Lost Mold Rapid Infiltration Forming of Mesoscale Ceramics: Part 1, Fabrication

Free-standing mesoscale (340 μm × 30 μm × 20 μm) bend bars with an aspect ratio over 15:1 and an edge resolution as fine as a single grain diameter (∼400 nm) have been fabricated in large numbers on refractory ceramic substrates by combining a novel powder processing approach with photoresist molds and an innovative lost-mold thermal process. The colloid and interfacial chemistry of the nanoscale zirconia particulates has been modeled and used to prepare highly concentrated suspensions. Engineering solutions to challenges in mold fabrication and casting have yielded free-standing, crack-free parts. Molds are fabricated using high-aspect-ratio photoresist on ceramic substrates. Green parts are formed using a rapid infiltration method that exploits the shear thinning behavior of the highly concentrated ceramic suspension in combination with gelcasting. The mold is thermally decomposed and the parts are sintered in place on the ceramic substrate. Chemically aided attrition milling disperses and concentrates the as-received 3Y-TZP powder to produce a dense, fine-grained sintered microstructure. Initial three-point bend strength data are comparable to that of conventional zirconia; however, geometric irregularities (e.g., trapezoidal cross sections) are present in this first generation and are discussed with respect to the distribution of bend strength.     

Piezoelectric Fibers with Uniform Internal Electrode by Co-Extrusion Process

Piezoelectric fibers with internal electrodes were fabricated by the co-extrusion process. The initial feedrods, which were composed of an outer piezoelectric PZN–PZT layer, a thin conducting PZN–PZT/Ag layer inside, and fugitive carbon black at the center, were co-extruded through a reduction die (1 mm) to form a continuous fiber. After thermal treatment and sintering, the PZN–PZT/Ag layer became the inner electrode, while the carbon black at the center was removed by oxidation to form an empty space. Three different types of fibers were produced: (i) solid fiber filled with an inner electrode, (ii) hollow fiber clad with a uniform inner electrode, and (iii) hollow fiber clad with a partial inner electrode. The piezoelectric properties of the fibers were evaluated in terms of their longitudinal strain (s₃₁) or transverse displacement. When the dimensions of the fiber were 840 μm (outer diameter) × 420 μm (inner diameter) × 40 mm (length), the longitudinal strains of the solid fiber with the inner electrode and hollow fiber clad with the uniform inner electrode were 5.25 × 10⁻⁵ and 8.5 × 10⁻⁵ m/m, respectively, under an applied voltage of 100 V (0.48 kV/mm) at a frequency of 100 Hz. For the hollow fiber clad with a partial inner electrode with the same dimensions, the transverse displacement was 80 μm under the same applied electric field.     

Characterization and control of plastic deformation in mesoscale premolded components to realize in‐mold assembled mesoscale revolute joints

This article reports a mold design strategy and a detailed mechanics‐based modeling approach to characterize and control the plastic deformation of premolded components during in‐mold assembly of mesoscale revolute joints. The following new results are reported in this article. First, a mesoscale mold design with varying cavity shape is described to perform in‐mold assembly of the mesoscale revolute joint. Second, a transient computational fluid dynamics (CFD) modeling approach to determine the forces experienced by the mesoscale parts due to injection molding is described. Finally, a mechanics‐based model approach developed using a combination of experimental materials property data and the CFD results as input to a finite element simulation of the deformation response of the mesoscale part is presented for the determination of critical mold design parameters that are necessary for repeatable fabrication of articulating mesoscale revolute joints. Using the advances reported in this article a mesoscale revolute joint has been successfully molded. To the best of our knowledge, this is the first demonstration of in‐mold assembly process using a varying cavity shape mold tocreate an articulating mesoscale revolute joint. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Technique for the R-Curve Determination of Y-TZP Using Indentation-Produced Flaws

A technique was developed to estimate the R -curve behavior of Y-TZP using indentation-produced flaws. This technique allows crack-growth resistance to be determined for short crack extensions (micrometers). For Y-TZP samples sintered at 1600°C for 1 h (mean grain size, 0.7 μm), the crackgrowth resistance increases rapidly over the first 10 μm. Peak toughness values determined from this technique correlate with chevron-notch fracture toughness if the ellipticity of the indentation-produced flaw is considered.     

