Ceramic Counterparts Research Articles

Emulsion processing of polymer-derived porous Si/C/(O) ceramic bodies

A novel route of miniemulsion processing was applied to preceramic polymers in order to fabricate porous Si/C/(O) bodies. Commercially available polysiloxane and polycarbosilane precursors and water were emulsified via intensive stirring or ultrasonication in the presence of surfactants, and subsequently shaped and cured in an autoclave. The resulting green bodies were dried and pyrolytically converted at 700 and 1050 °C under nitrogen into ceramic counterparts retaining their shape. A fine interconnected pore network derived from the aqueous phase allows escape of gaseous decomposition products during pyrolysis and avoids cracking. In the presence of transition metals such as Fe, Co, Ni or Pd nanofibers consisting of β-SiC/a-SiO 2 are formed. Effects of polymer viscosity and processing parameters on the microstructures, porosities and pore sizes of the resulting products were investigated by means of DLS, mercury porosimetry, SEM, TEM, XRD and FTIR.

Tribological properties of γ-Y 2Si 2O 7 ceramic against AISI 52100 steel and Si 3N 4 ceramic counterparts

Reciprocating ball-on-flat dry sliding friction and wear experiments have been conducted on single-phase γ-Y 2Si 2O 7 ceramic flats in contact with AISI 52100 bearing steel and Si 3N 4 ceramic balls at 5–15 N normal loads in an ambient environment. The kinetic friction coefficients of γ-Y 2Si 2O 7 varied in the range over 0.53–0.63 against AISI 52100 steel and between 0.51–0.56 against Si 3N 4 ceramic. We found that wear occurred predominantly during the running-in period and it almost ceased at the steady friction stage. The wear rates of γ-Y 2Si 2O 7 were in the order of 10 −4 mm 3/(N m). Besides, wear debris strongly influenced the friction and wear processes. The strong chemical affinity between γ-Y 2Si 2O 7 and AISI 52100 balls led to a thick transfer layer formed on both contact surfaces of the flat and counterpart ball, which changed the direct sliding between the ball and the flat into a shearing within the transfer layer. For the γ-Y 2Si 2O 7/Si 3N 4 pair, a thin silica hydrate lubricant tribofilm presented above the compressed debris entrapped in the worn track and contact ball surface. This transfer layer and the tribofilm separated the sliding couple from direct contact and contributed to the low friction coefficient and wear rate.

Chemical Synthesis of K0.5Na0.5NbO3 Ceramics and Their Electrical Properties

Powders of K0.5Na0.5NbO3 (KNN) were prepared by the Polymerized Complex Method (PC method), based on the Pechini-type reaction route. The crystallization process of KNN during the calcination of an amorphous precursor was observed, using TG-DSC analysis and X-ray powder diffraction. After calcination the powders were ball milled, pressed into pellets, and sintered at 1100°C, which is lower than the sintered temperature of the conventional solid-state method. Using this procedure, we achieved relatively high-density and very homogeneous KNN ceramics. The electric properties of the ceramics were investigated. The results showed that these values are superior to those of sintered KNN ceramic counterparts made by the conventional solid-state method.

Advanced Single Crystal Piezoelectric Transducers for Naval Sonar and Medical Ultrasound Applications

Single crystal piezoelectrics have received much attention due to their very high piezoelectric and electromechanical coupling coefficients. Naval sonar and medical ultrasound transducers, fabricated from this material, exhibit unprecedented increases in bandwidth, source level, and sensitivity compared to ceramic based devices. In this paper we report many new developments in the growth and manufacture of single crystals. These include increased diameter (> 3) crystal boules, improved compositional uniformity, increased thermal stability, and minimized surface damage. Manufacturing improvements are being applied to a broad range of crystal transducer applications. For navy sonar systems, crystals are resulting in very high bandwidths (> 100%) for transducer sizes that are 1/2 the resonating length and 1/4 the weight of ceramic counterparts with equivalent (or often lower) source level. Manufacturing improvements such as high tensile strength, achieved by precision polishing crystal surfaces, are enablin...

