S. J. Burns

Solidification of YBa2Cu3O7−δ from the melt

Abstract Nucleation and growth of 123 from the melt via a peritectic reaction into domains of aligned platelets is studied. Analysis of the microstructure of well-formed domains indicates that there is no orientation difference between adjacent platelets within a domain, suggesting that a domain grows from a single nucleus. The platelet boundaries are found to be filled-in with secondary phases that correspond to the liquid phase at high temperature, suggesting that constitutional supercooling effects may be operative. Samples quenched from temperatures considerably below the peritectic temperature contain only a few crystals, indicating the presence of a large nucleation barrier. The above observations, coupled with extensive microstructural examination of quenched solid-liquid interfaces, suggest that the 211 size, distribution and volume fraction not only control the growth rate of 123 along the fast growth ab -plane (by supply of yttrium), but also the growth rate along the slow growth c -direction since the nucleation barrier is reduced at 211/123 intersections. At high cooling rates there is a distinct change in the nucleation and growth processes. Structures characteristic of sympathetic or autonucleation and spherulitic growth are observed. These structures are distinct from the single crystal nature of well-formed domains. The growth mechanism which results in the formation of 123 domains and the final microstructure within a single domain, also explains the observed non-weak-link characteristics for current flow along the a , b - and c -directions, as determined by direct transport and magnetization measurements.

Mechanical properties of highly aligned YBa2Cu3O7−δ effect of Y2BaCuOx particles

Abstract The elastic modulus and hardness of highly aligned YBa 2 Cu 3 O 7−δ obtained by melt processing was determined using a highly spatially resolved mechanical properties microprobe. Ultra-low load indentation measurements on the (001) cleavage plane of aligned 123, indicated a Youngs modulus of 143 ± 4 GPa and a hardness of 10.0 ± 1.9 GPa. For measurements on a plane perpendicular to the cleavage plane, values of 182 ± 4 GPa for the modulus and 10.8 ± 1.7 GPa for the hardness were obtained. A lower modulus in the c -direction is perhaps a result of the layer-like structure of 123, with weak coupling between the layers. Measurements on the trapped single crystal 211 particles yielded a modulus of 213 ± 5 GPa and a hardness of 14.4 ± 2 GPa. Considerations of the thermal and elastic mismatch effects between the 211 particles and the 123 matrix, the large thermal expansion anisotropy of aligned 123, and microstructural examination of polished and fracture surfaces of the aligned samples indicate that the 211 particles perhaps serve to enhance the fracture resistance behavior of 123 by energy dissipation due to interfacial delamination and crack bridging.

Anisotropic hardness and fracture toughness of highly aligned YBa2Cu3O7−δ

The hardness and fracture toughness of aligned YBa2Cu3O7−δ obtained by melt processing was found to be highly anisotropic. Indentation measurements show that the (001), (100), and (010) planes are the preferred fracture planes in this material and that the critical stress intensity factor for propagating a crack on the (001) basal plane is the lowest, i.e., Kc001<Kc100 or Kc010. Indentation crack length measurements on the (001) basal plane with the impression diagonals oriented parallel to the [100] and the [010] directions, indicate that the fracture toughness of these planes is K100/010air∼0.7 MPa m1/2. The hardness in this orientation was found to be 6.7 GPa. Measurements on a plane perpendicular to the basal plane resulted in a lower hardness of ∼3.8 GPa. This reduction in hardness is influenced by the extensive preferential cleavage of the (001) basal plane boundaries. The extremely low values of the fracture toughness suggest that considerable toughening would have to be achieved in melt‐textured 1...

Fabrication of highly aligned YBa2Cu3O7−δ-Ag melt-textured composites

Abstract Liquid phase processing techniques with slow cooling through the peritectic transformation have been utilized to fabricate bulk aligned 123-Ag composites. Coarse (∼30–40 μm) and fine (∼5–10 μm) particles of silver are distributed between and within the 123 grains. Although the processing temperatures employed are much higher than the melting point of silver, particles of silver are retained behind the advancing solid-liquid interface. The peritectic reaction occurs at the solid-liquid interface and the rate of the reaction is limited by diffusion between the two solid phases, hence the transformation invariably results in some unreacted phases. Ellipsoidal, Y 2 BaCuO itx particles varying in size from 10–25 μm are trapped within the 123 phase. In addition, regions of CuO and BaCuO 2 are present in the sample as separate phases. Silver particles are also dispersed in these insulating regions of the microstructure. The 123-Ag composites have superior mechanical properties compared to melt-textured stoichiometric 123 samples. A detailed study describing the effects of processing on the microstructureof these aligned composites is reported.