Strain-Rate-Dependent Transformation Behavior of a 9 mol% Ce-TZP Ceramic

The strain rate dependence of the stress-induced t–m transformation behavior of a Ce-TZP ceramic with a grain size of 1.1 μm has been investigated both in four-point bend and Charpy impact tests. The crosshead speed was varied from 8.3 × 10⁻⁸ m/s to 8.3 × 10⁻³ m/s in the four-point bend tests. In the impact tests, the striking velocity was 2.5 m/s. Double notches ( a/w = 0.5) were made on one side of the specimen to measure the stress-induced t–m transformation zone size at the nonpropagated notch tip. It was found that a transition of the transformation zone size with crosshead speed occurred at about 10⁻⁶ to 10⁻⁵ m/s. Above this critical speed, the transformation zone was at least an order of magnitude larger in size. This new strain-rate-dependent transformation behavior has not been previously reported.     

Sintering and Properties of Monazite-Type CePO4

Monazite-type CePO₄ powder (average grain size 0.3 μm) was dry-pressed to disks or bars. The green compacts began to sinter above 950°C. Relative density ≧ 99% and apparent porosity <1% were achieved when the specimens were sintered at 1500°C for 1 h in air. The linear thermal expansion coefficient and thermal conductivity of the CePO₄ ceramics were 9 × 10⁻⁶/°C to 11 × 10⁻⁶/°C (200° to 1300°C) and 1.81 W/(m · K) (500°C), respectively. Bending strength of the ceramics (average grain size 4 μm) was 174 ± 28 MPa (room temperature). The CePO₄ ceramics were cracked or decomposed by acidic or alkaline aqueous solutions at high temperatures.     

Effects of geometry and injection‐molding parameters on weld‐line strength

Polymeric flows in microchannels are found to differ significantly from those in macrogeometries. Increasing the mechanical properties of microstructures is one of the most important issues in injection‐molding processes. Weld‐line characteristics of structures with different cross‐sections are investigated in this study. The effects of process parameters and cross‐sectional dimensions on the tensile strength of a weld line are discussed. A mold was designed in such a way that specimens with and without weld lines can be developed separately. Five specimens, with different cross‐sections, are injection‐molded simultaneously. Both polypropylene (PP) and high density polyethylene (HDPE) are used in this study. With the Taguchi method, four process variables: melt temperature, mold temperature, injection speed, and packing pressure were found to be the most influential. Experimental results show that the weld‐line strength from a standard test is not applicable in microinjection molding. The microstructure of weld lines is clearly observed from the micrographs. POLYM. ENG. SCI., 45:1021–1030, 2005. © 2005 Society of Plastics Engineers     

A potential method for electrochemical micromachining of titanium alloy Ti6Al4V

The micromachining of complex three-dimensional microstructures (bulk micromachining) on metals can be applied to fabricate many novel devices for micro electromechanical system (MEMS), which will greatly benefit the development of MEMS. In this paper, a new electrochemical micromachining method named the confined etchant layer technique (CELT) was explored on the micromachining of the titanium alloy. Micro-scale trapezoidal slots were replicated on titanium alloy by using a mold with the corresponding negative microstructures (trapezoidal teeth). The machining resolution reached 0.503 μm. The electrochemical mechanisms involved in the process are analyzed and the parameters that influenced the machining resolution are discussed. A. Attia is on leave from Physical Chemistry Department, National Research Centre, El-Tahrir St., Dokki, Cairo, Egypt.     

