Manfred Kahn

Subsurface defect detection in ceramics by high-speed high-resolution optical coherent tomography

We use optical coherence tomography with a new configuration to determine the size and location of subsurface defects in solid ceramic and composite ceramic materials. Cross-sectional subsurface regions either parallel or perpendicular to the surface were examined. We present experimental results showing that the size and distribution of small subsurface defects can be determined with depth and lateral resolutions of 10 and 4 microm, respectively.

Joining of ceramic tubes using a high-power 83-GHz millimeter-wave beam

High purity, high density alumina tubes have been successfully joined using a high-power millimeter-wave beam. This technique exploits the use of the beam-forming capability of an 83-GHz gyrotron-based system allowing the deposition of energy into a narrow region surrounding the joint area with minimal heating (<100/spl deg/C) of the metal fixturing (a modified microlathe). The power deposition and heating was modeled using a closed form analytical approach that has been compared with experimental results. The modeling results indicated areas of improvement that were implemented to make the process more effective. Conjoined tubes resulting from this technology meet the requirements for the dielectric-loaded accelerator (DLA) being developed by the Argonne National Laboratory.

Piezoelectric ceramic hydrostatic sound sensor

A piezoelectric ceramic hydrostatic sound sensor or transducer having highensitivity to hydrostatic pressure is made by placing a flat plastic disc between two flat layers of green ceramic material, compressing and fusing the layers, heating to a first temperature at which the plastic decomposes, leaving a flat void in the ceramic, and heating to a second temperature at which the ceramic sinters. The transducer is provided with electrodes on its top and bottom surfaces. In a further improvement, ceramic particles are provided which are entrapped in the void; they render the sound sensor sensitive to inertial forces. In yet another improvement, the inside walls of the void are coated with a conductive noble metal connected to a terminal wire, whereby an additional electrode is provided for sensing the electromechanical response of the transducer.

On the determination of the piezoelectric shear coefficient, d15, in a PZT ceramic

Abstract The polarization obtained in the direction of an arbitrary uniaxial stress is derived by applying tensor transformations to the polarization-stress relation: Pi , = dijkσjk . Two- and three-dimensional plots of the results are shown. The experimental work described includes the preparation of discs poled uniformly at 90° to their thickness and the measurement of d 333 as a function of orientation. Very good agreement with the calculated results was obtained. The effect of electrodes on the measurement of d 333 (d 33) and the use of a d 311 (d 31) parameter measurement to permit a d 113 (d 15) determination are also demonstrated. The d 333 measurements as a function of stress orientation permitted determination of the d 15 value with a resolution of ±8%. Its average value was within 5% of d 15 values derived from other techniques.

1-1-3 Piezocomposite for hydrophone transducer

Abstract A piezocomposite structure was prepared in a 1-1-3 configuration by casting a soft epoxy interlayer between PZT rods and a stiff polymer preform matrix to maximize the hydrostatic sensitivity of 1–3 type PZT polymer hydrophone transducers. By minimizing the adverse contribution of the lateral components of the hydrostatic pressure, a figure of merit of 7,200 (10−15 m2/N) was obtained in an ambient atmosphere. By encapsulating the composite with cover plates, a stable pressure dependence (up to 14 MPa) of the hydrostatic voltage and charge coefficients was obtained. In addition, the interface configuration between the composite and cover plate was modified to provide pressure amplification proportional to the reciprocal of the volume fraction of the PZT rods in the composite. As a result the modified interface structure in 1-1-3 composite transducer gave significantly increased piezoelectric coefficients, and the figure of merit at 2MPa hydrostatic pressure was enhanced to more than 22,000 (10−15 ...

