Process Mechanism for Vacuum-Assisted Microfluidic Lithography with Ceramic Colloidal Suspensions

The infiltration kinetics and the drying mechanism of suspensions involved in vacuum-assisted microfluidic lithography (μFL) have been investigated for the fabrication of complex micropatterned ceramic structures. Infiltration lengths of alumina suspensions with various solid loadings into microchannels were analyzed as a function of channel widths ranging from 10 to 100 μm. The use of well-dispersed ethanol-based suspensions with lower viscosities and wider channels allowed for easier and longer infiltration due to lower fluidic resistance in the channels. In contrast to the micromolding in capillaries (MIMIC) method, vacuum-assisted μFL has a distinct drying mechanism in which there is a critical level of solid loading of the suspension with respect to the volume of the microchannel for the fabrication of a defect-free pattern structure.     

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.     

Optimization of Dispersion and Viscosity of a Ceramic Jet Printing Ink

Different amounts (0.5–10.0 wt%) of an oligomeric dispersant were added to a ceramic ink containing 30 vol% of zirconia in butyl acetate. The sediment packing densities of the resulting suspensions were measured. The viscosities of the suspensions were determined using a syringe that was adapted as a capillary viscometer suitable for the measurement of the low viscosities encountered (up to 400 mPas). The highest sediment-packing density coincided with the lowest viscosity and occurred when 1–2 wt% of dispersant, based on the ceramic, was added. Investigation of the supernatant of the suspensions by thermogravimetric analysis indicated that this amount corresponded to the highest level of adsorption of the dispersant on the ceramic. The sediment from this suspension yielded a high density on subsequent sintering.     

Sintering Behavior of Gehlenite, Part II. Microstructure and Mechanical Properties

A self-toughened gehlenite (2CaO·Al₂O₃·SiO₂ or "C₂AS") ceramic with randomly distributed platelet grains was prepared by the organic steric entrapment (PVA) route. The gehlenite ceramic had a density of 2.698–2.875 g/cm³, corresponding to a relative density of 90%–96%. The platelet gehlenite grains had an average thickness of 3.6±0.8 μm and a width of 12.9±3.7 μm, respectively, with an average aspect ratio of 3.6. The three-point bending strength, fracture toughness, and Young's modulus attained were 142.1±12.1 MPa, 2.32±0.12 MPa·m^1/2, and 108±6.8 GPa, respectively. Fractography as well as Vickers indentation crack propagation profiles showed that crack deflection, crack blunting, and pinning effects due to the randomly distributed platelet grains were considered to be responsible for the good mechanical properties of the gehlenite ceramic.     

Screen printing of silver nanoparticle suspension for metal interconnects

Silver nanoparticle suspensions were synthesized by chemical reduction method using a formaldehyde reductant. Polyvinyl pyrrolidone (PVP) of two different molecular weights (M.W.=8,000 and 29,000) was used as a stabilizer for the suspensions. PVP of a smaller molecular weight could produce silver suspensions of nanoparticle size around 20 nm. Water-based conductive silver inks with different silver concentrations were prepared and tested for suitability for screen printing. We have successfully printed silver metal lines on glass substrates using a 400 mesh screen-mask with 60wt.% silver ink prepared in this study. Curing at a low temperature of 200 °C for an hour was found sufficient to reach the lowest resistivity value with the synthesized ink. For a line with a width and thickness of 0.5 mm and 2.12 μm, respectively, it exhibited a resistivity of 3.3×10⁻⁵ Ω·cm, which could serve as conducting lines for various electronic applications.     

Effect of Curing Conditions on the Capillary Porosity of Hardened Portland Cement Pastes

The capillary pore structure of hardened portland cement pastes cured by high-pressure steam, chemical acceleration, high-pressure steam with reactive SiO₂, water immersion, water immersion and high-pressure steam, and hot-pressing was measured using mercury porosimetry to 50,000 psi. Differences of > 2 orders of magnitude exist in the average capillary pore diameters of the cement pastes studied. The largest pores (∼1 to 3 μm in diameter) are associated with high-pressure steam-cured pastes. The smallest average capillaries observed were 0.02 μm for pastes steam-cured with reactive SiO₂. Hot-pressed pastes had essentially no porosity accessible to mercury. The application of pore size control to problems of polymer-impregnated concrete is discussed.     

