Piezoelectric properties of highly densified 0.01Pb (Mg1/2W1/2)O3–0.41Pb (Ni1/3Nb2/3)O3–0.35PbTiO3–0.23PbZrO3+0.1 wt% Y2O3+1.5 wt% ZnO thick films on alumina substrate

This paper reports on the formation of highly densified piezoelectric thick films of 0.01Pb(Mg_1/2W_1/2)O₃–0.41Pb(Ni_1/3Nb_2/3)O₃–0.35PbTiO₃–0.23PbZrO₃+0.1 wt% Y₂O₃+1.5 wt% ZnO (PMW–PNN–PT–PZ+YZ) on alumina substrate by the screen-printing method. To increase the packing density of powder in screen-printing paste, attrition milled nano-scale powder was mixed with ball milled micro-scale powder, while the particle size distribution was properly controlled. Furthermore, the cold isostatic pressing process was used to improve the green density of the piezoelectric thick films. As a result of these processes, the PMW–PNN–PT–PZ+YZ thick film, sintered at 890 °C for 2 h, showed enhanced piezoelectric properties such as P_r=42 μC/cm², E_c=25 kV/cm, and d₃₃=100 pC/N, in comparison with other reports. Such prominent piezoelectric properties of PMW–PNN–PT–PZ+YZ thick films using bi-modal particle distribution and the CIP process can be applied to functional thick films in MEMS applications such as micro actuators and sensors.     

Site-specific growth and density control of carbon nanotubes by direct deposition of catalytic nanoparticles generated by spark discharge

Catalytic iron nanoparticles generated by spark discharge were used to site-selectively grow carbon nanotubes (CNTs) and control their density. The generated aerosol nanoparticles were deposited on a cooled substrate by thermophoresis. The shadow mask on top of the cooled substrate enabled patterning of the catalytic nanoparticles and, thereby, patterning of CNTs synthesized by chemical vapor deposition. The density of CNTs could be controlled by varying the catalytic nanoparticle deposition time. It was also demonstrated that the density could be adjusted by changing the gap between the shadow mask and the substrate, taking advantage of the blurring effect of the deposited nanoparticles, for an identical deposition time. As all the processing steps for the patterned growth and density control of CNTs can be performed under dry conditions, we also demonstrated the integration of CNTs on fully processed, movable silicon microelectromechanical system (MEMS) structures.     

Aliphatic polycarbonate synthesis by copolymerization of carbon dioxide with epoxides over double metal cyanide catalysts prepared by using ZnX2 (X = F, Cl, Br, I)

As means of the chemical fixation of carbon dioxide and the synthesis polycarbonates, copolymerizations of carbon dioxide with various epoxides, such as cyclohexene oxide, cyclopentene oxide and propylene oxide were investigated in the presence of double metal cyanide (DMC) catalysts. The DMC catalysts were prepared by reacting K₃Co(CN)₆ with ZnX₂ (X = F, Cl, Br, I) together with tertiary butyl alcohol and poly(tetramethylene ether glycol) as complexing reagents and were characterized by various spectroscopic methods. The DMC catalysts showed high activity for epoxides and CO₂ copolymerization to yield aliphatic polycarbonates of narrow polydispersity and moderate molecular weight.     

Selective CO oxidation in the presence of hydrogen over supported Pt catalysts promoted with transition metals

Selective CO oxidation in the presence of excess hydrogen was studied over supported Pt catalysts promoted with various transition metal compounds such as Cr, Mn, Fe, Co, Ni, Cu, Zn, and Zr. CO chemisorption, XRD, TPR, and TPO were conducted to characterize active catalysts. Among them, Pt-Ni/γ-Al₂O₃ showed high CO conversions over wide reaction temperatures. For supported Pt-Ni catalysts, Alumina was superior to TiO₂ and ZrO₂ as a support. The catalytic activity at low temperatures increased with increasing the molar ratio of Ni/Pt. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Ni and Pt was determined to be 5. This Pt-Ni/γ A1₂O₃ showed no decrease in CO conversion and CO₂ selectivity for the selective CO oxidation in the presence of 2 vol% H₂O and 20 vol% CO₂. The bimetallic phase of Pt-Ni seems to give rise to stable activity with high CO₂ selectivity in selective oxidation of CO in H₂-rich stream.     

Strength and microscopic characteristics of alkali-activated fly ash-cement

The effects of different activator concentration, liquid/fly ash ratio, and curing temperature and time on the compressive strength of specimens prepared from low-calcium fly ash activated with sodium hydroxide without the use of Portland cement were investigated. SEM, XRD and mercury intrusion porosimetry (MIP) were used to observed the structural feature, reaction products, and porosity and pore-size distribution of the specimens from alkaliactivated fly ash, respectively. It was found that the degree of reactivity, as shown by the compressive strength, the activator concentration and the ratio of liquid/fly ash, and the curing temperature always result to be significative factors. The 7, 14, and 28-day compressive strengths of specimens prepared from alkali-activated fly ash by 5M NaOH solution at 50 °C are 152, 219, and 263 kgf/cm², while those from 6M solution are 184, 225, and 267 kgf/cm², respectively. In SEM observation, the fly ash activated by the 5M NaOH solution shows a more continuous matrix with solid and non porous due to subsequent gel restructuring by amorphous alkaline aluminosilicate produced from alkali-activated fly ash.     