High authority, telescoping actuators

A family of high authority actuators was developed at the Naval Research Laboratory. These actuators are based on displacement amplification within a compact, solid state, monolithic piezoelectric actuator, by using a telescoping tube design. In this design, concentric tubes are mechanically connected in series. This gives an effective actuator length that is equal to the sum of the lengths of the individual concentric elements. The high displacement output of this actuator permits efficient coupling of the actuator output into a load of similar impedance, and thereby much greater effective actuator output. Initial prototypes were made of commercially available PZT tubes of three different diameters and wall thickness. These tubes were pulsed through the thickness of the walls and the change in their lengths were used for actuation. Their actuation is therefore making use of the d31 piezoelectric coefficient. Alternatively, electrodes can be applied to the ends of the individual concentric tubes and their lengthwise displacement will subsequently be proportional to the d33 parameter of the material. The tubes were bonded at their ends to alumina plates using epoxy-based adhesive. The displacement obtained from the assembly is close to the sum of those of the three individual tubes at the same applied field. Other parameters such as blocking force and energy densities are also reported. These actuators have applications where high force and simultaneously large displacement are required and space is limited. Potential uses include high end aerospace as well as low tech commercial applications.

The development of mesoscale accelerometers with single crystal piezoelectric materials

Single crystal piezoelectric materials hold great promise as a candidate sensor element from which to develop low-noise, high-sensitivity vibration transducers. This paper outlines the integration of relaxor ferroelectric materials (PZNT and PMNT) into a mesoscale accelerometer. A robust bimorph design was selected given reliability considerations. Over a frequency range between 100-1000 Hz, the spectral noise of the single crystal-based accelerometer was approximately one order of magnitude lower (20 dB) than its PZT-based counterparts of equal dynamic mass and physical dimensions. This work was sponsored by the Office of Naval Research as part of a Phase I Small Business Innovative Research (SBIR) program.

Solid State Opening Switches

The generation of pulse power through the use of inductive energy storage devices frequently requires an opening switch that can operate repetitively, tolerates an instantaneous voltage increase, has a low inductance and carry a heavy load. The application of nonlinear, solid state materials to implement these types of opening switches is described in this chapter.

Simulation of elastic stresses and polarization in piezoelectric ceramic by the finite difference method

Abstract Stresses and polarization within a piezoelectric ceramic have been simulated via numerical boundary-value analysis. A Finite Difference algorithm was written to calculate the elastic response by using the Airy stress function method. Tetragonal anisotropy in the elastic, dielectric, and piezoelectric properties were included in the model. A two-dimensional plane stress analysis is used to calculate the solution for the distribution of stresses within the ceramic. The method assumes that a uniform poling field provides alignment of elastic, dielectric, and piezoelectric properties along a preferred axis. Stresses may be applied at the boundary of the model in either a uniform and in a nonuniform manner to simulate forces imposed on the specimen. The elastic response and the resulting polarization and the stresses created are used to calculate the piezoelectric stress coefficients d 33 and d 31. Simulation results were compared with test data taken on a Berlincourt d 33 meter. The conclusions from ...

Consolidation of Polycrystalline Yttria Powder By Millimeter-Wave Sintering for Laser Host Applications

Summary form only given. We report recent results of an investigation of millimeter-wave processing of yttria (Y2O3) for fabrication of transparent, high strength polycrystalline ceramic laser hosts for high energy laser (HEL) applications. The objective is to produce polycrystalline materials with optical quality comparable to that of a single crystal. It is difficult to produce yttria single crystals because of the phase transformation around 2000degC and the high melting temperature which is over 2400degC. While single crystals have high thermal conductivity and can operate at high powers, they are costly and limited in size and dopant concentration. Significant advantages of polycrystalline materials compared to single-crystals, are lower processing temperature, higher gain as a result of higher dopant concentrations, faster and less expensive fabrication, and the possibility of larger devices. Millimeter-wave processing has been proposed as an alternative method to solve the problems of both conventional vacuum sintering and low frequency microwave sintering, such as low heating rates, poor coupling, and unfavorable thermal gradients. A major component of the NRL millimeter-wave processing facility is a 20-kW, continuous-wave (CW), 83-GHz gyrotron oscillator (GYCOM, Ltd.). Translucent yttria has been successfully sintered with millimeter-wave beams with up to 99% theoretical density. A partially transparent yttria ceramic sample has also been achieved using the millimeter-wave sintering process. Several factors impact the quality of the sintered material including the presence of agglomerates, impurities, processing atmosphere, sintering aids, and thermal gradients. Efforts to improve the transparency are in progress.