型。然后,利用拉格朗日方法建立了系统的动力学方程,得出系统的自然振动频率。最后,通过ANSYS有限元方法对系统建立的模型进行了仿真分析和验证,此外,利用PID控制算法对微夹钳系统进行实验控制。实验结果表明:跟踪控制结果误差为2.4%;放大比为9.12倍。基本满足微纳尺度下的微夹持工作,其工作精度可达微米级别甚至纳米级别,符合设计要求。     

Low temperature synthesis of plate-like Na0.5Bi0.5TiO3 via molten salt method

In this study, plate-like Na_0.5Bi_0.5TiO₃ (BNT) templates with perovskite structure were obtained by two-step molten salt synthesis (MSS) method at a low temperature. Firstly, Bi₄Ti₃O₁₂ precursors were synthesized at 1030 °C in NaCl–KCl molten salt. Secondly, plate-like Na_0.5Bi_0.5TiO₃ particles with perovskite structure were obtained from plate-like layer-structured ferroelectric ceramic of Bi₄Ti₃O₁₂ by topochemical microcrystal conversion method. Result showed that excessive Na₂CO₃ was beneficial to facilitate the low temperature synthesis. In the case of an excess of 30 mol% Na₂CO₃, plate-like BNT particles could be obtained by synthesis at temperatures ranging from 760 °C to 800 °C, which indicated a flexible processing route. Also, it has been observed that plate-like BNT particles show a high aspect ratio with 1 μm in thickness and 10–20 μm in length. These Na_0.5Bi_0.5TiO₃ plate-like particles can be good candidates for the preparation of lead-free BNT-based piezoelectric ceramics with oriented grain microstructure.     

A compromise between piezoelectricity and transparency in KNN-based ceramics: The dual functions of Li2O addition

Transparent ceramics with good electrical performance have recently drawn broad interest as promising multifunctional materials. Here, we report that a superior transmittance (T = 75 % at 2000 nm) and good piezoelectricity (d₃₃ ∼ 150 pC/N) can be simultaneously realized in 0.93K_0.5Na_0.5NbO₃-0.07SrZrO₃ (KNN-SZ) ceramics by Li₂O regulation. The effect of Li₂O regulation has two parts: first, the presence of Li₂O facilitates the grain growth of KNN-SZ, considering that it melts at a relatively low temperature as a proper sintering aid; second, the introduced Li⁺ causes local lattice distortion, resulting in the coexistence of orthogonal and tetragonal (O–T) phases. The enlarged grains reduce the light scattering by grain boundaries for a higher optical transmittance; meanwhile, large grains stand as a prerequisite for the macroscopic domain structure favoured for decent piezoelectricity, which could also be partly caused by the coexistence of O–T phases. We believe that these findings might make KNN-based ceramics a preferable candidate for optoelectronic devices.     

Grain size dependence of hardness in nanocrystalline silicon carbide

The response of nanocrystalline silicon carbide (nc-SiC) to nanoindentation is investigated using molecular dynamics (MD) simulation. It is found that the hardness of the nc-SiC decreases with decreasing grain size, showing an inverse Hall-Petch relationship. The behavior is primarily attributed to the reduced number of intact covalent bonds with grain refinement. Dislocation nucleation and growth in nc-SiC are strongly suppressed by the grain boundaries (GBs). In addition to the dislocation region in the grains, the indentation-induced amorphization of nanograins proceeds preferentially from the GBs, leading to grain shrinkage until the grains are fully amorphized. The results provide an improved understanding of the mechanical properties in nc-SiC and other nanostructured covalent materials.     

Planar proton-conducting ceramic cells for hydrogen extraction: Mechanical properties,electrochemical performance and up-scaling

Proton-conducting ceramics, which selectively separate H₂ from any hydrogen-containing gas could play a role in the future of the growing hydrogen market. In recent years, membrane technologies related to H₂ extraction became attractive solutions to produce pressurized high-purity hydrogen. Yttrium-doped barium zirconate/cerate materials (BaCe_xZr_1-x-yY_yO_3-δ) are among the most studied and used materials. In this study, symmetrical cells consisting of a protonic electrolyte (BaCe_0·2Zr_0·7Y_0·1O_3-δ (BCZY27), 10–15 μm in thickness) surrounded by two cermet electrodes (BCZY27–Ni (50?50 vol%), 150 μm) were prepared for H₂ extraction applications. The cells were prepared via tape-casting and co-sintered at 1575 °C. The cells were up-scaled to an area of 135 cm². The fracture toughness of the cermet electrodes was determined to be 2.07 (±0.05) MPa · m^1/2 at room temperature using the double torsion technique. Impedance spectra were recorded on the symmetrical cells between 650 and 800 °C in 3% humidified 50% H₂/50% N₂ atmosphere and at 650 °C varying the hydrogen partial pressure (20% < pH₂<100%). In 50% H₂/50% N₂ with 3% H₂O the cells demonstrated an ohmic resistance of 0.59 and 0.44 Ω cm,² an average electrode polarization resistance of 0.10 and 0.09 Ω cm² (per one electrode) at 650 and 800 °C, respectively. Moreover, a stability test was performed over 400 h highlighting the stable electrochemical properties of the symmetrical membranes.     

