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In order to find out the influence of sintering additives on the electrical conductivity of Si 3 N 4 -based ceramics composites with dispersed carbon nano-fibers (CNFs) two different mixtures of sintering additives were tested – Al 2 O 3 /Yb 2 O 3 and MgSiN 2 /Yb 2 O 3 , respectively. Optimization of hot-pressing conditions was performed for each mixture. The results show that the electrical conductivity can be effectively increased up to 1315 S/m by replacement of traditional sintering aid – alumina, with magnesium silicon nitride, while the mechanical properties remained on the same level. Other advantages of using MgSiN 2 instead of alumina are the preservation of higher amounts of CNFs in the ceramic composite and lower densification temperature (1500 °C) compared to samples sintered with alumina-based sintering aids (1550 °C).

The electrical conductivity of dense and nanoporous zirconia-based thin films is compared to results obtained on bulk yttria stabilized zirconia (YSZ) ceramics. Different thin film preparation methods are used in order to vary grain size, grain shape, and porosity of the thin films. In porous films, a rather high conductivity is found at room temperature which decreases with increasing temperature to 120 °C. This conductivity is attributed to proton conduction along physisorbed water (Grotthuss mechanism) at the inner surfaces. It is highly dependent on the humidity of the surrounding atmosphere. At temperatures above 120 °C, the conductivity is thermally activated with activation energies between 0.4 and 1.1 eV. In this temperature regime the conduction is due to oxygen ions as well as protons. Proton conduction is caused by hydroxyl groups at the inner surface of the porous films. The effect vanishes above 400 °C, and pure oxygen ion conductivity with an activation energy of 0.9 to 1.3 eV prevails. The same behavior can also be observed in nanoporous bulk ceramic YSZ. In contrast to the nanoporous YSZ, fully dense nanocrystalline thin films only show oxygen ion conductivity, even down to 70 °C with an expected activation energy of 1.0 ± 0.1 eV. No proton conductivity through grain boundaries could be detected in these nanocrystalline, but dense thin films.

Ba(0.8)Sr(0.2)Co(0.8)Fe(0.2)O(3-delta) exists as a single cubic phase at 1000 degrees C in air. Upon slow cooling, it transforms, between 900 and 700 degrees C, to a mixture of cubic, hexagonal, and rhombohedral phases, as evidenced by X-ray diffraction and transmission electron microscopy. The reversible phase change is accompanied by a large change of the oxygen content (Delta delta = 0.14) of Ba(0.8)Sr(0.2)Co(0.8)Fe(0.2)O(3-delta) and a volume change of 1.2 vol% at 700-900 degrees C. The multiphase material shows lamellar shaped structures in the HRTEM micrographs due to the partial transformation of the material from the cubic to the hexagonal and rhombohedral symmetries. The electrical conductivity changes from p-type semiconductivity between room temperature and 700 degrees C to metal-like conductivity at higher temperatures up to 1000 degrees C. The electrical conductivity is irreversibly changed upon thermal cycling due to the formation of microcracks.

Ceramic thin films are used in various applications for their structural or functional properties. Below a thickness of approximately 25 μm, ceramic films are typically deposited, sintered, and used on a supporting substrate due to their fragility. Here, we present a set of easy and versatile green film deposition and sintering techniques that allow the fabrication of free-standing micrometer-thin alumina and yttria-stabilized zirconia foils. Because of their extremely low thickness, the foils were transparent and flexible but still strong enough that they were self-supporting and could be handled manually. The presented concepts are not limited to the materials used here and can potentially be extended to most ceramic materials and glasses. So prepared free-standing ultrathin ceramic foils allow new assembly and fabrication methods of devices and novel materials that contain thin ceramic membranes.

