S. Jin

Large current density from carbon nanotube field emitters

Field emitters made from carbon nanotubes exhibit excellent macroscopic emission properties; they can operate at a very large current density, as high as 4 A/cm/sup 2/. At electric fields as low as 4-7 V//spl mu/m, they emit technologically useful current densities of 10 mA/cm/sup 2/. The emission originates from nanotube ends with a characteristic structured ring pattern.

In situ-grown carbon nanotube array with excellent field emission characteristics

In situ catalytic thermal decomposition method was used for producing aligned multiwalled carbon nanotubes (MWNTs) in bulk quantities on stable and electrically conducting substrates. Very low turn-on electric fields of 0.75 V/μm and low threshold fields of ∼1.6 V/μm (for current density of 10 mA/cm2) were obtained from the MWNT arrays grown on TiN substrate. Furthermore, large emission current densities of 1–3 A/cm2 were obtained at reasonably low fields of less than ∼8 V/μm. These enhanced emission properties are tentatively attributed to the oriented and high-density nature of the emitting carbon nanotube structure and the high-conductivity, stable nature of the TiN substrate onto which the nanotubes are attached.

Ultrahigh frequency permeability of sputtered Fe–Co–B thin films

Amorphous FeCoB alloys can have high resistivity and relatively high saturation magnetization which are desired for ultrahigh frequency devices such as future write heads and wireless inductors. In this study, FeCoB films are observed to have a low easy axis coercivity (1–2 Oe), a field deposited induced anisotropy of ∼35 Oe and 4πMs∼17u200a500u200aG when sputtered in a typical configuration. However, samples sputtered in an off axis arrangement are found to have an additional uniaxial anisotropy as a result of the oblique incidence of the atomic flux onto the substrate. This extra anisotropy increases with increasing oblique angle. For these same samples, there is little change in the easy axis coercivity, saturation magnetization, and the Hoffmann structure factor (S∼0.05u200aergs/cm2). The obtained large anisotropy fields (>35 Oe) are found to push the ferromagnetic resonance frequency to at least above 1 GHz at the expense of the initial permeability.

High frequency properties of Fe–Cr–Ta–N soft magnetic films

High frequency magnetic properties of a new, soft magnetic Fe–Cr–Ta–N alloy film have been investigated. Thin films with a composition of Fe-4.6% Cr-0.2% Ta-7.4% N (in atomic %) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., Hc as low as 1.2 Oe (95.3 A/m) and 4πMs∼20 kG. The easy-axis M–H loop is square. The hard-axis loop is linear and closed, with the anisotropy field Ha=20–100 Oe (1.59–7.95 kA/m). By virtue of their high 4πMs and relatively high Ha, these soft magnetic films exhibit high permeability and low loss in the GHz frequency range with the undesirable ferromagnetic resonance interference suppressed to beyond 2 GHz.

Field emission properties of diamond and carbon nanotubes

Both diamond and carbon nanotubes are efficient field emitters. Both materials can emit electrons at very low electric fields (3–7 V/μm for a current density of 10 mA/cm2). Moreover, nanotube emitters are capable of delivering very high emission currents densities, with current density routinely exceeding 1 A/cm2. Improvements of emission uniformity are critical in realizing the potential of these carbon materials in enabling practically useful cold cathode devices.

Soft-magnetic properties of Fe–Co–B thin films for ultra-high-frequency applications

Fe67Co18B15 amorphous thin films were sputter deposited on glass substrates under a uniform applied magnetic field. The films exhibit excellent soft-magnetic properties of 4πMs∼17.5 kG, Hc∼0.3 Oe, and Hk (anisotropy field) ∼22 Oe. The ultra-high-frequency performance on these films was studied and the relative permeability μ′ is shown to be as high as ∼740 and almost constant to at least 1 GHz. The combination of high 4πMs and relatively high Hk in these films is believed to be partly responsible for the excellent ultra-high-frequency behavior. The soft-magnetic properties of these films show a strong dependence on the film deposition bias voltage, which presumably changes the microstructure of the films and the related magnetic anisotropy.

