Deformation Bands in Ceria-Stabilized Tetragonal Zirconia/Alumina : II, Stress-Induced Aging at Room Temperature

A stress-induced aging phenomenon is observed to occur at room temperature in deformation bands introduced into a 8.5 mol% ceria-stabilized tetragonal zirconia/alumina (CeTZP/A1₂O₃) composite by flexural loading. The aging occurs with time after unloading and in laboratory air. Over a period of 100 days, the concentration of monoclinic zirconia within a deformation band increases and, in addition, the wedge-shaped deformation band grows with time. Accompanying these two changes are an increase in the tensile stress in the remaining tetragonal zirconia within the deformation band and a consequential increase in the overall compressive stress within the band. The average value of the monoclinic concentration within the deformation band is found to increase parabolically with time, suggesting the mechanism responsible for the observed aging is diffusion limited. Away from the deformation bands, no aging is observed to occur, suggesting aging is stress dependent. Although a water-vapor-mediated mechanism cannot be ruled out, it is proposed that the observed aging is in fact due to a tensile stress assisted chemical reduction of Ce⁴⁺ to Ce³⁺ whose rate is controlled by the indiffusion of oxygen vacancies driven by the tensile stress gradient. It is further proposed that the deformation band grows with time, since the region ahead of the band is under tension and hence subject to an enhanced rate of reduction     

A photoelectron velocity map imaging spectrometer for experiments combining synchrotron and laser radiations

A velocity map imaging/ion time-of-flight spectrometer designed specifically for pump-probe experiments combining synchrotron and laser radiations is described. The in-house built delay line detector can be used in two modes: the high spatial resolution mode and the coincidence mode. In the high spatial resolution mode a kinetic energy resolution of 6% has been achieved. The coincidence mode can be used to improve signal-to-noise ratio for the pump-probe experiments either by using a gate to count electrons only when the laser is present or by recording coincidences with the ion formed in the ionization process.     

Residual Stresses in Alumina/Zirconia Composites: Effect of Cooling Rate and Grain Size

Alumina/zirconia composites with various compositions at the zirconia-rich part of the phase diagram have been prepared with various grain sizes of the starting alumina powders. After firing under identical conditions, the pellets have been cooled systematically, changing the cooling rates from 0.5 to 8000 K/min. Subsequently, the residual stresses in alumina have been determined by monitoring the frequency shifts of the R ₂ luminescence line of alumina (14 430 cm⁻¹). The data indicate that the stress in alumina is compressive in all cases, with increasing absolute values of the stress with decreasing alumina content. Within the same composition, the residual stress as a function of the cooling rate presents a minimum for values between 10 and 100 K/min, with no clear dependence on the alumina or zirconia grain size. An interpretation of the experimental data in terms of a Coble-type diffusional relaxation applies for intermediate cooling rates (from 10 to 800 K/min), but it fails to account for the large stresses at low cooling rates. The width of the stress distribution is narrow (∼150 MPa) and constant for all compositions and grain sizes at low cooling rates, but it increases for cooling rates >10 K/min, depending on the grain size but not on the composition. For fast cooling rates, a correlation is found when reporting the average width of the stress distribution as function of the average sintered grain-size distribution of alumina. Overall, zirconia grain size seems to influence the average stress, whereas alumina grain size determines the stress distribution.     

Local Measurements of Temperature Changes Associated with the Stress-Induced Tetragonal to Monoclinic Transformation in Zirconia

An experimental arrangement capable of monitoring temperature changes from 0.01 to 0.1 K has been successfully tested for registering the temperature evolution occurring during the tetragonal to monoclinic transformation of an alumina/ceria-stabilized tetragonal zirconia polycrystal (Al₂O₃/Ce-TZP). The arrangement is based on a very small thermistor. The data obtained have been used for evaluating the thermal diffusivity of the Al₂O₃/Ce-TZP composites.     

Simplified method for the construction of an orthonormal base forCPFSK signals

The authors present a simplified method to derive a 2M dimensional orthonormal base for M-ary CPFSK signals based on the well-known Gram-Schmidt procedure. On the basis of this method, the simulated bit error performance of 4, 8 and 16-ary CPFSK modulation in additive white Gaussian noise channels is presented     

Mechanical and chemical consequences of the residual stresses in plasma sprayed hydroxyapatite coatings

