Jung-Hung Chang

A 1-D analysis of ejector performance

A 1-D analysis for the prediction of ejector performance at critical-mode operation is carried out in the present study. Constant-pressure mixing is assumed to occur inside the constant-area section of the ejector and the entrained flow at choking condition is analyzed. We also carried out an experiment using 11 ejectors and R141b as the working fluid to verify the analytical results. The test results are used to determine the coefficients, h p, h s, f p and f m defined in the 1-D model by matching the test data with the analytical results. It is shown that the1-D analysis using the empirical coefficients can accurately predict the performance of the ejectors. q 1999 Elsevier Science Ltd and IIR. All rights reserved.

Empirical correlation for ejector design

In the present study, two empirical correlations from the test results of 15 ejectors are derived for the performance prediction of ejectors using R141b as the working fluid. The ratio of the hypothetical throat area of the entrained flow to the nozzle throat area Ae/At, the geometric design parameter of the ejector A3/At, and the pressure ratios Pg/Pe and Pc*/Pe are used to correlate the performance of the ejector. The prediction of the entrainment ratio v using the correlations is within ^ 10% error. A method of calculation for the ejector design using the correlations is also developed. R141b is shown in the present study to be a good working fluid for an ejector. The measured v for the ejectors used in the present study can reach as high as 0.54 at Pga 0.465 MPa (848C), Pc*a 0.087 MPa (288C) and Pea 0.040 MPa (88C). For Pga 0.538 MPa (908C), Pc*a 0.101 MPa (328C) and Pea 0.040 MPa (88C), v reaches 0.45. q 1999 Elsevier Science Ltd and IIR. All rights reserved.

A combined-cycle refrigeration system using ejector-cooling cycle as the bottom cycle

A combined-cycle refrigeration system (CCRS) that comprises a conventional refrigeration and air-conditioning system using mechanical compressor (RAC/MC) and an ejector-cooling cycle (EJC) is proposed and studied. The EJC is driven by the waste heat from the RAC/MC and acts as the bottom cycle of the RAC/MC. A system analysis shows that the COP of a CCRS is significantly higher than a single-stage refrigeration system. Improvement in COP can be as high as 18.4% for evaporating temperature of the RAC/MC Te at −5°C. A prototype of the CCRS was built and tested in the present study. Experimental results show that at Te=−4.5°C, COP is improved by 14% for a CCRS. For Te at 5°C, COP can be improved by 24% for a CCRS with higher condensing temperature of the RAC/MC. The present study shows that the CCRS using the ejector-cooling cycle as the bottom cycle of the RAC/MC is viable. Further improvement in COP is possible since the prototype is not designed and operated at an optimal condition.

Formation of perfect ohmic contact at indium tin oxide/N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine interface using ReO3.

A perfect ohmic contact is formed at the interface of indium tin oxide (ITO) and N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) using ReO3 as the interfacial layer. The hole injection efficiency is close to 100% at the interface, which is much higher than those for interfacial layers of 1,4,5,8,9,11-hexaazatripheylene hexacarbonitrile (HAT-CN) and MoO3. Interestingly, the ReO3 and MoO3 interfacial layers result in the same hole injection barrier, ≈0.4 eV, to NPB, indicating that the Fermi level is pinned to the NPB polaron energy level. However, a significant difference is observed in the generated charge density in the NPB layer near the interfacial layer/NPB interface, indicating that charge generation at the interface plays an important role in forming the ohmic contact.

Growing GaN LEDs on amorphous SiC buffer with variable C/Si compositions

The epitaxy of high-power gallium nitride (GaN) light-emitting diode (LED) on amorphous silicon carbide (a-SixC1−x) buffer is demonstrated. The a-SixC1−x buffers with different nonstoichiometric C/Si composition ratios are synthesized on SiO2/Si substrate by using a low-temperature plasma enhanced chemical vapor deposition. The GaN LEDs on different SixC1−x buffers exhibit different EL and C-V characteristics because of the extended strain induced interfacial defects. The EL power decays when increasing the Si content of SixC1−x buffer. The C-rich SixC1−x favors the GaN epitaxy and enables the strain relaxation to suppress the probability of Auger recombination. When the SixC1−x buffer changes from Si-rich to C-rich condition, the EL peak wavelengh shifts from 446 nm to 450 nm. Moreover, the uniform distribution contour of EL intensity spreads between the anode and the cathode because the traping density of the interfacial defect gradually reduces. In comparison with the GaN LED grown on Si-rich SixC1−x buffer, the device deposited on C-rich SixC1−x buffer shows a lower turn-on voltage, a higher output power, an external quantum efficiency, and an efficiency droop of 2.48 V, 106 mW, 42.3%, and 7%, respectively.