Recent advances in metallic interconnects for solid oxide fuel cells

With the reduction in solid oxide fuel cell (SOFC) operating temperatures into the range 650–800°C, cost effective metallic materials, in particular high temperature oxidation resistant alloys, have become promising candidates, in place of ceramic counterparts, for construction of interconnects in SOFC stacks. For a lifetime of thousands of hours, the alloys must demonstrate excellent surface, structural and electrical stability, and compatibility with other components in SOFC stacks in a very challenging environment. The present paper provides a critical review of metallic interconnects for SOFCs with a focus on recent progress in materials development, as well as advances in understanding materials degradation and interfacial phenomena under the SOFC operating conditions. It also attempts to provide guiding principles for the development of optimised, cost effective metallic interconnect materials that can demonstrate satisfactory life time stability and performance.

Dielectric properties of barium strontium titanate (BST)/yttrium aluminate (YAlO3) thick films under DC bias field

The DC bias dependence of the dielectric properties of BST thick films on YAlO3 substrates has been investigated for possible tunable microwave device applications. Rare earth aluminate (e.g. YAlO3) substrates were used to overcome the interfacial interactions between BST thick films and alumina substrates during high temperature sintering. The results show that the BST films exhibit good chemical compatibility with YAlO3 substrates at sintering temperatures up to 1,500 °C. Improved density and enhanced grain growth in the films have been obtained compared to BST films on alumina substrates sintered at lower temperatures (<1,250 °C). Consequently, the permittivity and tunability are increased significantly. The low frequency losses in the ferroelectric region are also increased due to the contribution of domain wall motion. However, compared to their bulk ceramic counterparts, the films still exhibit a relaxor-like behaviour and DC bias hysteresis in both ferroelectric and paraelectric regions, which indicates some extent of microstructural heterogeneity and existence of micro-/nano- polar phases within the films. Possible reasons are discussed in terms of the substrate constraint induced stress effect in the films during high temperature sintering due to the thermal expansion coefficient mismatch between the BST films and YAlO3 substrates.

Fabrication and ferroelectric properties of highly dense lead-free piezoelectric (K0.5Na0.5)NbO3 thick films by aerosol deposition

Lead-free piezoelectric thick films of (K0.5Na0.5)NbO3 were fabricated by aerosol-deposition method. The thickness of KNN film was 7.1μm and fully dense films were obtained. The dielectric constants ε3T∕ε0 of the as-deposited and annealed films at 1kHz were 116 and 545, respectively, which are higher than any previously reported values for lead-free piezoelectric thin/thick films, either without or with heat treatment. The ferroelectric properties were improved after annealing and the maximum values of Pr=8.1μC∕cm3 and Ec=100kV∕cm were achieved. These values are markedly superior to those of sintered KNN ceramic counterparts.

The correlation between the hardness and tribological behaviour of electroplated chromium coatings sliding against ceramic and steel counterparts

Electroplated chromium (Cr) coatings with different hardness were electrodeposited on steel substrate through adjusting the concentration of rare earth in a Sargent bath (CrO3 250 g/l, H2SO4 2.5 g/l). The results show that with a Si3N4 ceramic counterpart, the wear rate of electroplated Cr coatings increases with the increase of the hardness owing to the delamination wear mechanism; while with a steel counterpart, the wear rate of electroplated Cr coatings decreases with increasing hardness thanks to the abrasion wear mechanism. The wear mechanism may be dictated by the adhesive force between the sliding pairs and the possibility of abrasive particles formation that weak adhesive force between the ceramic counterpart and electroplated Cr coatings can hardly result in the adhesion, transfer of material and formation of abrasive particles on the tribosurface of the electroplated Cr coating, and the frictional shear energy tends to transform to dislocation and deformation energy stored inside the material instead of friction heat; therefore, the delamination wear is more likely to occur on the tribosurface. Whereas, the strong adhesive force between the steel counterpart and electroplated Cr coatings will result in the adhesion, transfer of material and formation of the abrasive particles on the tribosurface, the frictional shear energy would be allowed to transform to cutting and plough energy; hence, the adhesive and abrasive wear may occur.