Grain-Size Dependence of Thermal-Shock Resistance of Yttria-Doped Tetragonal Zirconia Polycrystals

Thermal-shock fracture behavior of yttria-doped tetragonal zirconia polycrystals (Y-TZP) of various grain sizes was evaluated by the quenching method using water as the quenching solvent. The tetragonal-to-monoclinic phase transformation behavior of Y-TZP around cracks introduced by thermal stress was investigated by using Raman microprobe spectroscopy. The critical quenching temperature difference (Δ T _c ) of Y-TZP ceramics increased from 250° to 425°C with increasing grain size of zirconia from 0.4 to 3.0 μm, while the fracture strength decreased from 900 to 680 MPa. The improvement of Δ T _c of Y-TZP with increasing grain size of zirconia corresponded with the quantity of tetragonal-to-monoclinic phase transformation around cracks introduced by thermal stress.     

Residual wall thickness of water‐powered projectile‐assisted injection molding pipes

Water‐powered projectile‐assisted injection molding (W‐PAIM) is an innovative molding process for the production of hollow shaped polymer parts. The W‐PAIM utilizes high pressure water as a power to drive a solid projectile to displace the molten polymer core to form the hollow space. The residual wall thickness (RWT) and its distribution are the important quality criteria. The experimental and numerical investigations were conducted. Experimental specimens showed that the RWT of a W‐PAIM pipe was much thinner than that of a water‐assisted injection molding pipe. The cross‐section size of the projectile defined the basic penetration section size. The software FLUENT was used to obtain the instantaneous distributions of the flow field, which revealed the forming mechanism of the RWT. The experiments indicated that the processing parameters, such as melt temperature, melt injection pressure, mold temperature, and water injection delay time had obvious effects on the RWT, while the water pressure had little effect on it. The RWT of curved pipes was thin at the inner concave side while thick at the outer convex side. The RWTs at the bend portion are influenced by the deflection angle and bending radius, which is due to the pressure difference between the two sides. POLYM. ENG. SCI., 59:295–303, 2019. © 2018 Society of Plastics Engineers     

Quantitative Electron Microscopic Investigation of the Pore Structure in 10:90 Colloidal Silica/Potassium Silicate Sol-Gels

Transmission electron microscopy (TEM), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and nitrogen sorption technique (BET) were utilized to characterize the microstructure of a 10:90 wt% colloidal silica/potassium silicate gel as first described by Shoup. Gels in the unsintered state (15% theoretical density) were prepared for microscopy by the techniques of ultramicrotomy, Pt/C replication, and pore casting. Electron microscopic images of the ultramicrotomed thin sections (70 nm) show that the unfired gel possesses three distinct species of pores which are referred to as the micropores, mesopores, and macropores. The average micropore diameter was found to be 4 nm as determined by nitrogen desorption. Quantitative stereological analysis of the ultramicrotomed sections indicated that the average circular and lengthwise dimensions of the cylindrical mesopores were 0.15 and 0.39 μm, respectively. Similarly, this same analysis determined the average spherical macropore diameter to be 0.83 μm. In contrast, MIP results suggested that these gels possessed a unimodal pore size distribution centered around the 0.2-μm pore size. The discrepancy between MIP and microscopy can be explained by viewing the void space as a pore-throat network. Experimental evidence for this type of pore geometry was obtained from stereo pairs of Pt/C replicas and thick microtomed sections (0.5 μm) which gave information about particle connectivity and pore casts which depicted the pore connectivity in three dimensions.     

Ceramic micro parts. Part 1: How thermal debinding can be utilized to enhance surface finish and mechanical properties

Thermal debinding represents the most critical processing step of powder injection moulding (PIM) of ceramics. Defects such as cracks and pores might be caused, when the process cannot be controlled properly. Considering low-pressure injection moulding (LPIM), however, thermal debinding opens up the possibility to enhance the mechanical properties of ceramic micro parts. In this study, the unique effect of surface defect healing is presented, which takes place during debinding. It enables improved surface finish and results in increased mechanical strength and reliability. As a model, 3Y-TZP micro bending bars with dimensions of 200 μm × 200 μm × 1200 μm were selected. It could be revealed that thermal debinding can be utilized to increase the characteristic 3-point-bending strength up to 3235 MPa with Weibull modulus of 21.4. This result corresponds to macroscopic bending strength of 1727 MPa, which can be achieved only by exhausting fabrication methods and surface post-processing.     

