Influence of injection temperature and pressure on the properties of alumina parts fabricated by low pressure injection molding (LPIM)

The quality of ceramics parts made by powder injection molding (PIM) method is influenced by a range of factors such as powder and binder characteristics, rheological behavior of feedstock, molding parameters and debinding and sintering conditions. In this study, to optimize the molding parameters, the effect of injection temperature and pressure on the properties of alumina ceramics in the LPIM process were thoroughly studied. Experimental tests were conducted on alumina feedstock with 60 vol% powder. Injection molding was carried out at temperatures and pressures of 70–100 °C and 0.1–0.6 MPa respectively. Results showed that increase in injection temperature and pressure and the resulting increase in flow rate leads to the formation of void which impairs the properties of molded parts. The SEM studies showed that injection at temperature of 100 °C results in evaporation of binder components. From the processing point of view, the temperature of 80 °C and pressure of 0.6 MPa seems to be the most suitable condition for injection molding. In addition, the effects of sintering conditions (temperature and time) on the microstructure and mechanical properties are discussed. The best final properties were found using injection molding under the above stated conditions, thermal debinding and sintering at 1700 °C during 3 h.     

Preparation of monodisperse PEG hydrogel microparticles using a microfluidic flow-focusing device

A microfluidic assisted preparation method of nearly monosized poly(ethylene glycol) (PEG) microparticles has been described. Three types of microfluidic flow-focusing devices with different geometries were fabricated using polydimethylsiloxane (PDMS). Microdroplets of PEG hydrogel were successfully prepared in the microfluidic flow-focusing devices by adjusting the flow rates of the continuous phase, namely, mineral oil, and the dispersed phase, viz., hydrogel solution. Then, the microdroplets of PEG hydrogel were cured by UV irradiation. Various experimental conditions pertaining to the geometry of the microfluidic flow-focusing device, flow rates of the dispersed and continuous phases, and concentration of PEG hydrogel solution were investigated and optimized to fabricate monosized PEG hydrogel microparticles. The prepared PEG microparticles were nearly monosized in the range of 40 μm to 200 μm in diameter according to the above experimental conditions. Then, PEG hydrogel particles laden with microbeads of 6 μm diameter were fabricated using the microfluidic flow-focusing devices with the optimized conditions.     

High-aspect-ratio,free-form patterning of carbon nanotube forests using micro-electro-discharge machining

This paper reports post-growth processing of vertically aligned carbon nanotube forests for the formation of high-aspect-ratio, three-dimensional microstructures in the material. High-frequency pulses of electrical discharge are generated to locally machine the nanotubes in order to create target shapes in a forest. Machining is performed in both dielectric oil and air. The optimal processing is demonstrated in air with a pulse voltage and peak current of 30 V and 60 mA, respectively, providing a discharge gap of ~ 10 μm. The minimized discharge energy and gap are shown to achieve an aspect ratio of 20 with the smallest feature of 5 μm in forests. Multilayer, three-dimensional geometries with vertical and angled surfaces are successfully obtained without disordering the vertical orientation of the nanotubes. Scanning electron microscopy and energy-dispersive X-ray spectroscopy are used for the surface analysis of the micromachined forests, revealing the dependence of their surface characteristics on the discharge conditions.     

Stabilization of highly-loaded boron carbide aqueous suspensions

Injection molding of boron carbide (B₄C) slurries affords the production of complex-shaped personal armor. To injection mold, however, requires preparation of a well dispersed, flowable suspension with >45 vol% B₄C loadings to reduce porosity that must be removed during sintering. In the present study, the preparation of highly-loaded B₄C suspensions is investigated using zeta potential and rheological measurements, varying dispersant type, molecular weight, and amount. Of those dispersants investigated, polyethylenimine (PEI) with a molecular weight of 25,000 g/mol was found to produce suspensions with up to 56 vol% B₄C and the requisite rheological properties suitable for injection molding. A PEI concentration of 1.83 mg/m² was established as the appropriate to produce highly-loaded B₄C suspensions. The effect of a prior B₄C powder treatment (ethanol washed or attrition milled) on rheological properties of the suspensions was also investigated. The PEI was completely burned out in argon, nitrogen, and air at 450 °C.     