Fabrication and Measurement of Micro Three-Dimensional Photonic Crystals of SiO2 Ceramic for Terahertz Wave Applications

Three-dimensional (3D) photonic crystals with a diamond structure made of a dense SiO₂ ceramic were successfully fabricated using a CAD/CAM micro-stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO₂ paste (before sintering) was around 15 μm. After the SiO₂-resin photonic crystals were formed via micro-stereolithography, they were converted to pure SiO₂ ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO₂ photonic crystals was measured by terahertz-time-domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ– X 〈100〉 direction, between 490 and 630 GHz in the Γ– K 〈110〉 direction, and between 400 and 510 GHz in the Γ– L 〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.     

Formation and Sintering Characteristics of Aluminum Borate Whiskers

A boric acid-stabilized aluminum acetate powder was decomposed to aluminum borate (Al₁₈B₄O₃₃) by calcining at 1000°C. The powder was then ball-milled, compacted, and fired to temperatures of 1500°, 1600°, and 1700°C for a period of 1 h. The resulting aluminum borate ceramic had a whiskerlike grain morphology, with the whisker length approaching 20 μm and a diameter of 2–3 μm. The sintered compact showed no shrinkage upon firing, had a porosity of ∼50%, a narrow pore size distribution, and an average pore size of 1–3 μm.     

Spark Plasma Sintering of Nanocrystalline Niobium Nitride Powders

Nanocrystalline niobium nitride (NbN) powders were sintered by spark plasma sintering under a nitrogen atmosphere at temperatures from 1040° to 1230°C. Fully dense bulk NbN ceramic with grain sizes of 0.5–1.0 μm was obtained at 1130°C. The effects of sintering temperature on the density, phase content, electrical conductivity, Vickers hardness, and microstructure of the NbN ceramic were discussed.     

Fabrication of Functionally Graded and Aligned Porosity in Thin Ceramic Substrates With the Novel Freeze–Tape-Casting Process

Functionally graded and continuously aligned pore structures have been fabricated by a modified tape-casting process for use as solid oxide fuel cell electrodes, catalysts, sensors, and filtration/separation devices. Pore gradients from <5 to 100 μm and aligned pore tubules have been directly fabricated in various ceramic materials with thin substrate sections approximately 500–1500 μm utilizing both low-toxicity aqueous-based slips and organic solvents. This process allows for the generation of pores without the use of thermally fugitive pore formers in a single processing step with no need for tape lamination. The incorporation of tape casting, unidirectional solidification, and the freeze-drying process results in uniformly acicular pores aligned with the direction of the moving carrier film. Processing and microstructure variability will be discussed as it pertains to the effects of solids loading, freezing temperatures, and solvent type. Applications for this ceramic processing technology will also be discussed.     

Piezoelectric Multilayer Ceramic/Polymer Composite Transducer with 2–2 Connectivity

A multilayer piezoelectric ceramic/polymer composite with 2–2 connectivity was fabricated by thermoplastic green machining after co-extrusion. The multilayer ceramic body was composed of piezoelectrically active lead zirconate titanate (PZN)–lead zinc niobate (PZN)-lead zirconate titanate (PZT) layers and electrically conducting PZN–PZT/Ag layers. After co-extruding the thermoplastic body, which consisted of five piezoelectric layers interspersed with four conducting layers, it was computer numeric-controlled machined to create periodic channels within it. Following binder burnout and sintering, an 18 vol% array of 190 μm thin PZT slabs with a channel size of 880 μm was fabricated. The channels were filled with epoxy in order to fabricate a PZN–PZT/epoxy composite with 2–2 connectivity. The piezoelectric coefficient (effective d ₃₃) and hydrostatic figure of merit ( d _h× g _h) of the PZN–PZT/epoxy composite were 1200 pC/N and 20 130 × 10⁻¹⁵ m²/N, respectively. These excellent piezoelectric characteristics as well as the relatively simple fabrication procedure will contribute in widening the application range of the piezoelectric transducers.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

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.     

Novel Microcellular Ceramics from a Silicone Resin

Microcellular silicon oxycarbide open cell ceramic foams were fabricated from a silicone resin. Microcellular foams, with a cell size ranging from ∼1–80 μm, were fabricated using poly(methyl methacrylate) microbeads as sacrificial templates. The compression strength of the foams decreased with increasing cell size.     