Wick Debinding of Molybdenum-Silicon-Boron Extrudates

Wick debinding was investigated as possible means of binder removal for the Mo-Si-B extrudates. With increased temperature the amount of binder wicked out of the extrudate increased because of a decrease in viscosity, 2513 mPa·s at 75°C down to 7 mPa·s at 250°C. The permeability of the binder in the 1 μm wicking powder was higher than that in the 0.05 μm powder, 3.1 × 10⁻¹³ m² compared to 6.024 × 10⁻¹⁵ m². The amount of binder removed at a given temperature was considerably lower, which could be attributed to the larger capillary pressure difference between the 0.05 μm wick and the extrudate (1.55 × 10⁵ Pa compared to 1.46 × 10⁴ Pa for the 1 μm powder). Wicking removed approximately 80% of the binder in <10 h at 250°C with no defect formation.     

Grain-Size Dependence of Sliding Wear in Tetragonal Zirconia Polycrystals

Using a pin-on-plate tribometer with the reciprocating motion of SiC against yttria-doped tetragonal zirconia polycrystal (Y-TZP) plates, the friction and wear of Y-TZP ceramics were investigated as a function of grain size in dry N₂ at room temperature. The results showed that the overall wear resistance increased as the grain size of Y-TZP ceramics decreased. For grain sizes ≤0.7 μm, the wear results revealed a Hall-Petch type of relationship ( d ^−1/2) between wear resistance and grain size. In this case, the main wear mechanisms were plastic deformation and microcracking. For grain sizes ≥0.9 μm, the wear resistance was proportional to the reciprocal of the grain diameter. In this regime, delamination and accompanying grain pullout were the main mechanisms. In this case, the phase transformation to monoclinic zirconia had a negative effect on the wear resistance of TZP ceramics. The coefficient of friction tended to be higher for fine-grained TZP-SiC couples than for coarse-grained TZP-SiC couples, whereas, for a specific regime of grain size, the coefficient of friction was almost independent of the grain size.     

Low-Temperature Aging of t'-Zirconia: The Role of Microstructure on Phase Stability

Polycrystalline, tetragonal ( t ') zirconia samples containing 3 and 4 mol% yttria were fabricated by annealing pressureless-sintered samples in air at ∼ 2100°C for 15 min. The grain size of these fully tetragonal samples was on the order of 100 to 200 μm. Domain structure of the samples and of a 3-mol%-yttria-doped tetragonal zirconia single crystal was examined by transmission optical microscopy under polarized light and by transmission electron microscopy. The orientations of the domain/colony boundaries were in accord with the predictions of group theory. As-polished surfaces of polycrystalline t ' materials showed no monoclinic phase even after 1000 h at 275°C in air. By contrast, conventionally yttria-doped tetragonal zirconia polycrystalline (Y-TZP) ceramics of grain size >0.5 μm showed substantial transformation. Surface grinding enhanced the resistance to degradation of Y-TZP but decreased that of t ' materials. Even then, the t ' materials exhibited better resistance to degradation than the Y-TZP ceramics. Excellent resistance of the t ' materials to low-temperature aging despite a very large grain size and the opposite effect of grinding on phase stability are all explained on the basis of ferroelastic domain structure of these materials.     

Growth of Potassium Titanate Fibers in Flowing N2 Gas

The length of potassium titanate fibers produced by several conventional methods averages 50 μm, with a maximum of 100 μm. Extremely long fibers (most >1000 μm long) were obtained by calcination in N₂ gas flowing at 5.2×10^-4 m/s.     