Fabrication of focused poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) copolymer 40–50 MHz ultrasound transducers on curved surfaces

Copolymer films such as poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) have lower acoustic impedance compared to their ceramic counterparts, allowing for a better acoustic match to tissues in the human body. Because of this, copolymer ultrasonic transducers are capable of yielding the desirable characteristics of broad bandwidth and short pulse duration that allow better image resolution to be achieved. In the past, such transducers in the frequency range from 40 to 80 MHz have frequently been fabricated by spin coating the copolymer film onto a flat substrate and then applying the film to a curved backing using an adhesive layer. The adhesive layer may cause spurious signals at these frequencies, in addition to the film damage that may occur as a result of such processing. In order to avoid these problems, a copolymer film can be directly spin coated onto a curved substrate. The resulting devices had an operating frequency of over 40 MHz and approximately a 75% bandwidth. The potential of several approaches that could be further explored to increase the level of performance of such devices is also discussed.

Characterization of nanostructured coatings based on oxides for tribological applications

The aim of this project was to develop a wear-resistant coating for tribological applications in various environments. Zirconia nanocrystallites were encapsulated into an amorphous Cu matrix by the simultaneous deposition of Zr and Cu with pulsed reactive magnetron sputtering. The incorporation of ZrO2 in a Cu matrix leads to a nanocomposite coating with improved wear resistance, temperature stability and chemical inertness. The deposition parameters (cathodes power, gas flow) were correlated to the mechanical properties of the coating. The nanoparticles were deposited onto metal-grids and analyzed by transmission electron microscopy. Sliding tests were performed under N2 and with Isopar M as lubricant. The coatings were tested against steel 100Cr6 and ceramic Al2O3 counterparts. The wear mechanism was studied in each situation.

Evaluation of the Moisture Sensitivity of Molding Compounds of IC’s Packages

Plastic encapsulated devices are of great interest against their ceramic or metallic counterparts, as they permit significant cost reductions. However, they are more sensitive to moisture ingress, which can lead to reliability problems. Moisture can penetrate either through the polymer or along the interface between the leads and the encapsulant. Samples of 3 different polymers usually used for IC’s encapsulation, either pure or containing a lead frame with the lead terminations, were aged under different RH conditions. Results show that the penetration through the polymer is preponderant face to the progression along the leads. In a first approach, this penetration can be considered as a diffusion process, following the Fick’s law; the diffusion coefficient D and the saturation coefficient S can be deduced from the measurements as a function of the relative humidity. These values can be used in finite element simulations, in order to evaluate the moisture ingress as a function of time within an IC of a given geometry. Moisture diffusion within these new resins is a very long process. In fact, a first “saturation” occurs after a few hours of aging, but if samples are left longer in the oven, a new diffusion process occurs, not leading to a real saturation. Hence processes different than pure fickian diffusion occur in the polymers.

Preparation and characterization of novel piezoelectric and pyroelectric polymer electrets

The preparation and characterization of novel piezoelectric and pyroelectric electrets is discussed with respect to their electret properties. Charged heterogeneous electrets, like closed-cell microporous polypropylene (pp) foams and 'soft-hard' fluoropolymer hybrids are shown to yield large quasi-static and dynamic piezoelectric responses, indistinguishable from true piezoelectricity. Piezoelectric coefficients comparable to that of ceramic counterparts have been achieved. For the investigation of the piezoelectric and pyroelectric properties of the electret materials, the thermal-pulse technique has been extended towards temperature-dependent investigations and measurements on planar, cylindrical or spherical samples. The potential of the thermal-pulse technique not only for the measurement of electric-field and polarization distributions, but also for acoustical, electromechanical and pyroelectric properties is outlined.

Thin films of novel ferroelectric composites

Abstract Composites of BSTO combined with other non electrically active oxides have demonstrated adjustable electronic properteis which can be tailored for use in various electronic devices.[1,2] These novel composites of barium strontium titanate (BSTO) and oxide III compounds have already exhibited promising results in their ceramic form.[3] The additive oxides modify the dielectric constant, tunability (change in the dielectric constant with applied voltage), and dielectric loss of the material. One application has been for use in phased array antennas and insertion has been accomplished into several working antenna systems.[4] To further accommodate the frequencies required by these phased array antennas, thin films of the composites have been fabricated. Preliminary studies have indicated that thin films of such composites exhibit similar behavior as their bulk ceramic counterparts.[5] The purpose of this study is to investigate the properties of the BSTO/oxide III based compounds in thin film form.

