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Prospect of Making Ceramic Shell Mold by Ceramic Laser Fusion
Manufacturing prototypical castings by conventional investment casting not only takes
several weeks, but also is prohibitively expensive. Z Corporation in USA, EOS GmbH and
IPT in Germany employ the techniques of 3DP and SLS respectively to make directly ceramic
shell molds for metal castings. Although those techniques dramatically reduce time
expenditure and production cost, each layer cannot be thinner than 50 µm because of using
powder to pave layers. The dimensional accuracy and roughness of the castings still cannot
meet the specification of precision casting. Therefore, in this paper the ceramic laser fusion
(CLF) was used to pave layers. Each layer can be thinner than 25 µm, so that the step effect
can be diminished and the workpiece surface can be smoother; drying time will be shortened
dramatically. Moreover, the inherent solid-state support formed by green portion has the
capability of preventing upward and downward deformation of the scanned cross sections. In
order to make shell mold which meets the roughness requirement (Rq=3.048µm) of the
precision casting, following issues have to be further studied: (1) design a proper ceramic
shell mold structure, (2) design a paving chamber for paving a complete green layer which
can be easily collapsed, (3) cut down drying time, (4) optimize laser scanning process
parameters with the smallest distortion, (5) eliminate sunken area, (6) reduce layer thickness
to less than13µm, (7) control power to guarantee the energy uniformly absorbed by workpiece,
and (8) develop a method which can directly clean green portion in cavity from gate.Mechanical Engineerin
Strange meson-nucleon states in the quark potential model
The quark potential model and resonating group method are used to investigate
the bound states and/or resonances. The model potential consists of
the t-channel and s-channel one-gluon exchange potentials and the confining
potential with incorporating the QCD renormalization correction and the
spin-orbital suppression effect in it. It was shown in our previous work that
by considering the color octet contribution, use of this model to investigate
the low energy elastic scattering leads to the results which are in pretty
good agreement with the experimental data. In this paper, the same model and
method are employed to calculate the masses of the bound systems.
For this purpose, the resonating group equation is transformed into a standard
Schr\"odinger equation in which a nonlocal effective interaction
potential is included. Solving the Schr\"odinger equation by the variational
method, we are able to reproduce the masses of some currently concerned
states and get a view that these states possibly exist as
molecular states. For the system, the same calculation gives no support to
the existence of the resonance which was announced
recently.Comment: 15 pages, 4 figure
Spin gap behavior in CuScGeO by Sc nuclear magnetic resonance
We report the results of a Sc nuclear magnetic resonance (NMR) study
on the quasi-one-dimensional compound CuScGeO at
temperatures between 4 and 300 K. This material has been a subject of current
interest due to indications of spin gap behavior. The temperature-dependent NMR
shift exhibits a character of low-dimensional magnetism with a negative broad
maximum at 170 K. Below , the NMR shifts and
spin lattice relaxation rates clearly indicate activated responses, confirming
the existence of a spin gap in CuScGe% O. The experimental
NMR data can be well fitted to the spin dimer model, yielding a spin gap value
of about 275 K which is close to the 25 meV peak found in the inelastic neutron
scattering measurement. A detailed analysis further points out that the nearly
isolated dimer picture is proper for the understanding of spin gap nature in
CuScGeO.Comment: 4 pages, 6 figures, submitted to Phys. Rev.
Dirac-Schr\"odinger equation for quark-antiquark bound states and derivation of its interaction kerne
The four-dimensional Dirac-Schr\"odinger equation satisfied by
quark-antiquark bound states is derived from Quantum Chromodynamics. Different
from the Bethe-Salpeter equation, the equation derived is a kind of first-order
differential equations of Schr\"odinger-type in the position space. Especially,
the interaction kernel in the equation is given by two different closed
expressions. One expression which contains only a few types of Green's
functions is derived with the aid of the equations of motion satisfied by some
kinds of Green's functions. Another expression which is represented in terms of
the quark, antiquark and gluon propagators and some kinds of proper vertices is
derived by means of the technique of irreducible decomposition of Green's
functions. The kernel derived not only can easily be calculated by the
perturbation method, but also provides a suitable basis for nonperturbative
investigations. Furthermore, it is shown that the four-dimensinal
Dirac-Schr\"odinger equation and its kernel can directly be reduced to rigorous
three-dimensional forms in the equal-time Lorentz frame and the
Dirac-Schr\"odinger equation can be reduced to an equivalent
Pauli-Schr\"odinger equation which is represented in the Pauli spinor space. To
show the applicability of the closed expressions derived and to demonstrate the
equivalence between the two different expressions of the kernel, the t-channel
and s-channel one gluon exchange kernels are chosen as an example to show how
they are derived from the closed expressions. In addition, the connection of
the Dirac-Schr\"odinger equation with the Bethe-Salpeter equation is discussed
Impact of Substrate Outgassing on the Growth of Carbon Nanotubes Using the Single-Pulse Discharge Method
Carbon nanotubes (CNTs) were fabricated in air using the electrical discharge machining method. The main parameters for this process were substrate temperature, peak current (Ip), and pulse duration (τ). The substrate was baked at 50°C and this temperature was maintained for 12 h under vacuum chamber; it was then cooled to room temperature and stored in vacuum for outgassing. During single-pulse discharge in air, the substrate was heated from room temperature to the test temperatures (50 and 70°C). The results indicated that the length, density, and purity of CNTs grown on outgassed substrates were better than those of CNTs grown without outgassing. Additionally, CNTs grown with Ip = 3 A and τ = 1200 μs were of better quality than those grown with other combinations of parameters. The size of the discharge pit was effectively reduced by 30% (80 μm). This finding may help in controlling the amount of peak current used during the process, thereby reducing the problems of heat-affected zones and electrode consumption. Consequently, there was substantial improvement in the zonal selectivity and reticular density of the CNTs grown using the single-pulse discharge method
Jet array impingement flow distributions and heat transfer characteristics. Effects of initial crossflow and nonuniform array geometry
Two-dimensional arrays of circular air jets impinging on a heat transfer surface parallel to the jet orifice plate are considered. The jet flow, after impingement, is constrained to exit in a single direction along the channel formed by the jet orifice plate and the heat transfer surface. The configurations considered are intended to model those of interest in current and contemplated gas turbine airfoil midchord cooling applications. The effects of an initial crossflow which approaches the array through an upstream extension of the channel are considered. Flow distributions as well as heat transfer coefficients and adiabatic wall temperatures resolved to one streamwise hole spacing were measured as a function of the initial crossflow rate and temperature relative to the jet flow rate and temperature. Both Nusselt number profiles and dimensionless adiabatic wall temperature (effectiveness) profiles are presented and discussed. Special test results which show a significant reduction of jet orifice discharge coefficients owing to the effect of a confined crossflow are also presented, along with a flow distribution model which incorporates those effects. A nonuniform array flow distribution model is developed and validated
Non-volatile resistive switching in dielectric superconductor YBCO
We report on the reversible, nonvolatile and polarity dependent resistive
switching between superconductor and insulator states at the interfaces of a
Au/YBaCuO (YBCO)/Au system. We show that the
superconducting state of YBCO in regions near the electrodes can be reversibly
removed and restored. The possible origin of the switching effect may be the
migration of oxygen or metallic ions along the grain boundaries that control
the intergrain superconducting coupling. Four-wire bulk resistance measurements
reveal that the migration is not restricted to interfaces and produce
significant bulk effects.Comment: 4 pages, 4 figures, corresponding author: C. Acha ([email protected]
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