J. E. Graebner

Unusually high thermal conductivity in diamond films

Chemical‐vapor‐deposited (CVD) polycrystalline diamond films have recently been reported with a thermal conductivity that is only 25% less than that of high quality single‐crystal natural diamond. By studying a series of such films of various thicknesses grown under virtually identical conditions, we have discovered a significant (factor of four) through the thickness gradient in thermal conductivity. The observed gradient is attributed mainly to phonon scattering by the roughly cone‐shaped columnar microstructure. For 350 μm films, the material near the top (growth) surface has a conductivity of at least 21 W/cm °C, i.e., comparable to the best single crystals. This remarkable dependence of thermal conductivity on microstructure has important implications for thermal management of microelectronic devices using CVD diamond.

Enhanced flux pinning by phase decomposition in Y-Ba-Cu-O

Significantly improved flux pinning has been achieved in bulk YBa2Cu3O7−δ superconductor (‘‘123’’ compound) containing fine‐scale defects (<∼50 A thick). The measured Jc intragrain of ∼105 A/cm2 at 77 K, H=0.9 T is about ten times higher than the typical values for bulk Y‐Ba‐Cu‐O. The improved structure was produced by rapid decomposition at 920 °C of the YBa2Cu4O8 (‘‘124’’) precursor. This new and simple processing route could lead to a commercially viable processing technique for flux‐pinning enhancement in bulk Y‐Ba‐Cu‐O.

The low temperature electrical properties of 1T-TaS2

Abstract Measurements of the electrical resistivity of 1 T -TaS 2 to 0.03 K show that the increase in resistivity below ∼ 50 K is extrinsic.Below 2 K the resistivity is described by ϱ = ϱ 0 exp T 0 /T) 1 3 . Because of this fractional power law behavior, we conclude that the increase is due to Anderson localization by random impurity and/or defect potentials. Other difficulties in understanding the properties of 1 T -TaS 2 are also pointed out.

Critical currents and magnetization in c -axis textured Bi-Pb-Sr-Ca-Cu-O superconductors

Transport critical currents and magnetization behavior in c‐axis textured Bi‐Pb‐Sr‐Ca‐Cu‐O superconductor ribbons have been studied. The highly oriented layer structure was achieved by a combination processing of spray coating on silver foil, cold rolling, and partial melting. Transport Jc values as high as 2.3×105 A/cm2 at 4.2 K, H=8 T (H⊥ab) have been obtained. The high Jc at H≥5 T is maintained to temperatures near 20 K but it vanishes completely at or above ∼30 K, thus showing the limitation in useful, high‐field operating temperatures for the Bi‐system superconductors. A comparison of Jc (transport) and Jc (magnetization) indicates that the size scale of the circulating supercurrent loop in the Bean model nearly corresponds to the whole sample dimension rather than the orders‐of‐magnitude‐smaller grain size. This demonstrates that the a‐b grain boundaries in the melt‐processed ribbons are not weakly coupled. The time decay of magnetization has also been studied.

Massive thinning of diamond films by a diffusion process

Substantial thinning or polishing of diamond films is a nontrivial problem because of the extraordinary hardness of diamond. In this letter, we report the observation of massive thinning of CVD diamond film (from ∼220 to ∼120 μm thickness) by simple diffusional transfer of carbon from diamond to iron foil at 900 °C. The thinning process also creates relatively smooth surfaces by eliminating much of the roughness from the top faceted surface of the film. A very sharp Raman peak at 1332 cm−1 indicates that the high quality of the diamond bonding is not compromised by the thinning heat treatment. This technique may be useful for conveniently removing the undesirable part of the diamond films, such as the rough growth facets or the fine‐grained bottom layer with inferior thermal properties.