All-ceramic dental restorations exhibit enhanced esthetics and biocompatibility as compared to traditional metal-based prosthesis. However, long-term fatigue and subcritical crack growth in the presence of water and cyclic loading can decrease the strength of ceramic components over time. We investigated the cyclic fatigue in water of three dental materials currently used as frameworks in all-ceramic restorations: a 3 mol%-yttria partially stabilized zirconia (3Y-TZP, Cercon, Degudent GmbH), a Al 2 O 3 –ZrO 2 –Glass composite (Inceram-Zirconia, Vita Zahnfabrik GmbH) and a Li 2 O·2SiO 2 glass ceramic (Empress 2, Ivoclar Vivadent AG). Fatigue and fast fracture tests were performed to determine the Weibull distribution of lifetime and initial mechanical strength for each framework component. In spite of its noticeable susceptibility to fatigue in water, the 3Y-TZP material was found to be particularly suitable for the preparation of posterior all-ceramic bridges due to its high initial mechanical strength. Guidelines are provided for the selection of materials and the design of all-ceramic posterior bridges exhibiting lifetime longer than 20 years under severe wet and cyclic loading conditions.

Objectives Ceramic materials applied in dentistry may exhibit significant subcritical crack growth due to the severe cyclic loading in the aqueous environment encountered in the mouth during mastication. The authors report on the subcritical crack growth behavior of three dental restoration systems (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) under cyclic loading in water, in order to establish guidelines for the use and design of long-lifetime all-ceramic posterior bridges. Methods Inert strength and lifetime tests under cyclic loading in an aqueous environment were performed in a mechanical bending apparatus and evaluated with Weibull statistics. Results Subcritical crack growth occurred predominantly in the outer veneer layer of the veneer–framework composites. The apatite-based veneer (IPS Eris) was more susceptible to subcritical crack propagation compared to the feldspathic glass veneers (Cercon S and Vita VM7). Significance Dental restoration systems containing apatite-based veneers and weak frameworks (Empress 2/IPS Eris) are not recommended for the fabrication of all-ceramic bridges in the molar region. Conversely, veneer–framework systems consisting of feldspathic glass veneers and tough zirconia-based frameworks (TZP/Cercon S and Inceram-Zirconia/Vita VM7) may exhibit lifetimes longer than 20 years if the bridge connector is properly designed.

The aim of this study was to evaluate the cyclic fatigue behavior of zirconia (3Y-TZP) in water and derive guidelines for the design of zirconia-based dental bridges with extended lifetime.The subcritical crack growth parameters under aqueous and cyclic loading conditions were determined from Weibull distributions of the initial mechanical strength and of the lifetime of TZP specimens. The strength and lifetime data were obtained using a specially designed bending machine under simple and oscillatory loading conditions, respectively.The TZP components submitted to cyclic loading in water exhibited subcritical crack propagation at stress levels significantly ( approximately 50%) lower than the critical stress intensity factor (K(IC)=5.6 MPam(1/2)). In spite of this susceptibility to subcritical crack growth, calculations based on the fatigue parameters and on the stress applied on the prosthesis indicate that posterior bridges with zirconia frameworks can exhibit lifetimes longer than 20 years if the diameter of the bridge connector is properly designed.This in vitro study indicates that partially stabilized zirconia (3Y-TZP) can withstand the severe cyclic loading and wet conditions typically applied in the molar region of the mouth and is therefore an appropriate material for the fabrication of all-ceramic multi-unit posterior bridges.

ZnO-based varistor ceramics were sintered under various conditions to optimize their mechanical strength. For highest strength, the optimum sintering temperature was 1070°C or below. At higher maximum temperature, the strength decreased because of grain coarsening and the increasingly inhomogeneous distribution of secondary phases thereby induced. Fracture typically started from holes associated with hollow or poorly compacted sprayed granules. All series contained the same type of critical flaws, but, depending on the sintering temperature, the fracture toughness changed, which led to different strengths. At sintering temperatures above 1050°C, the density started to decrease slightly because of swelling attributed to the pressure of gas entrapped in closed pores.

A complex high-temperature furnace has been produced from presintered α-Al2O3precompacts. The furnace is designed for the Mar345 imaging-plate detector system. Up to now, the design complexity of ceramic parts has been limited by the difficulty of machining hard ceramic materials or by the moulds used in hot-pressing and casting. However, the machining of soft presintered ceramic materials enables the realisation of much more demanding designs, provided that the final sintering shrinkage of the ceramic is homogeneous and predictable. The design of the furnace and its realisation and application in an X-ray diffraction study with synchrotron radiation are presented.

