Hirohiko Murakami

Synthesis of YBa2Cu4O8 Powders by Sol-Gel Method under Ambient Pressure

The 124 superconductor YBa2Cu4O8 was synthesized using a sol-gel method with starting solutions contained stoichiometric compositions of Y/Ba/Cu=1/2/3 and 1/2/4. Heattreatment was done below 800°C in 1 atm oxygen atmosphere; neither special equipment nor additives were required. Our result suggests that the 123 phase is thermodynamically metastable in the temperature region that had been thought to be the equilibrium region of 123-orthorhombic phase.

Low-Temperature Synthesis of YBa2Cu3O7-x and YBa2Cu4O8 by Sol-Gel Method

The 123 and 124 superconductors were synthesized by sol-gel methods at low temperature. Heat treatment was done below 800°C; neither special equipment nor additives were required. The 123 pellets, which were prepared at 650°C under vacuum condition (0.1 to 1.0 Pa) followed by annealing in flowing oxygen at 750° and 800°C, exhibited zero resistance at 70 and 80 K, respectively. Both the 124 pellets, which were prepared at 780°C in 1 atm oxygen atmosphere followed by either furnace cooling to room temperature or quenching from 780°C to liquid nitrogen temperature, had almost the same transition temperature (Tc(onset)=83-80 K, and Tc(zero)=76–69 K).

Growth of hydrogenated Si clusters using a quadrupole ion trap

To investigate how stability of small Si clusters is affected by hydrogenation, SinH+xions (n=2–10) were grown from silane gas in a recently developed ion trap and the mass spectra were measured. Some compositions have been found to be particularly stable besides those already known for nonhydrogenated Si clusters such as Si6 and Si10. These clusters, Si5H10+, Si6H12+, and Si10H12–17+, are inferred to have the bulklike tetrahedrally coordinated structures whose dangling bonds are terminated by H atoms.

Field emission characteristics of carbon nanofiber improved by deposition of boron nitride nanocrystalline film

An improvement in field emission characteristics of a graphite nanofiber (GNF) has been attempted. Boron nitride (BN) films are synthesized by plasma-assisted chemical vapor deposition. It is demonstrated that electron emission occurs at a low anode voltage due to depositing the BN nanocrystalline film on flat Si substrates. Deposition of the BN nanocrystalline film is applied to the GNF to improve the field emission characteristics of the GNF. In addition to a reduction in the average turn-on electric field, the emission current increases by two orders of magnitude in comparison with that of an as-grown GNF.

Phase diagram of YBa2Cu3O7-x, Y2Ba4Cu7O15-x and YBa2Cu4O8 superconductors

The phase diagrams of Y-system superconductors with compositions of 123 and 124 are given as a function of temperature and partial pressure of oxygen over the range 700 to 1000°C and 0.5P(O2)10 atm. The stable regions of the phases YBa2Cu3O7-x(123), Y2Ba4Cu7O15-x(247) and YBa2Cu4O8(124) were determined. The phase boundaries of 247/124 (starting composition of 124) and 123/124 (starting composition of 123) were found to be represented by the following equations, respectively: log P(O2)(atm)=(7.89±0.08)-(0.86±0.01)×104/T(K) log P(O2)(atm)=(12.1±1.6)-(1.25±0.18)×104/T(K). The 123 phase is stable only in its semiconductor phase (tetragonal phase) and the superconducting phase (orthorhombic phase) is metastable at all temperatures. The 124 phase is more stable than the 123 orthorhombic phase.

Spin-on Cu films for ultralarge scale integrated metallization

We have developed the spin-on Cu metal (SOM) process to fill trenches and vias down to 0.3 μm. SOM is a liquid material that contains an organic solvent and dispersed ultrafine particles as a source of Cu. This solution was applied to a Si wafer using a spin coater to form a film. Coated wafers were baked at 623–673 K for 10 min in a reducing atmosphere. The end result is a Cu film. Contrary to the conventional deposition techniques, the SOM process is simple but advantageous to its gap filling, planarization, and cost consideration.

Ion trap for mass-selective production of nanoclusters

To form nanoclusters with a specified atomic structure, an electrical trapping technique of charged particles was developed that allows us to confine, grow, and mass selectively eject cluster ions. Theoretical calculations and experiments using inert gas ions indicate that the developed ion trap can confine ions with a wide range of mass values using the external field of an ac quadrupole, and send out the ions mass selectively through the internal region of the quadrupole. To demonstrate performance, Si6H12+ clusters were selectively grown using SiH4 as a starting material. In the trap, confined SiHx+ ions were allowed to react with neutral SiH4 molecules or SiHy radicals, and SinHx+ clusters were grown, from which Si6H12+ clusters were automatically ejected by tuning trapping parameters.

Electron field emission from boron nitride nanofilm and its application to graphite nanofiber

Hexagonal polycrystalline boron nitride (BN) films are synthesized on Si substrates by plasma assisted chemical vapor deposition. In the case of the BN films thicker than 20 nm, the turn-on electric field of the electron emission is strongly influenced by the surface roughness rather than the film thickness. On the other hand, in the case of the BN film with a thickness of 8–10 nm, it is found that the turn-on electric field as low as 8.3 V/μm is achieved in spite of the surface of the BN nanofilm being flat as well as the Si substrate. A significant reduction in the effective potential barrier height is suggested. The tunneling controlled field emission is proposed for the BN nanofilm with positive space charge. The BN nanofilm is deposited onto the graphite nanofiber sample. A significant improvement of the field emission characteristics is demonstrated.

Magnetic anisotropy of Ni nano-crystals prepared with gas-deposition method

Abstract Ni nano-crystals were prepared with Gas-deposition method. As deposited films were composed of fine particles with approximately 10 nm in diameter. Thermal annealing was applied to increase the grain size. Coercive forces of the nano-crystals vary in accordance with the 6 powers of grain size. This is explained by the Random Anisotropy Model. Effective local magnetic anisotropy is deduced to be 1.9∼3.5 × 10 4 J/m 3 and the magnitude is 2∼5 times larger than that of bulk Ni. From the field cool magnetization measurement, Hc shift due to exchange anisotropy was not observed. The oxygen atoms in nano-crystals are not responsible for the global anisotropy but may reduce the exchange stiffness.

Injection of Mass-Selected Ions into a Quadrupole Ion Trap.

A simple ion trap has been developed, which captures and confines ions using the fringing field of a linear ac quadrupole. The trap consists of a quadrupole electrode that has one end surrounded by a cage electrode. Ions generated outside the trap are injected through the internal field of the quadrupole and subsequently trapped in the external field extending to the cage. Theoretical calculations and experiments show that when Ar+ ions are injected into the trap at an ac frequency, f, of 1.2 MHz, the chirping of f to 0.6 MHz over ~50 µ s improves the trap lifetime. This effect arises due to the adiabatic change in the trapping potential, which closes the injection pathway of the ions into the trap.