Internal stress wave measurements in solids subjected to lithotripter pulses

Semiconductor strain gauges were used to measure the internal strain along the axes of spherical and disk plaster specimens when subjected to lithotripter shock pulses. The pulses were produced by one of two lithotripters. The first source generates spherically diverging shock waves of peak pressure approximately 1 MPa at the surface of the specimen. For this source, the incident and first reflected pressure (P) waves in both sphere and disk specimens were identified. In addition, waves reflected by the disk circumference were found to contribute significantly to the strain fields along the disk axis. Experimental results compared favorably to a ray theory analysis of a spherically diverging shock wave striking either concretion. For the sphere, pressure contours for the incident P wave and caustic lines were determined theoretically for an incident spherical shock wave. These caustic lines indicate the location of the highest stresses within the sphere and therefore the areas where damage may occur. Results were also presented for a second source that uses an ellipsoidal reflector to generate a 30-MPa focused shock wave, more closely approximating the wave fields of a clinical extracorporeal lithotripter.

Anisotropic thermal conductivity of rare earth--transition metal thin films

Thermal conductivity measurements were performed on several amorphous rare earth transition metal thin films of varying microstructure. The thermal conductivity perpendicular to the plane of the film, measured by the thermal comparator method, was compared with the thermal conductivity value measured parallel to the plane of the film. The latter value was obtained by converting electrical conductivity values to thermal conductivity via the Wiedemann--Franz relationship. As expected, the columnar microstructure induced during the sputter deposition of the thin films causes an anisotropy in the thermal conductivity values, with the in-plane values consistently lower than the out-of-plane values. The effect is most pronounced for the more columnar films deposited at higher pressure, for which the in-plane thermal conductivity, 0.3 W/mK, is an order of magnitude lower than the out-of-plane thermal conductivity, 4.3 W/mK. The thermal conductivity out of the plane of the film decreased with increasing deposition pressure, due to the decreasing film density.

Model for laser damage dependence on thin-film morphology.

The high laser damage thresholds often reported for porous thin films are discussed in terms of point defects or small absorbing inclusions as sites of thin-film damage initiation. The model is based on the internal pressure built up upon laser heating at short times. The competing effects of short pressure relaxation distances and low thermal conductivity inherent in porous films are discussed. The model predicts that at thicknesses less than or equal to the neck or column diameter of a porous film, the effect of lower thermal conductivity should dominate and cause the films to exhibit lower laser damage thresholds than their denser counterparts.

DEPENDENCE OF BIREFRINGENCE AND RESIDUAL STRESS NEAR LASER-INDUCED CRACKS IN FUSED SILICA ON LASER FLUENCE AND ON LASER-PULSE NUMBER

Measurements of birefringence induced in fused-silica specimens by a crack produced by a 351-nm/500-ps Nd:glass laser as a function of laser fluence F(L) and of number of laser shots N are presented. The varying dimensional parameter is found to be the crack depth a and can be put in the form a(mm) = (0.0096 ? 0.0021)N[(F(L)/F(exit/th)) - 1](2/3) with F(L) >/= F(exit/th)(F(exit/th) is the exit-surface damage threshold). The retardance data are converted into units of stress, thus permitting the estimation of residual stress near the crack. The results of the measured residual stress can be cast in the form varsigma(r)(MPa) approximately (. ? .)[(F(L)/F(exit/th)) - 1](1/2) N(2/3) with F(L)>/= F(exit/th). A theoretical model giving the stress field around a crack is developed for comparison and shows reasonable agreement with the experiment. Good agreement with experimental data of others is also obtained. The effect of residual stresses on fracture strength is pointed out. The results obtained show that the presence of birefringence/residual stress in a fused-silica specimen with a crack on its surface has a strong effect on fracture and should be taken into account in any formulation that involves the failure strength of optical components used in inertial-confinement-fusion experiments.

