John E. Keem

High performance isotropic permanent magnet based on Nd‐Fe‐B

We have fabricated a macroscopically isotropic permanent magnet material (Ovonic Hi‐RemTM) which significantly exceeds conventionally understood performance limits for a material with saturation magnetization Ms =16 kG. Enhanced magnetic properties, including a remanent magnetization Mr =9–10 kG or more, and a maximum energy product in excess of 20 MGOe, are observed without preferred orientation. This material has been fabricated by rapid solidification of an alloy structurally and compositionally similar to Nd2Fe14B, but which depends on critical alloying additions and process parameters to obtain optimum magnetic performance.

Chemical states study of Si in SiOx films grown by PECVD

Abstract Thin films of SiO x have been grown by low temperature (350°C) plasma enhanced chemical vapor deposition (PECVD), and the local chemical bond of Si and O has been investigated by infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Comparisons between the IR spectra of Si-rich oxides (SiO x , x 2 and low oxygen content a-Si:H:O alloys demonstrates a continuous variation in the character of the Si-O stretching, bending and rocking vibrations as a function of the oxygen concentration. In particular, the frequency of the dominant stretching vibration displays a linear frequency shift with the relative oxygen concentration. A similar behavior prevails in the XPS spectra where the position of the Si2p core states shifts monotonically with increasing oxygen concentration from a value of about 98.8 eV in a-Si to 103.1 eV in a-SiO 2 . The IR and XPS then reflect the average chemical bonding in these films. In contrast, the AES spectra of suboxides display two strong features in the SiL 23 VV spectrum. We show that one of these features at approximately 78 to 80 eV is associated with ejection of electrons from Si-O valence bonding states, and a second at about 90 eV is associated with ejection of electrons from Si-Si bonding states. Therefore, the AES spectra reflects the details of the local bonding, i.e., the relative numbers of Si-O and Si-Si bonds, while the IR and XPS spectra reflect the average chemical bonding. In this context, the combination of the three techniques confirms that the suboxide fims produced by this low temperature PECVD process are homogeneous, in contrast to being two phases with regions of a-Si and a-SiO 2 .

Thermal Stability of W/C Multilayer Films

W(10A)/C(40A), W(15A)/C(15A) and W(40A)/C(10A) periodic multilayer films were prepared by magnetron sputtering and subsequently annealed at 730°C. The resulting change of the layered and crystal structures was studied by X-ray diffraction. The resulting structures largely depended on the thickness ratio of W to C. For example, in the W(15Å)/C(15Å) sample a-W as formed without any peaks of carbon crystals; in W(40Å) C(10Å)only W2C (orthorhombic) peaks appeared.

Observation of rf-induced dc voltages in sputtered binary superconducting films

Abstract We have observed that an rf current of frequency 1–5 MHz, passing through a sputtered binary superconducting film, can induce a dc voltage as large as 1 mV across the film for temperatures near the critical temperature T C . The largest induced voltage has been observed in films which are microscopically inhomogeneous, such as Mo 63 C 37 films which exhibit columnar microstructure. We suggest that the effect may be due to an inverse ac Josephson effect in a randomly connected series array of many microscopic superconducting weak links that are expected to exist close to T C .

Superconducting properties of amorphous transition metal alloys

Abstract We have co-sputtered amorphous films of several Mo and W-based superconducting alloys. Measurements of Tc, ( d H c2 d T )Tc and the normal state resistivity near Tc were made on a number of these alloys. Our results and other data from the literature are studied to examine the correlation between Tc and the dressed density of states at the Fermi level.