To assess the effect of different peri- and intertubular dentin mineralization conditions and etching on shear bond strength in vitro.One hundred fifty crowns of extracted bovine incisors were embedded in resin and ground to expose the buccal coronal dentin. Sixty specimens were subjected to a demineralizing solution (DS) and another 60 teeth to a bacterial-based laboratory caries model (S. mutans, SM). Thirty specimens of each demineralization protocol (DS and SM) were randomly selected and remineralized (-R). Thirty sound dentin specimens served as control (C). Resin composite buildups (Tetric) were bonded after application of one of the following adhesives: a one-step self-etching adhesive (Xeno III), and a self-etching adhesive (Syntac Classic) without (three-step) and with prior additional 35% phosphoric acid etching (etch-and-rinse, four-step). Teeth were subjected to shear bond strength testing in a universal testing device at a crosshead speed of 0.5 mm/min (Ultradent method).Bond strength value for group C ranged from 6.3 to 8.4 MPa (p0.05). DS and DS-R samples showed in creased bond strength with the one-step adhesive (11.6 MPa, p0.05), whereas the three-step adhesive with additional etching showed decreased bond strength (3.2 MPa, p0.05). SM samples showed the lowest bond strength of all adhesive systems (range 1.1 to 1.5 MPa, p0.05). Remineralization showed no effect on the latter group.The degree of mineralization of the dentin is important for adhesion. Additional etching with phosphori acid reduced bond strength of a three-step adhesive.

The immunosuppressive cyclic undecapeptide cyclosporine (Cs) represents a useful model for studying the molecular basis of antibody-antigen interactions. The three-dimensional structure of the Cs molecule is known and a large panel of monoclonal antibodies (mAbs) to Cs has been well characterized by cross-reactivity studies with numerous Cs analogs. In the present study, the sequences of the variable regions of seven mAbs to Cs were determined and a striking relationship was found between the expressed variable region genes and the Cs recognition pattern. An analysis of the length and hydrophobic content of the hypervariable regions and sequence similarities suggested that the heavy chain plays a major role in Cs recognition. Different fine specificities were observed for mAbs exhibiting identical light chains, while two antibodies differed by only a single amino acid located in the heavy chain. The presence of a duplication of 12 nucleotides within the heavy chain third hypervariable region of two antibodies suggests the existence of an additional mechanism for creating antibody diversity.

High-strength ceramics are required in dental posterior restorations in order to withstand the excessive tensile stresses that occur during mastication. The aim of this study was to investigate the fracture behavior and the fast-fracture mechanical strength of three veneer-framework composites (Empress 2/IPS Eris, TZP/Cercon S and Inceram-Zirconia/Vita VM7) for all-ceramic dental bridges.The load bearing capacity of the veneer-framework composites were evaluated using a bending mechanical apparatus. The stress distribution through the rectangular-shaped layered samples was assessed using simple beam calculations and used to estimate the fracture strength of the veneer layer. Optical microscopy of fractured specimens was employed to determine the origin of cracks and the fracture mode.Under fast fracture conditions, cracks were observed to initiate on, or close to, the veneer outer surface and propagate towards the inner framework material. Crack deflection occurred at the veneer-framework interface of composites containing a tough framework material (TZP/Cercon S and Inceram-Zirconia/Vita VM7), as opposed to the straight propagation observed in the case of weaker frameworks (Empress 2/IPS Eris).The mechanical strength of dental composites containing a weak framework (K(IC)3 MPam(1/2)) is ultimately determined by the low fracture strength of the veneer layer, since no crack arresting occurs at the veneer-framework interface. Therefore, high-toughness ceramics (K(IC)5 MPam(1/2)) should be used as framework materials of posterior all-ceramic bridges, so that cracks propagating from the veneer layer do not lead to a premature failure of the prosthesis.

The complete amino acid sequence of mature C8..beta.. has been derived from the DNA sequence of a cDNA clone identified by expression screening of a human liver cDNA library. Comparison with the amino acid sequence of C9 shows an overall homology with few deletions and insertions. In particular, the cysteine-rich domains and membrane-inserting regions of C9 are well conserved. These findings are discussed in relation to a possible mechanism of membrane attack complex formation.