The Role of Blade Sharpness in Cutting Instabilities of Polyethylene Terephthalate

Industrial polyester ®lms are produced in large rolls that need to be cut to speci®c dimensions to make ®nal products. The deformation and secondary fractures during the cutting or slitting processes reduce the amount of usable material. An ideal cut should be perpendicular to the web, leave minimal debris, be performed at high speeds and have no deformation at the edges. The dominant mechanical factors in cutting polyester ®lms are tension in the web, blade sharpness, cutting speed and material properties [1]. Arcona and Dow [2] examined the role of blade sharpness in the slitting of polyethylene terephthalate (PET) using commercially available scissors. One blade was ®xed and the other driven by a variable speed motor. The cutting force was measured with a load cell attached to the scissors. Impressions of the blade, made with vinyl polysiloxane, were sectioned and viewed under a light microscope to determine the blade radius. The cutting speed and the angle of the blade entering the web varied over the length of the cut. Their results showed the cutting force strongly decreasing with cutting speed and increasing with blade radius. Our experiments on cutting a PET web, with rate and knife angle independently controlled, showed that for a ®xed blade angle, the cutting force varied by less than 5% when the cutting speed changed from 10 mm sy1 to 10y2 mm sy1 [3]. We concluded that the blade cutting angle can cause the web to buckle locally but that the cutting force is nearly speed-independent for sharp knives. In this letter, we have varied the knife blade radius and cut a PET web again with a wide selection of cutting speeds. At very large cutting radii, we observed a negative strain rate±sensitivity [4] type of effect. This rate instability in the cutting force shows a decrease in cutting force for increasing cutting rate. The effects of blade sharpness and cutting speed were examined on 178 2 im PET. The test ®xture measured the cutting force and allowed independent control of the cutting speeds and blade angle [3]. Stanley #11-921 knife blades were dulled using a special tool designed to round the edge at the blade tip. The blades were dulled by abrasion on 600 grit silicon carbide paper. The blade direction was reversed at the midpoint of dulling to ensure a symmetric edge. It was found that 100 passes over silicon carbide paper increased the radius of curvature of the blade by approximately 1.4 im. The blade tip radius was measured using 3M Express 7322H vinyl polysiloxane impression material. The impressions were made with the blade vertical and held still during the hardening process so the radius was not skewed. The mold was sectioned into pieces approximately 1.5 mm thick. These sections were viewed under a light microscope, and the radius was determined by ®tting the blade tip to a circle template and recording the microscopes magni®cation. This method was veri®ed by mounting a section of the blade in a molding material and metallurgically polishing it for viewing under the light microscope. The radii for the two methods differ by less than 1 im, which con®rms the validity of the radius found using the impression material. Sections of the blade were also viewed in a scanning electron microscope (SEM), and it was found that the radii matched those measured by the impression material. All testing was conducted using the slitting apparatus, seen in Fig. 1, mounted on a screwdriven test machine (Instron Model 1115). The slitting apparatus was designed to deliver a long, straight slit to the PET and accurately record the force needed to make the cut [3]. The web was

Effect of Ag/Ag2O additions on the resistive behavior of preformed YBa2Cu3O7−δ compacts in the low temperature sintering regime

Abstract Contacts between the individual ceramic particles dictate the resistive behavior of bulk, polycrystalline, preformed YBa 2 Cu 3 O 7−δ compacts in the low temperature sintering regime. Ag/Ag 2 O additions to performed 123 are investigated, with an aim to lower the sintering temperature required to achieve macroscopic zero resistivity in particulate cermet superconductors. The addition of Ag/Ag 2 O results in enhanced resistive properties due to improved interparticle contacts. The thermal decomposition of Ag 2 O in the presence of 123 is unaltered, as shown by DTA and TGA analysis. Hence, the presence of Ag 2 O at temperatures higher than its normal decomposition temperature appears unlikely. For sintering temperatures below 600°C, internal oxidation due to dissociation of Ag 2 O may serve as an efficient mechanism for oxygenating the 123 phase. However, for sintering temperatures close to 800°C, Ag additions are as effective as Ag 2 O additions and the benefit of oxygen donation by Ag 2 O appears to be small. The addition of Ag/Ag 2 O and the use of high compaction pressures, reduce the minimum sintering temperature to attain macroscopic zero resistivity from 900–950°C to ∼ 800°C.