Brown coal derived carbonaceous gels. Part 2. Drying mechanisms and shrinkage

Carbonaceous gels have been shown to form rapidly from colloidal dispersions of brown coal and produce a resilient network structure. Upon drying, these gels form hard glassy structures under the capillary stresses of contraction. The evolution of this increasingly less compliant microstructure was followed by yield stress and modulus measurements as drying proceeded. Moisture and shrinkage profiles were determined and a drying model suggested based upon ceramic counterparts. An approximate solution to a differential equation which relates the strain rate due to shrinkage, the pressure driving the drying liquid flow, and the pressure gradient to the microstructure via the permeability was examined over the initial 2 h drying period and shown to represent the solids concentration at the surface but not the gradient of the concentration profiles.

1–3 Piezocomposite transducer materials for commercial underwater applications

Fugro-UDI Limited has gained several years experience in developing piezo-composite materials for use in underwater imaging sonars operating in the frequency range 100–500 kHz. A substantial amount of this work has concentrated on multielement arrays for military applications such as mine counter measures. The low mechanical quality factor (Qm) and high electro-mechanical coupling coefficient (Kt) of the material has provided wideband and efficient transducer materials. These parameters and others such as dielectric constant and electrical impedance can be tailored by varying the type and quality of both the ceramic material and polymer phase within the piezocomposite device. This has contributed to reducing the development and manufacturing costs of piezocomposite transducers. This presentation will discuss the various transducers developed by Fugro-UDI for commercial applications ranging from simple echo-sounder transducers, to sidescan transducers, through to multielement arrays for electronically scanned sonars. Sensitivities and beam patterns for piezocomposite devices will be compared with their ceramic counterparts. Other important transducer parameters such as pressure and temperature dependence of the piezocomposite materials will be presented.

Dielectric, piezoelectric and acoustical properties of high performance piezorubber composite hydrophone

It is well known that conventional piezoelectric ceramics are hard and brittle materials in nature. These possess high acoustic impedance causing great acoustical mismatching when used in conjunction with radiating media like water or human tissues. To overcome this problem a number of composite systems based on piezoceramic and polymers have been suggested with different connectivity patterns [1, 2]. Such composites offer better acoustic coupling in low density medium like water. Being relatively flexible, most of the composites can be used to form non-planar transducers without the need for machining or grinding. The concept of composite materials has also been extended for the cancellation of reflected and transmitted sound in one dimension by single and double layers of piezoelectric material subjected to a normally incident wave in a fluid [3]. Active piezocomposite coatings for active underwater sound reduction have also been developed [4]. However, the present work is aimed to develop a piezorubber composite hydrophone with much higher piezoelectric voltage constant (g33) than their ceramic counterparts [5-7]. Special attention was focused on the development of hydrophones with omnidirectional broad band characteristics. This was achieved by adopting a 0-3 connectivity pattern consisting of lead zirconate-lead titanate ceramic powder loaded in a rubber matrix. The effects of the relative amounts of individual constituents, grain size of the ceramic powder and poling conditions on dielectric and piezoelectric properties have been studied. Composite hydrophones of cylindrical shape have been fabricated and their electroacoustic sensitivity has been evaluated analytically as well as experimentally. The donor doped lead zirconate-lead titanate (PZT) was synthesized using a conventional mixed oxide route by pre-reacting the oxides of lead, zirconium and titanium at 900 °C. The powders were pressed into disc-shaped specimens and sintered at 1220 °C for 4 h. These sintered discs were then crushed to form powder. The X-ray diffraction (XRD) patterns of the powders confirmed the formation of a perovskite phase. These materials were used to obtain powders of average particle size 30, 50 and 80/xm. The ceramic filler powder was uniformly dispersed in commercial chloroprene rubber dissolved in toluene. A small quantity of carbon (1.0 wt%) was added to facilitate the poling. The resulting mixture has been used to prepare specimens of desired dimensions. The mixture was