Effect of dissolved CO2 on the crystallization behavior of linear and branched PLA

In this study, the crystallization behavior of polylactide (PLA) was investigated in the presence of dissolved CO₂ using high-pressure and regular differential scanning calorimeter. The isothermal and non-isothermal melt crystallization results showed that increasing the CO₂ pressure decreased the crystallization half-time. During isothermal and low-cooling-rate non-isothermal crystallization, a very high crystallinity was achieved at 15 bar CO₂ pressure by facilitating more perfect crystal formation with the plasticization effect of CO₂ while limiting the crystal nucleation rate. At higher CO₂ pressures, a larger number of less close-packed crystals were formed due to chain entanglement, and consequently, the final crystallinity was decreased. The non-isothermal results at high cooling rates showed the total crystallinity decreased for all CO₂ contents, because of less time given for crystallization. Also the effects of the CO₂ pressure and the cooling rate on T_c and T_g were investigated.     

Melt crystallization of poly(ethylene terephthalate): Comparing addition of graphene vs. carbon nanotubes

Poly(ethylene terephthalate) (PET)-based nanocomposites with graphene or multi-wall carbon nanotubes (MWCNT) were prepared by melt mixing. Aspect ratio, A_f, and interparticle distance, λ, of graphene in the nanocomposites were obtained from melt rheology and transmission electron microscopy respectively. λ of PET/graphene nanocomposites was much smaller than λ in PET/MWCNT. For PET/graphene with highest A_f, λ became <1 μm at more than 0.5 wt% graphene. Non-isothermal crystallization behavior from the melt was investigated by differential scanning calorimetry. The crystallization temperatures suggest that the nucleation effect of graphene was stronger than that of MWCNT. The half crystallization time of PET/graphene became longer than PET/MWCNT with increasing graphene loading, suggesting that confinement by graphene suppressed the crystal growth rate. XRD analysis indicated that smaller crystals formed in PET/graphene than in PET/MWCNT. From Raman spectroscopy, the π–π interaction between PET and graphene was stronger than that between PET and MWCNT. This stronger interaction in PET/graphene appears to result in formation of crystals with higher perfection.     

Microwave dielectric properties of barium substituted screen printed CaBi2Nb2O9 ceramic thick films

In the present study, Aurivillius-structured Ba²⁺ substituted CaBi₂Nb₂O₉ (CBNO) ceramic powder was synthesized by co-precipitation method. The CBNO thick films were delineated by screen printing method on alumina substrates using co-precipitated ceramic powder. The overlay method was adopted to measure the microwave dielectric properties of prepared thick films. Single phase layered perovskite structure of the prepared thick films was confirmed by X-ray Diffraction. The effects of Ba²⁺ substitution on the surface morphology, bonding, and microwave dielectric properties of thick films were systematically presented. The maximum value of microwave dielectric constant for the CBNO thick films at 11.8 GHz is 15.6 for Ba²⁺=0.8 substitution. The shift in the stretching vibration modes of the Nb-O bond of NbO₆ octahedron in the Raman spectra with a substitution of Ba²⁺ in CBNO was observed. The substitution of Ba²⁺ on A-site of CBNO improves the microwave dielectric properties of prepared thick films. This work may provide a new approach to enhance the microwave dielectric performance of Aurivillius-structured ceramic thick films.     

Hydrophobicity and transparency of Al2O3-based poly-tetra-fluoro-ethylene composite thin films using aerosol deposition

Alumina and polytetrafluoroethylene (Al₂O₃-PTFE) composite films were fabricated by a simple aerosol deposition (AD) process, to confirm its applicability for various display screens requiring water resistant, anti-smudge and easy-to-clean properties. The surface morphologies, hydrophobic properties, and transparencies of the composite films with different PTFE contents, varying from 0.01 to 1?wt% were investigated. As a result, the composite films with over 0.3?wt% PTFE showed a sudden rise in surface roughness and low transmittance, despite having the highest contact angle of 128° at a PTFE content of 0.3?wt%. From the energy dispersive spectrometer analysis, the crash-cushioning effect of PTFE and agglomerated PTFE particles were determined to be major causes of surface roughness and opacity. In contrast, the transmittance showed a tendency to be enhanced, with an increasing PTFE content in the range of 0.01, 0.05, and 0.1?wt% PTFE, respectively. Especially, the film with 0.1?wt% of PTFE had contact angle of 111° and exhibited a high transmittance of over 75%, which was inferred to be an appropriate amount of PTFE, with a high elongation filling up the surface and the internal defects, leading to an enhancement of transparency. Consequently, these results implied that the AD-prepared Al₂O₃-PTFE composite coatings are promising candidates for various display applications.     

The effect of curing temperature on thermal,physical and mechanical characteristics of two types of adhesives for aerospace structures

The effects of cure temperatures on the thermal, physical and mechanical characteristics of two types of thermosetting structural epoxy film adhesives were determined in detail. The aim of this paper is to assess the effect of cure temperatures (82–121 °C) on the degree of cure of the two adhesives and the relevant void formations that need to be addressed in bonded part production and repair. Two thermal parameters were used to characterize the advancement of the reaction, such as degree of cure and glass transition temperature. The joint properties with respect to the cure temperatures were characterized by void content and bond-line thickness measurements and lap shear strength tests. Experimental results presented that all lap shear strengths were well within minimum shear strength (29 MPa) required by the specification of the film-type adhesive. However, the lap shear strength testing after aging at 82 °C and 95%R.H for 1000 h showed that the improved durability when the adhesive is cured at 121 °C did not occur for the 82 °C cure. Low curing conversion (75–77% degree of cure) combined with high voids (over 2 areal%) has a catastrophic effect on the bonding qualities at the metal-adhesive interface and due to lack of cohesion in the adhesive. The changes in the interface caused by the low temperature curing may contribute to an increased susceptibility of the bonded joint to moisture and consequent bond-line degradation.