PURPOSE: To describe a preliminary investigation of a model of naltrexone therapy and counselling for use by primary care providers and evaluate its impact on drinking behaviors in a cohort of alcohol-dependent subjects. PATIENTS AND METHODS: The subjects enrolled in this study were 29 alcohol-dependent individuals. They were managed within a primary care treatment model located at a university-affiliated substance research program in New Haven, Connecticut. Subjects were assigned to a primary care provider for treatment of their alcohol dependence and were placed on naltrexone at a dose of 50 mg per day. They were seen for an initial "new patient" visit and 7 "brief" follow-up visits during the 10-week study. The primary outcomes for this study were completion of treatment, change in drinking behaviors from baseline, change in liver enzymes from baseline, provider ratings of improvement, and patient ratings of improvement and satisfaction with treatment. RESULTS: Of the 29 subjects: 21 (72%) completed treatment, and 10 (35%) relapsed to heavy drinking. All drinking behaviors improved significantly from baseline: percent of days abstinent increased from 36.6% to 88.8% (P < 0.0001), percent days abstinent from heavy drinking increased from 48.7% to 97.3% (P < 0.0001), and mean number of drinks per occasion decreased from 9.5 to 2.5 (P < 0.0001). The mean serum gamma glutamyl transferase (GGT) for the group decreased from 67.1 U/L to 45.3 U/L (P < 0.0001). CONCLUSIONS: In this preliminary investigation, treatment of alcohol dependence with our model of naltrexone and counselling by primary care providers appeared to be both feasible and effective.     

Indirect Determination of the Piezospectroscopic Coefficients of Ceria-Stabilized Tetragonal Zirconia Polycrystals

A method is proposed for the indirect determination of the stress dependence (expressed as piezospectroscopic (PS) coefficients) of spectroscopic bands of ceramic materials/phases. This method is based on the intimate mixture (intimate at the microstructural, grain-size level) of two phases/materials when the stress in one is independently known; it is used to determine the PS coefficients of the most intense Raman bands in ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP). Different amounts of Ce-TZP were mixed with alumina and the composite pellets sintered; subsequently, the stress in alumina was determined through the PS coefficient of its R2 luminescence band and the stress in Ce-TZP derived from the static equilibrium condition. The frequency shifts of each Raman band of Ce-TZP have been plotted against the stress and the slopes provide the PS coefficients. The method has the advantage of not requiring any type of loading device (i.e., diamond anvil-cell, bending jig). Finally, the limits are also discussed, the most important one being the requirement of immiscibility of the two materials/phases.     

The FAST module: an add-on unit for driving commercial scanning probe microscopes at video rate and beyond

We present the design and the performance of the FAST (Fast Acquisition of SPM Timeseries) module, an add-on instrument that can drive commercial scanning probe microscopes (SPM) at and beyond video rate image frequencies. In the design of this module, we adopted and integrated several technical solutions previously proposed by different groups in order to overcome the problems encountered when driving SPMs at high scanning frequencies. The fast probe motion control and signal acquisition are implemented in a way that is totally transparent to the existing control electronics, allowing the user to switch immediately and seamlessly to the fast scanning mode when imaging in the conventional slow mode. The unit provides a completely non-invasive, fast scanning upgrade to common SPM instruments that are not specifically designed for high speed scanning. To test its performance, we used this module to drive a commercial scanning tunneling microscope (STM) system in a quasi-constant height mode to frame rates of 100 Hz and above, demonstrating extremely stable and high resolution imaging capabilities. The module is extremely versatile and its application is not limited to STM setups but can, in principle, be generalized to any scanning probe instrument.     

Observation of Subcritical Spall Propagation of a Thermal Barrier Coating

Observations are reported of the room-temperature propagation of a spalling failure mode of a thermal barrier coating (TBC) from its bond coat after oxidation. The coating is a Y₂O₃-stabilized ZrO₂ coating formed by electron-beam deposition on a Ni-Co-Cr-Al-Y bond coat. The spall shape evolution and stress redistribution as the spall propagates are reported. The failure propagates primarily as an interface crack between the bond coat and the thermally grown aluminum oxide (TGO) formed on the underside of the TBC during oxidation. The observations are consistent with subcritical propagation of an interface crack between the TGO and bond coat assisted by the presence of moisture. An estimate of 9 J/m² is made of the fracture resistance in air of the interface.     

Cyclolinear organosilicon copolymers with monocyclic fragments in the side chain

The reaction of heterofunctional condensation of organodichlorosiloxycyclotri(tetra, penta) siloxanes and organodichlorosililcarbocyclotrisiloxane with dihydroxydimethylsiloxanes in the presence of pyridine was investigated. It was shown that at small lengths of the linear dimethylsiloxane link (n ≤ 4) the reaction of heterofunctional condensation runs both intermoleculary with formation of polymers and intramoleculary with formation of bicyclo‐organosiloxanes. It was established that insertion of cyclic fragments in the side chain hinders the chain transfer reactions that proceed with release of the D‐type cycles during thermal depolymerization. The conformational and hydrodinamic properties of some polymethylsiloxane copolymers with cyclosiloxane fragments in the side chain have been studied. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 583–594, 1999