Preparation of Na-beta″-alumina film by tape casting process

This study focused on the preparation of Na-beta″-alumina film by a tape casting process. The effects of solvent, dispersant, binder, plasticizer contents, as well as milling time on the rheological properties of the slurry were investigated. The dispersion of the ceramic powder in the slurry was optimized by ball milling an azeotropic mixture of methyethylketone (MEK) and ethanol (EtOH) as the solvent, 2 wt% triethanolamine as the dispersant, 7 wt% PVB as the binder for 4 h. Green Na-beta″-alumina thick films with thickness of 100–300 μm and homogeneous microstructure were obtained and further sintered at 1600 °C to obtain a relatively dense membrane with a thickness of ca. 100 μm. The conductivity of the sintered ceramic was comparable to that obtained by the conventional isostatic pressing and sintering method.     

Gelcasting of alumina suspensions containing nanoparticles with glycerol monoacrylate

An acrylic monomer of low toxicity containing two hydroxyl groups has been synthesized and used for gelcasting in water. The results have been compared to those achieved with the use of a commercially available monomer (2-hydroxyethyl acrylate). Due to the chemical structure of the synthesized monomer, no addition of the crosslinking agent was necessary for gelation and similar results in terms of rheology of suspensions, density and microstructure of the bodies were obtained with respect to those obtained with the commercial monomer. However, higher time for gelation was observed.Two alumina powders with very different particle sizes were used in this study: a commercial submicron-sized powder (d₅₀ = 0.35 μm) and a nanometer-sized alumina obtained by freeze-drying from aluminum sulphate solutions. The rheological behavior of concentrated suspensions was studied in order to establish their stability and to analyse the effect of the different monomers used in the process. Once the suspensions were optimized, the influence of the size of the powder on the gelation process was studied. The sintered density of submicrometer-sized alumina was higher (99%) than that measured when the bimodal suspension was used due to the difficulty to obtain highly concentrated suspensions from nanometric powder.     

Fabrication of Fine‐Scale 1–3 Piezoelectric Arrays by Aqueous Gelcasting

Soft mold technique combined with a modified gelcasting process was developed to fabricate microscale piezoelectric pillar arrays. This technique involves the fabrication of high‐aspect‐ratio silicon master, generation of negative polydimethylsiloxane mold, and replica molding piezoelectric pillar array by gelcasting. A gelling system based on the polymerization of Hydantion epoxy resin and 3,3′‐Diaminodipropylamine (DPTA) was employed. Weibull analysis revealed that 20.0 wt% Hydantion epoxy resin was the optimized concentration while the highest Weibull modulus and characteristic flexural strength of 14.8 and 90.9 MPa were obtained for lead zirconate titanate sintered bodies, respectively. Excellent piezoelectric pillar arrays with lateral dimensions around 8 μm and aspect ratio higher than five were successfully fabricated.     

Processing and impact behavior of Al/SiCp composites fabricated by the pressureless melt infiltration method

In the current investigation, pressureless melt infiltration was applied to fabricate the Al/SiC composites based on the SiC porous preforms. The process was conducted by introducing the aluminum melt into the SiC preforms at 950 °C under the nitrogen atmosphere, without the aid of pressure. To explore development of melt infiltration, initial preforms were produced with variable SiC fractions (40, 50, and 60 vol.%) using three different SiC powders with the mean particle size of 20, 50, and 90 μm. While the infiltration of aluminum melt into the preforms with 40 vol.% initial SiC volume fraction (SiC particle size of 90 μm) resulted to the composites with final density of 0.94 theoretical density (TD), this value drops down to ～0.9 TD for the composites produced by preforms with the SiC (90 μm) volume fraction of 60 vol.%. On the other hand, composites fabricated by 50 μm SiC powder (SiC volume fraction of 40 vol.%) demonstrated the final density of ～0.91 TD. The impact resistance tests performed on the composites demonstrated an enhancement in the value of impact energy with an increase of SiC powder particle size. Results, additionally, revealed a significant superiority of impact energy for the composites fabricated by a combined melt infiltration and sintering (MIS) procedure compared to those produced by infiltration at 950 and 1350 °C.     