Aqueous Tape Casting of Zirconium Diboride

Aqueous tape casting of ZrB₂ powder with sintering additives was investigated. The dispersion of ZrB₂ suspensions in aqueous media was studied and characterized in terms of zeta potential, sedimentation, and rheological measurements. A well-stabilized suspension with a high solid content (up to 45 vol%) was prepared in the alkaline pH region with 0.4 wt% Lopon 885 as the dispersant. Several suspensions with different compositions of binder and plasticizer were prepared for comparison. Crack-free green tapes with a maximum thickness of approximately 250 μm were obtained with a binder content of 18–23 wt%. The green tapes had high qualities, such as homogeneity, good flexibility, and a smooth surface. Results showed that the slurries at selected formulations met the needs of the tape-casting process.     

Interconnected Hydroxyapatite Nanofibers in the Biomimetic Coating of a Bioinert Ceramic Substrate

A bioinert ceramic substrate, α-Al₂O₃, has been coated with hydroxyapatite (HAp) by the biomimetic route using simulated body fluid (SBF) solution at room temperature. The substrate was incubated at 37°C in SBF for 6 days with a periodic replacement with freshly prepared SBF at 48-h intervals. After 6 days, continuous nanofiber-like structures of HAp (5–35 μm in length, 0.05 μm diameter) were obtained, connecting the intra- and interglobular clusters, within the coated mineral layer on the substrate surface. This is a unique and new observation, and this phenomenon has been demonstrated by a simple fractal growth model.     

Sintered Hydroxyapatite Latticework for Bone Substitute

Hydroxyapatite suspensions were extruded through a range of nozzles from 100 to 330 μm diameter from a stepper-driven syringe. Regular latticework scaffolds were built by fused deposition modeling (FDM), a solid freeforming method, on a three-axis table using 330 μm diameter filament. The scaffolds were sintered at 1250°C to 97% density retaining the hydroxyapatite crystal structure. The scaffolds were introduced to cultured human osteoblast cells and, after 48 h, the surfaces were thoroughly covered with active cells.     

Effects of solid loading on the fabrication of ceramic microparts by soft molding

The effects of solid loading on the fabrication of ceramic microparts by soft molding were studied. Alumina microchannel parts of different dimensions (60–160 µm) were fabricated from well-dispersed suspensions with different solid loadings (70, 75 and 80 wt%). The structural integrity of the green microchannel parts was examined to study the moldability of the suspensions. It was found that the minimum feature size and linear shrinkage of the microchannel parts decreased with increasing solid loading, while the green density and sintered density showed the opposite trend. The reasons for incomplete filling and demolding failures were also discussed.     

Calcium- and Lanthanum-Modified Lead Titanate (PCLT) Ceramic and PCLT/Vinylidene Fluoride-Trifluoroethylene 0-3 Nanocomposites

Calcium- and lanthanum-modified lead titanate (PCLT) powders with size in the nanometer range were prepared by a sol–gel process. The PCLT gel was annealed at 850°C to produce powder with an average particle diameter of 80 nm. A dense and fine-grained PCLT ceramic, with grain size of ∼0.7 μm, was prepared by sintering the sol–gel-derived powder at 1150°C. The piezoelectric and pyroelectric properties of the PCLT ceramic varied linearly with the degree of poling in the ceramic. PCLT/vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) 0-3 nanocomposites with PCLT volume fractions of 0.1–0.5 were fabricated, using PCLT powders imbedded in a P(VDF-TrFE) matrix. The ceramic data were used to model the piezoelectric and pyroelectric properties of the PCLT/P(VDF-TrFE) composites, and good agreements were obtained.     

Two Photon Polymerization of Polymer–Ceramic Hybrid Materials for Transdermal Drug Delivery

Three-dimensional microneedle devices were created by femtosecond laser two photon polymerization (2PP) of organically modified ceramic (Ormocer®) hybrid materials. Arrays of in-plane and out-of-plane hollow microneedles (microneedle length=800 μm, microneedle base diameter=150–300 μm) with various aspect ratios were fabricated. The fracture and penetration properties of the microneedle arrays were examined using compression load testing. In these studies, the microneedle arrays penetrated cadaveric porcine adipose tissue without fracture. Human epidermal keratinocyte viability on the Ormocer® surfaces polymerized using 2PP was similar to that on control surfaces. These results suggest that 2PP is able to create microneedle structures for transdermal drug delivery with a larger range of geometries than conventional microfabrication techniques.