Growth and measurements of ferroelectric lead zirconate titanate on diamond by pulsed laser deposition

Pb(Zr0.53Ti0.47)O3 (PZT) on diamond is a potentially robust structure for surface acoustic wave (SAW) device applications. We have studied the growth and physical characteristics of PZT on diamond and other substrates by pulsed laser deposition. Under a broad range of processing conditions we explored, PZT deposited directly on diamond is almost exclusively pyrochlore-type, which is nonferroelectric. Growth of ferroelectric perovskite PZT is promoted via the use of a PbTiO3 buffer layer within a narrow window of processing parameters [i.e., P(O2)=100–200u200amTorr, T=550–650u200a°C, 1–2 J/cm2]. Similar results were also obtained for deposition of PZT on Si, Pt-coated Si, and Pt-coated diamond substrates. The dielectric constants of the perovskite PZT films are 500–650 at 1 V and 100 kHz. The piezoelectric coefficients of these films are in the range of 50×10−12–350×10−12u200am/V. The SAW velocity of perovskite PZT films is similar to that of highly oriented sputter deposited ZnO films. The acoustic attenuation in per...

Significantly enhanced creep resistance in low-melting-point solders through nanoscale oxide dispersions

Nanosized, nonreacting, and noncoarsening oxide dispersoids have been incorporated into solder alloys to create an improved solder structure with an ultrafine grain size of ∼2000–5000 A and significantly enhanced mechanical properties. As much as three orders of magnitude reduction in the steady-state creep rate has been achieved. These solders also exhibit improved (4–5 times higher) tensile strength at low strain rates and improved ductility under high strain rate deformation. It is demonstrated that with a dispersion of TiO2 particles, the Pb–Sn eutectic solder with a low melting point of 183u2009°C can be made more creep resistant than the 80Au–20Sn eutectic solder with a much higher melting point of 278u2009°C.

Shock wave induced changes in superconductivity in YBa2Cu3O7−δ

Shock wave loading produced significant changes in superconductivity in sintered YBa2Cu3O7−δ causing a broad transition with Tc(R=0)∼40 K. The normal‐state resistivity increased by 20–50× with the ρ‐T curve exhibiting a semiconducting behavior. Comparative thermogravimetric analysis, however, indicated no loss of oxygen (δ∼0.1) in the shock‐loaded sample, and x‐ray powder diffraction analysis showed no major changes except for slight line broadening. The observed semiconductive behavior and degredation in superconductivity is thus attributed to a rather subtle disturbance in crystal structure, the nature of which is not clearly understood at present. The lack of sufficient recovery in Tc by post‐shock oxygen processing at temperatures as high as 750u2009°C and subsequent cooling implies that the atomic‐scale disturbance may not necessarily be related to a simple oxygen disorder. Such defects, if controlled properly, may be advantageously turned into desirable flux‐pinning sites for improved critical currents.Shock wave loading produced significant changes in superconductivity in sintered YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} causing a broad transition with {ital T}{sub {ital c}}({ital R}=0){similar to}40 K. The normal-state resistivity increased by 20--50{times} with the {rho}-{ital T} curve exhibiting a semiconducting behavior. Comparative thermogravimetric analysis, however, indicated no loss of oxygen ({delta}{similar to}0.1) in the shock-loaded sample, and x-ray powder diffraction analysis showed no major changes except for slight line broadening. The observed semiconductive behavior and degredation in superconductivity is thus attributed to a rather subtle disturbance in crystal structure, the nature of which is not clearly understood at present. The lack of sufficient recovery in {ital T}{sub {ital c}} by post-shock oxygen processing at temperatures as high as 750 {degree}C and subsequent cooling implies that the atomic-scale disturbance may not necessarily be related to a simple oxygen disorder. Such defects, if controlled properly, may be advantageously turned into desirable flux-pinning sites for improved critical currents.

Fe-Cr-N soft magnetic thin films

Soft magnetic properties of new Fe-Cr-N and Fe-Cr-Ta-N alloy films have been investigated. Thin films with compositions in the range of Fe-2∼8% Cr-0∼1% Ta-5∼15% N (in at.u2009%) were prepared by reactive sputtering in a nitrogen-containing atmosphere. The films, most likely nanocrystalline, exhibit excellent soft magnetic properties in the as-deposited condition without any post heat treatment, e.g., Hc∼1–2 Oe (79.4–158.8 A/m) and 4πMs∼15–20 kG. The easy-axis M-H loop is square. The hard-axis loop is linear and closed, with the anisotropy field Ha=20–60 Oe (1.59–4.77 kA/m). The combination of high 4πMs and relatively high Ha in these films is conducive to the suppression of the undesirable ferromagnetic resonance (FMR) interference up to the GHz frequency range.