Thermal conductivity and the microstructure of state-of-the-art chemical-vapor-deposited (CVD) diamond

Abstract Two new techniques have been developed for making high-accuracy measurements of the thermal conductivity κ in chemical-vapor-deposited (CVD) diamond films: (1) a steady state technique for measuring κ∥ (heat flowing in a direction parallel to the plane of the sample) and (2) a laser flash technique for measuring κ⊥ (heat flowing perpendicular to the plane of the film). By measuring κ∥ and κ⊥ for a series of high-quality CVD diamond films of different thicknesses, we are able to extract local values for these variables as a function of height z above the substrate surface. Both show a large gradient with respect to z, with κlocal⊥ rising more rapidly with z than κlocala for approximately the first 200 μm. This is consistent with phonon scattering from impurities and defects if they are preferentially located near grain boundaries of the columnar structure. For z ⪆ 300 μ m , the local conductivity is nearly isotropic with the very high value of 23–24 W cm−1°C−1 at 25°C, to be compared with 22 W cm−1°C−1 for the best type IIa single-crystal diamond so far reported. These results have direct implications for the thermal management of microelectronic devices if the remarkable conductivity of diamond is to be used most effectively.

Polishing of CVD diamond by diffusional reaction with manganese powder

Abstract Because of their extraordinary mechanical hardness, polishing of CVD diamond films is often time consuming and costly. In this work, substantial polishing of diamond films has been accomplished by simple diffusional reaction at 900 °C with manganese powder. The rough growth facets (typically about 20–60 μm variation in height) on the top surface of the films have been essentially removed. The observed polishing effect is attributed to the diffusional transfer of carbon atoms from diamond to manganese which has a large solid solubility for carbon (approximately 12 at.% at about 900 °C). The reaction of manganese with diamond appears to be much faster than that of iron. In addition, much of the manganese reaction product left on the diamond surface is easily etched away in acid, which is not the case with iron. Patterning of diamond films by selective deposition of manganese thin films and reaction with diamond is also described. The polishing techniques utilizing manganese may conveniently be used to remove undesirable parts of the film, such as the top growth facets or the fine-grained bottom layer with inferior physical properties, and for simultaneous processing of a large number of films.

Superconducting properties of YBa2Cu3O7−δ with partial rare earth substitution

Abstract Superconducting properties of the YBa 2 Cu 3 O 7−δ compound with partial rare earth substitution (20 atomic pct substitution for yttrium) have been studied. Among the 14 rare earth elements investigated, La, Ce and Pr caused suppression in T c , while other elements had no effect. Some of the rare earths (Sm, Gd, Eu), when partially substituted for Y, resulted in a slight improvement in intragrain J c (flux pinning) as measured magnetically. Although the rare earth elements have widely different ionic radii, all the (RE) Ba 2 Cu 3 O 7−δ phases have been found to be soluble in YBa 2 Cu 3 O 7−δ with no detectable segregation observed by SEM and energy dispersive X-ray analysis.

Sources of thermal resistance in chemically vapor deposited diamond

Abstract Measurement of the strong gradient (with respect to the distance z from the substrate surface) in the local thermal conductivity of CVD diamond over a wide temperature range (4–400 K) provides a powerful tool for identifying the microscopic sources of thermal resistance. IR absorption and elastic recoil measurements in the same samples reveal the primary impurity to be hydrogen. However, quantitative comparison with the measured point-defect thermal resistivity, as well as with nuclear magnetic resonance studies, indicates that hydrogen is not by itself a strong source of thermal resistance. Rather, it is usually associated with defects or other impurities which do cause thermal resistance and as such the hydrogen is only a secondary indicator of thermal resistance. Mass-density measurements reveal a lower mass density near the substrate surface than near the growth surface. Quantitative arguments are given for the preferential location of point defects at grain boundaries and for the preferential alignment of dislocations and twin intersections with the growth direction, thus accounting for the large anisotropy observed in the conductivity in the range z ≈ 30 to 100 μm .

Local thermal conductivity in chemical‐vapor‐deposited diamond

The thermal conductivity has been measured in the temperature range 5–400 K for a series of chemical‐vapor‐deposited diamond samples differing only in thickness. By analyzing the conductance, a local conductivity is extracted as a function of height z above the substrate on which the samples were grown. An analysis of the temperature dependence of the conductivity at any given height yields the phonon scattering mechanisms as a function of z. Point defects and extended defects of approximately 1.5 nm diam appear to be the most important phonon scattering entities at room temperature, but their scattering strengths decrease with height above the substrate. At a height z≊300 μm, the material is sufficiently perfect that scattering from extended defects at room temperature is negligible and scattering from point defects is only slightly higher than that attributable to naturally occurring 13C. The observed anisotropy of the thermal conductivity is consistent with aggregation of point defects and extended def...