Theoretical studies of hydrogen storage in binary Ti-Ni, Ti-Cu, and Ti-Fe alloys

Theoretical studies have been carried out to examine hydrogen storage in some binary transition metal alloys which include titanium as one of the alloying elements. Quantum mechanical calculations at the Extended Hückel level of approximation have been performed on numerous clusters of compositions Ti18Ni18, Ti18Ni18H, Ti18Ni18H12, Ti24Ni12, Ti24Ni12H, Ti24Ni12Hi12, Ti16Cu16, Ti16Cu16H, Ti24Cu2, Ti16Fe16, Ti16Fe16H9, and Ti16Fe16H32, to yield information on energetics, densities of states, charge distributions, and the effects of hydrogenation on these properties. In addition, ab initio calculations at the split valence level of approximation have been performed on several smaller clusters. The hydrogens have been shown to acquire a partially anionic character in all cases. Another conclusion is that the preference of H for certain types of sites (for example the tetrahedral Ti4 sites in crystalline TiCu) is more likely to be related not to the intrinsically greater stability of a hydrogen atom located in such a site, but to more general topological and electronic considerations. Qualitative concepts related to the classification, spatial distribution, and sizes and shapes of “hole” sites which could become occupied by hydrogen atoms, have been shown to correlate with hydrogen storage capacity for crystalline materials. These qualitative concepts have been extended to amorphous materials and corroborate the observations that under optimized conditions amorphous alloys can be found with better reversible hydrogen storage properties than the crystalline or microcrystalline systems. Distorted tetrahedral and octahedral holes have been examined in detail, and parameters (volume, area, “tetrahedrality”, and “octahedrality”) have been introduced to describe their sizes and shapes. An algorithmic surveying technique has been introduced, and shown to provide useful information about the limiting amounts of hydrogen uptake.

New method for achieving accurate thickness control for uniform and graded multilayer coatings on large flat substrates

High performance in normal incidence soft X-ray optical systems requires accurate control of the d-spacing across the surface of each mirror in the system. As a first step towards being able to fabricate any desired d-spacing variation, we demonstrate the ability to produce large (25 x 150 mm) flat Mo/Si multilayer coated mirrors with a d-spacing uniformity of +/- 0.4 percent. Instead of applying the approach most often taken to minimize the d-spacing variation physical shielding of the deposition source, we use a mask with a corrected profile positioned just in front of the rotating substrate to compensate for the nonuniform deposition flux. Results obtained from hard (lambda = 0.154 nm) and soft (wavelength of interest) X-ray mapping of the surface are presented along with a discussion of the technique used to control the d-spacing distribution.

Measured X-ray Performance of Synthetic Multilayers Compared To Calculated Effects of Layer Imperfection

In connection with the manufacture of OVONYXTM multilayer x-ray optical elements, investigations have been carried out into the effects of various types of layer imperfections on the x-ray optical properties of multilayers. These have included extensive numerical modeling of real multilayers (using a computational scheme based on the complete dynamical theory) including simulations of interface diffusion, deviations from constant d-spacing, and interface roughness. Results are presented for several examples of Hf-Si and W-Si multilayers, including comparison of measurements at Cu-Kato the theoretical model.

MULTILAYER X-RAY REFLECTORS: EFFECTS OF LAYER IMPERFECTIONS

Periodic multilayer structures, consisting of largely amorphous alternating layers of heavy and light elements, have been fabricated by sputtering techniques. Investigations have been carried out into the effects of various types of layer imperfections on the x-ray optical properties of these multilayers. These have included extensive numerical modeling of real multilayers (using a computational scheme based on the complete dynamical theory) with simulations of diffused interfaces and deviations from constant d-spacing. Results are presented for examples of W–C and Hf–Si multilayers, with comparison of measurements at Cu–K to the theoretical model.

Superconducting Properties of Amorphous Multilayer Metal-Semiconductor Composites

There has been considerable interest in the past decade in developing novel materials using precision microlayering techniques. Much of this has been oriented towards crystalline epitaxy, using pairs of materials with lattice spacings that almost match. However, for most other material pairs, layer interfaces tend to be dominated by dislocations, agglomeration and interdifussion, if allowed to thermally equilibra Le.