换气管弯管抽芯脱模机构设计

介绍了一种顶部为S异形弯管状,竖直固定段侧壁带三角形扣位的塑料制品的脱模机构。弯管段设计由两段碰接组合侧向抽芯完成脱模,竖直段由带推杆的滑块组合机构完成脱模,弯管段两端部特征设计由两个斜滑块完成脱模。抽芯机构结构合理,易加工,降低生产成本,工作稳定可靠。     

Vacuum-Assisted Microfluidic Lithography of Ceramic Microstructures

The method to fabricate complex shaped micro-patterned ceramic structures has been developed. Vacuum-assisted infiltrating the suspensions to the micro channels generated by the contact of polydimethylsiloxane mold to the substrate enables simple micro patterning of ceramics with complex structures in a relatively large area in short time. The use of well-dispersed ethanol-based suspensions of solid loading ∼20 vol% plays an important role in a successful pattern formation without defects. The current process, called microfluidic lithography, is applicable to the entire range of ceramic materials which can be processed to colloidal suspension with relatively low viscosity. It is demonstrated that the interdigitated ceramic structures with 50 μm in the width composed of Al₂O₃ and NiO on a Si substrate were fabricated in an area of 5 mm × 5 mm.     

基于MPI和灰色关联异型出风罩注塑参数优化

针对某异型出风罩注塑成型工艺,以聚碳酸酯/丙烯腈-丁二烯-苯乙烯共聚物(PC/ABS)工程塑料合金为填料,运用Moldflow软件对其注塑过程进行模流分析,通过田口实验设计研究了熔体温度、保压时间、保压压力、注射时间和模具温度对塑件收缩率和翘曲变形量的影响,得到它们对塑件收缩率的影响次序为:保压时间>熔体温度>保压压力>注射时间>模具温度,对翘曲变形量的影响次序为:保压压力>注射时间>熔体温度>保压时间>模具温度。基于灰色关联分析,获得了最优组合工艺参数,即:熔体温度280℃、模具温度为65℃、注塑时间2.1 s、保压时间11 s、保压压力21 MPa。优化后的仿真结果表明,塑件的体积收缩率为6.523%、翘曲变形量为0.80 mm,比灰色关联次序中位组合的样本数据分别降低6.9%和15.8%,并获得最大注射压力为20.34 MPa、最大锁模力为3.25×10^5 N,为后期模具的设计和注塑参数设定提供了有力的参考,缩短了模具开发周期。     

The Wulff Shape of Alumina: IV. Ti4+-Doped Alumina

Controlled-geometry cavities, initially ≈20 μm × 20 μm × 0.5 μm, were introduced into     -plane titanium-doped (≈210 ppma;≈500 wt. ppm) sapphire substrates using photolithographic methods, and subsequently internalized by diffusion bonding. The samples were annealed in air for prolonged periods at 1600° and 1800°C to convert the titanium to the 4+ state and to allow the pore shapes to adjust. Pores with an equivalent spherical radius of ≈3.6 μm reached a quasi-equilibrium shape within 160 h at 1600°C and within 48 h at 1800°C. The Wulff shape was determined using optical microscopy, scanning electron microscopy, and atomic force microscopy. The Wulff shape of Ti⁴⁺-doped alumina includes well-defined c(0001),     , and     facets and smoothly curved sections. The     and a     facets, which are components of the Wulff shape of undoped sapphire, are not discernable. In contrast to undoped alumina, for which the r-plane has the lowest energy, the c-plane is the lowest energy plane in Ti⁴⁺-doped alumina. The surface energy sequence of the stable c, r, and p surfaces differs from that in undoped alumina. The Wulff shape varies with temperature. Samples equilibrated at 1800°C were re-annealed at 1600°C. Pore shape changes were reversible, indicating that the observed pore shapes were close to the equilibrium (Wulff) shape.