Exploration on the origin of enhanced piezoelectric properties in transition-metal ion doped KNN based lead-free ceramics

In this work, we studied effects of Ni₂O₃ and Co₂O₃ doping on crystal structures, microstructures, orthorhombic and tetragonal phase transition temperature (T_o-t), and electrical properties of [Li_0.06(Na_0.57K_0.43)_0.94][Ta_0.05(Sb_0.06Nb_0.94)_0.95]O₃ (LNKTSN) lead-free ceramics. The experimental results showed that the Ni₂O₃ addition with appropriate amount could shift the T_o-t downwards to the room temperature, and thus obviously increasing the room-temperature piezoelectric coefficient (d₃₃), dielectric coefficient (ε_r) and electromechanical coupling coefficient (k_p) of the LNKTSN ceramics. These were consistent with previous experimental results obtained in Fe₂O₃ doped LNKTSN ceramics. On the contrary, Co³⁺ doping shifted continuously the T_o-t upward and deteriorated obviously piezoelectric properties of LNKTSN ceramics. Fe, Co and Ni had similar ion radii and were expected to result in the same (donor or acceptor) doping effects on electrical properties of LNKTSN ceramics. The different doping effects between Co³⁺ (deterioration) and Ni³⁺ or Fe³⁺ (improvement) on the electrical properties of LNKTSN ceramics suggested that the coexistence of orthorhombic and tetragonal phases at room temperature due to downward shift of T_o-t, rather than ion doping (donor or acceptor doping) effects was the main cause for enhanced room-temperature piezoelectric properties. This conclusion can be extended to all KNN-based materials in general, thus offering principle guide for future development of new lead-free materials with good piezoelectric properties.     

A novel approach to fabricate porous alumina ceramics with excellent properties via pore-forming agent combined with sol impregnation technique

An innovative approach for fabricating porous alumina ceramics (PACs) with improved mechanical and thermal properties using walnut shell powders as pore-forming agent combined with alumina sol impregnation is reported in the present work. It is demonstrated that uniform distribution of spherical pores can be observed in as-prepared PACs by using above technical route. The decrease of walnut shell powder sizes significantly promotes the enhancement of crushing strength and reduction of thermal conductivity of the PACs. Meanwhile, the impregnated alumina sol is favoring for the formation of spherical micro-pores, then further improves their mechanical and thermal insulation performances. The lowest thermal conductivity and highest crushing strength of resulting sample reach 0.16?W/m?K and 29.2?MPa, respectively. This novel method offers new possibilities to fabricate high-quality PACs.     

Transient liquid phase diffusion process for porous mullite ceramics with excellent mechanical properties

Porous mullite ceramics were fabricated by the transient liquid phase diffusion process, using quartz and fly-ash floating bead (FABA) particles and corundum fines as starting materials. The effects of sintering temperatures on the evolution of phase composition and microstructure, linear shrinkage, porosity and compressive strength of ceramics were investigated. It is found that a large amount of quartz and FABA particles can be transformed into SiO₂-rich liquid phase during the sintering process, and the liquid phase is transient in the Al₂O₃-SiO₂ system, which can accelerate the mullitization rate and promote the growth of mullite grains. A large number of closed pores in the mullite ceramics are formed due to the transient liquid phase diffusion at elevated temperatures. The porous mullite ceramics with high closed porosity (about 30%) and excellent compressive strength (maximum 105?MPa) have been obtained after fried at 1700?°C.     

Porous alumina ceramics with enhanced mechanical and thermal insulation properties based on sol-treated rice husk

To improve the properties of porous alumina ceramics, which were typically prepared by adding pore-forming agents, rice husk (RH) as pore-forming agent was pretreated with zirconia sol. The effects of sol-treatment on the thermal conductivity and compressive strength of the resultant ceramics were characterized. Furthermore, the pore size distribution, pore shape, microstructure, and phase evolution also were studied. The results showed that the RH pretreatment optimizes the microstructure of the ceramic pores. Moreover, complete morph-genetic RH is clearly observed in the pores, which is established as a key factor in improving the properties of the resultant ceramic. The thermal insulation properties are determined to significantly improve, although the thermal conductivity increases slightly with the increment of zirconia sol concentration from 5 to 10?wt%. Meanwhile, after sintering at 1550?°C, the compressive strength is significantly greater for the specimen prepared with 10?wt% zirconia sol-treated RH (65.56?MPa) than that with untreated RH (43.37?MPa). Hence, it was demonstrated that the use of zirconia sol-pretreated RH as a pore-forming agent could enhance the mechanical and thermal insulation properties of porous alumina ceramics.     

Preparation and temperature-resistance characteristics of novel dense SiAlCN ceramics

The compact green bodies, prepared via a novel solid-liquid mixing method of precursors, were successfully pyrolyzed to obtain the dense bulk SiAlCN ceramics at 1000 °C. It can be seen from their SEM that they have uniform and dense microstructure, indicating that this method can be used to prepare bulk ceramics. In order to verify that they can be used as sensor heads, their temperature-resistance characteristics and repeatability were tested. The results show that the conductive mechanism belongs to Arrhenius's Tailed-State and Extended-State in the temperature range of 500–650 °C and 650–930 °C, respectively. And it shows that SiAlCN ceramics can be used as the sensor heads for high-temperature sensors.