Dice-and-fill processing and characterization of microscale and high-aspect-ratio (K,Na)NbO3-based 1–3 lead-free piezoelectric composites

Piezoelectric composites with high-aspect-ratio lead-free piezoelectric microrods embedded in a polymer matrix were fabricated by applying the modified dice-and-fill method to a (K,Na)NbO₃-based piezoelectric ceramic, 0.9475[Li_0.02(K_0.48Na_0.53)_0.98](Nb_0.80Ta_0.20)O₃–0.0525AgSbO₃–0.5 wt% MnO₂ (abbreviated as LKNNT–AS–M) with enhanced piezoelectricity but low mechanical strength. The Pb-free LKNNT–AS–M/polymer composite specimens containing ∼10–23 vol% ceramic pillars with sectional width ∼50 μm, separation ∼55–110 μm and high aspect ratio ∼11, were prepared and investigated with an emphasis on their electrical properties. The developed composites possess a higher thickness factor k_t ∼63–67%, lower planar factor k_p ∼27–34% and enhanced k_t/k_p ratio ∼2.3. The composites also possess favorable properties which includes low electrical (tanδ ∼0.029) and mechanical (Q_m ∼6) losses, low acoustic impedance Z ∼4.2–7.8 Mrayl and ∼5 times higher voltage coefficient g₃₃ compared to LKNNT–AS–M piezoceramic. The promising results indicate that the LKNNT–AS–M/polymer composites have the potential to be used as active elements in high performance ultrasonic transducers.     

Calcium phosphate substrates with emulsion-derived roughness: Processing,characterisation and interaction with human mesenchymal stem cells

Calcium phosphates (CaP) have been the subject of several studies that often lack a systematic approach to understanding how their properties affect biological response. CaP particles functionalised with a pH-responsive polymer (BCS) were used to prepare microporous substrates (porosity between 70 and 75% and pore sizes of 5–20 μm) through the aggregation of oil-in-water emulsions by controlling solid loading, emulsification energy, pH, drying and sintering conditions. The combined effect of surface roughness (roughness amplitude, R_a between 0.9–1.7 μm) and chemistry (varying Hydroxyapatite/β-Tricalcium phosphate ratio) on human mesenchymal stem cells was evaluated. HA substrates stimulated higher cell adhesion and proliferation (especially with lower R_a), but cell area increased with β-TCP content. The effect of surface roughness depended of chemistry: HA promoted higher mineralising activity when R_a ～ 1.5 μm, whereas β-TCP substrates stimulated a more osteogenic profile when R_a ～ 1.7 μm. A novel templating method to fabricate microporous CaP substrates was developed, opening possibilities for bone substitutes with controlled features.     

Preparation of kanamycin powder by an optimized spray freeze-drying method

The physical and antibiotic properties of kanamycin powders obtained by spray freeze drying (SFD) were compared with those of raw kanamycin. The SFD procedures were optimized to prepare kanamycin for use as an inhaled drug. Scanning electron microscopy (SEM) and a laser particle size analyzer were applied to estimate physical structure and properties of the particle. In addition, the disk diffusion method was used to compare the antibiotic activity of raw kanamycin and that produced by SFD. According to SEM, the kanamycin particles had various sizes and shapes with porous structures at different SFD conditions. The diameters of the kanamycin powders were between 13.5 μm and 21.8 μm, and their aerodynamic particle sizes were between 3.58 μm and 6.39 μm. The antibiotic activities of the raw and spray freeze-dried kanamycin samples were not significantly different (P > 0.05). The optimized conditions for annealing temperature, annealing time, kanamycin concentration, pressure, and nozzle tip lift were ? 15 °C, 5 h, 10% kanamycin, 100 kPa, and, 1 mm, respectively.     

Effect of repeated composite sol infiltrations on the dielectric and piezoelectric properties of a Bi0.5(Na0.82K0.18)0.5TiO3 lead free thick film

Bi_0.5(Na_0.82K_0.18)_0.5TiO₃ lead free thick films have been produced using a combination of screen printing and subsequent infiltration of corresponding composite sol. Their structure, dielectric, ferroelectric and piezoelectric properties were investigated with variation in the number of composite sol infiltrations and the nanopowder loading in composite sol. Dielectric constant, remanent polarization, and piezoelectric coefficient have been shown to increase with increasing numbers of composite sol infiltration. Dielectric and ferroelectric properties of the thick films are found to be strongly dependent on the powder concentration of composite sols. The resulting 40 μm thick films infiltrated with 1.5 g/ml composite sols have maximum relative permittivity of 569 (at 10 kHz), remanent polarization of 21.3 μC/cm², coercive field of 80 kV/cm, and longitudinal effective piezoelectric coefficient d_33eff of 109 pm/V. The performance of these lead free piezoelectric thick films is comparable to the corresponding bulk ceramics.     

Notable grain-size dependence of converse piezoelectric effect in BaTiO3 ceramics

Converse piezoelectric effect is of critical importance to device applications like actuators, but no systematical investigation concerning the influence of microstructure on it has been reported for BaTiO₃ ceramics so far. Piezoelectric and ferroelectric properties were inclusively investigated for a group of BaTiO₃ ceramics that are fabricated through solid-state reaction route and show various average grain sizes in this study. It was found that the piezoelectric properties of these BaTiO₃ ceramics display significant grain-size dependences. The direct piezoelectric coefficient d₃₃ increases with decreasing the average grain size (GS) from 170 pC/N at 40 μm, reaches a maximum value of 413 pC/N at 1.2 μm, and then decreases with a further reduction of GS. Converse piezoelectric effect was characterized by measurement of unipolar strain versus electric field (S–E) curve, and the converse piezoelectric coefficient d₃₃*(E) was quantitatively calculated from the slope of S–E curve at relatively large E. Interestingly, d₃₃*(E) is nearly twice as large as d₃₃ and shows a quite similar trend of change with GS to d₃₃. It increases largely from 350 pm/V to 870 pm/V when reducing the GS value from 40 μm to 1.2 μm, and then decreases to 480 pm/V with the further GS reduction to 0.7 μm. Meanwhile, the remanent polarization P_r shows an increase with the decreasing of GS, reaches a maximum at 3.3 μm, and then decreases with the further GS reduction. Domain structure is considered to play an essential role in determining the notable grain-size dependence of converse piezoelectric effect.     

Influence of barium titanate content and particle size on electromechanical coupling coefficient of lead-free piezoelectric ceramic-Portland cement composites

The 0–3 lead-free piezoelectric ceramic-Portland cement composites were prepared by mixing and pressing the Portland cement (PC) and barium titanate (BT) ceramic powder. The influences of BT particle size and BT content on the electromechanical coupling coefficient (K_t) of the composites were investigated. The results indicate that the particle size of BT used to produce the composite under the conditions of the same BT content (at 50% BT) and fabrication technique has an influence on the K_t values. The electromechanical coupling coefficient was found to increase with the particle size of BT used where the values of K_t are found to be at 10.8% and 14.1% for composites with median particle size of 75 μm and 425 μm, respectively. Furthermore, K_t of composites increase with increasing content of BT (at the same particle size of 425 μm) when the content of BT reaches 70%, K_t is 16.6%. In addition, the acoustic impedance of the composite also increase with an increase of BT content.     

Isobam assisted slurry optimization and gelcasting of transparent YAG ceramics

Gelcasting is a simple near-net shaping method to fabricate large-sized and/or complicated-structural ceramics. In this paper, a transparent yttrium alumina garnet (YAG) ceramic was successfully fabricated by gelcasting with a nontoxic, water soluble copolymer (isobutylene and maleic anhydride, Isobam) as both dispersant and gelling agent. The rheological behaviors of the slurries with different solids loading and Isobam contents were systematically investigated. The optimized slurry of 0.5 wt% Isobam and the solid loading of 68 wt% had the low viscosity and high stability, resulting in better homogeneity of the green body and better optical quality of transparent ceramics. A nearly pore-free structure of the sintered YAG ceramics with average grain size about 10.0 µm was obtained possessing an in-line transmittance of 75.7% at the wavelength of 1064 nm for a sample 2.5 mm thick.     

Fabrication of large diameter SiC monofilaments by polymer route

We demonstrate the possibility to fabricate SiC monofilaments with large diameters of 100 μm by a polymer route using a dry-spinning process. The properties of the spinning solution and the parameters of the spinning process were optimized to achieve a circular cross section of the spun filaments despite their large diameter. The evolution of the diameter and the mechanical properties of the filaments with pyrolysis temperature were studied. Filament shrinkage started above 400 °C. A radial shrinkage of about 25% was measured for pyrolysis temperatures of 1200 °C. The mechanical properties significantly start to increase at pyrolysis temperatures above 600 °C. At a diameter of 100 μm the filaments show a tensile strength of 620 MPa and a tensile modulus of 138 GPa after pyrolysis at 1200 °C. A decrease in the filament diameter leads to an improvement of the mechanical properties. We demonstrate the fabrication of these SiC monofilaments on spools.     

Influence of powder size on characteristics of air plasma sprayed silicon coatings

Silicon powders with different medium sizes (114 μm, 79 μm and 31 μm, respectively) were used to fabricate coatings by air plasma spraying. The velocity and temperature of in-flight silicon particles during plasma spraying were determined. The composition and microstructure of the coatings were characterized and some physical properties of the coatings were measured. The obtained results showed that the size of silicon particles had great influence on their velocity and temperature in plasma flame. The oxidation of silicon particles in the spraying process was observed and is higher for particles of smaller sizes. Areas of silicon oxide in micrometer size are embedded and randomly distributed in the coating. The surface roughness and void content of silicon coatings increase with an increase in the particle size of the powders. The microhardness and oxygen content of coatings decrease with an increase in the particle size. However, the size of silicon particles has little impact on the deposition efficiency of silicon under the same deposition conditions.     

Computational fluid dynamics simulations of submicrometer and micrometer particle deposition in the nasal passages of a Sprague-Dawley rat

Computational fluid dynamics (CFD) simulations were conducted in a model of the complete nasal passages of an adult male Sprague-Dawley rat to predict regional deposition patterns of inhaled particles in the size range of 1 nm to 10 μm. Steady-state inspiratory airflow rates of 185, 369, and 738 ml/min (equal to 50%, 100%, and 200% of the estimated minute volume during resting breathing) were simulated using Fluent?. The Lagrangian particle tracking method was used to calculate trajectories of individual particles that were passively released from the nostrils. Computational predictions of total nasal deposition compared well with experimental data from the literature when deposition fractions were plotted against the Stokes and Peclet numbers for micro- and nanoparticles, respectively. Regional deposition was assessed by computing deposition efficiency curves for major nasal epithelial cell types. For micrometer particles, maximum olfactory deposition was 27% and occurred at the lowest flow rate with a particle diameter of 7 μm. Maximum deposition on mucus-coated non-olfactory epithelium was 27% for 3.25 μm particles at the highest flow rate. For submicrometer particles, olfactory deposition reached a maximum of 20% with a particle size of 5 nm at the highest flow rate, whereas deposition on mucus-coated non-olfactory epithelium reached a peak of approximately 60% for 1–4 nm particles at all flow rates. These simulations show that regional particle deposition patterns are highly dependent on particle size and flow rate, indicating the importance of accurate quantification of deposition in the rat for extrapolation of results to humans.     

Micronization and characterization of squid lecithin/polyethylene glycol composite using particles from gas saturated solutions (PGSS) process

Lecithin was isolated from squid viscera residues after supercritical carbon dioxide (SC-CO₂) extraction at 25 MPa and 45 °C. The particle formation of squid lecithin with biodegradable polymer, polyethylene glycol (PEG) was performed by PGSS using SC-CO₂ in a thermostatted stirred vessel. By applying different temperatures (40 and 50 °C) and pressures (20–30 MPa), conditions were optimized. Two nozzles of different diameters (250 and 300 μm) were used for PGSS and the reaction time was 1 h. The average diameter of the particles obtained by PGSS at different conditions was about 0.74–1.62 μm. The lowest average size of lecithin particle with PEG was found by the highest SC-CO₂ density conditions with the stirring speed of 400 rpm and nozzle size of 250 μm. The inclusion of lecithin in PEG was quantified by HPLC. Acid value and peroxide value was measured after micronization of lecithin.     

Piezoelectric thick films and their application in MEMS

Piezoelectric thick films (up to 10 μm thick) and piezoelectric micromachined ultrasonic transducer (pMUT) have been successfully demonstrated at low temperatures of 650 °C using a composite thick film processing route. Submicron-sized PZT powder was dispersed into sol–gel solution to form homogeneous slurry for spin-coating on silicon substrate. Issues associated with recipe of the slurry, deposition process and sintering of films have been summarized with a view to optimizing the properties of the films and pMUTs. Typical microstructure, ferroelectric and piezoelectric properties of the composite films are given. Thermal stability of bottom electrode, a key issue about device fabrication, has also been investigated. The ultrasound-radiating performance of the pMUT element in response to a continuous alternating current driving voltage has been reported. The generated sound pressure level is 116.8 dB at 76.3 kHz at a measuring distance of 12 mm. The pMUT is suitable for application of airborne object recognition.