Fabricating graphite nano-sheet powder by slow electrochemical exfoliation of large-scale graphite foil as a mode-locker for fiber lasers

Without the need of single-layer graphene, the graphite nano-sheet powder electrochemically exfoliated from graphite foil can also be employed as a stable saturable absorber and mode-locker for fiber lasers. High-quality graphite nano-sheets containing few graphene layers can be obtained by slow electrochemical exfoliation without the need of post annealing procedure. With reducing the electrochemical exfoliation bias of the graphite foil based anode from + 6 and + 3 volts, the electrochemically exfoliated graphite nano-sheets reveals a decreased D-band intensity in Raman scattering spectrum, and the 2D-band intensity is concurrently enlarged by two times to support the improved quality with suppressed oxidation during the exfoliation reaction. The X-ray photoelectron spectroscopy also confirms the suppression of the C-O bonds in the graphite nano-sheets obtained with decreasing the exfoliation bias. After centrifugation, the average diameter of the exfoliated graphite nano-sheets extracted from the acetone solution is shrunk from 7 μm to 100 nm as the anode bias decreases from 6 to 3 volts. Both the quality and size distribution of the graphite nano-sheets are improved with such slow but refined electrochemical exfoliation. In application, the graphite nano-sheets obtained at different exfoliation bias show relatively stable saturable absorption and passive mode-locking performance in Erbium doped fiber lasers (EDFLs). Benefiting from the advantages of high-gain and strong pulse compression in the EDFL, the graphite nano-sheets with different modulation depths only behave as a mode-locking starter and show trivial influence to the pulse shortening in the mode-locked EDFL, indicating that the strong soliton compression mechanism dominates the generation of 430-450 fs pulsewidth in the EDFL passively mode-locked by graphite nano-sheets.

Enhancement in current efficiency in organic light-emitting diodes with incorporation of subphthalocyanine

A highly efficient hole injection material, boron subphthalocyanine chloride (SubPc), was incorporated in organic light-emitting diodes. Device performance is greatly enhanced by inserting an ultrathin layer of SubPc between anodes and N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidene (NPB). Electronic structures and chemical reaction at the interface between NPB and SubPc are also investigated by photoemission spectroscopy with synchrotron radiation sources. Extra states are observed at the forbidden gap of SubPc with deposition of NPB, resulting from the broken bonds between boron and chlorine on SubPc with presence of NPB. These gap states are attributed to the improvement of device performance.

Comprehensive study of medium-bandgap conjugated polymer merging a fluorinated quinoxaline with branched side chains for highly efficient and air-stable polymer solar cells

A new medium-bandgap conjugated copolymer comprising a rigidly fused benzo[1,2-b:4,5-b′]-dithiophene (BDT) unit and a fluorinated quinoxaline moiety through a thiophene π-spacer has been rationally designed and synthesized by Stille coupling polymerization and thoroughly evaluated for use as a donor material in bulk-heterojunction polymer solar cells (BHJ PSCs). A comprehensive study of the structure-function relationship in the PSCs was also explored. The PDBTQEH copolymer exhibits good solubility in a wide range of organic solvents and has a high hole mobility. Introduction of an highly electronegative fluorine atoms to quinoxaline moiety further lowers both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of polymer, which is beneficial for attaining higher open-circuit voltage (Voc) and long-term stability. Conventional architecture BHJ PSCs using PBDTQEH:PC71BM (1 : 1, w/w) displays a high power conversion efficiency (PCE) of 5.90%. Compared with the same composition, the device in the inverted configuration reveals a rather high PCE of 6.36% with a Voc of 0.78 V, a short-circuit current density (Jsc) of 12.72 mA cm−2, and a high fill factor (FF) of 64.3%. The inverted device also demonstrates outstanding air stability; without any encapsulation, the solar efficiency of the device remains above 74% of the original value after storage in air for 1000 h.

Alternating Current Driven Organic Light Emitting Diodes Using Lithium Fluoride Insulating Layers

We demonstrate an alternating current (AC)-driven organic light emitting diodes (OLED) with lithium fluoride (LiF) insulating layers fabricated using simple thermal evaporation. Thermal evaporated LiF provides high stability and excellent capacitance for insulating layers in AC devices. The device requires a relatively low turn-on voltage of 7.1 V with maximum luminance of 87 cd/m2 obtained at 10 kHz and 15 Vrms. Ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy are employed simultaneously to examine the electronic band structure of the materials in AC-driven OLED and to elucidate the operating mechanism, optical properties and electrical characteristics. The time-resolved luminance is also used to verify the device performance when driven by AC voltage.

Strong optical nonlinearity of the nonstoichiometric silicon carbide

Enhanced nonlinear refractive indices and absorption coefficients of nonstoichiometric SixC1−x with varying C/Si composition ratios from 0.51 to 1.83 grown by low-temperature plasma-enhanced chemical vapor deposition are demonstrated. When increasing the [CH4]/[CH4 + SiH4] fluence ratio from 70% to 92%, two Raman scattering signals at 795 and 970 cm−1 contributed by transverse and longitudinal optical modes of Si–C bonds are simultaneously enhanced in stoichiometric and C-rich SixC1−x samples, whereas the Si-associated Raman peak at 520 cm−1 decreases with increasing C/Si composition ratio. The C-rich SixC1−x film shows additional Raman scattering peaks at 1330 and 1580 cm−1, which are individually contributed by the diamond-like C–C bonds and the G peak of the graphite-like C–C bonds, respectively. The nonlinear refractive index increases to 1 ± 0.1 × 10−11 cm2 W−1 upon increasing the C/Si composition ratio up to 1.83, which is four orders of magnitude higher than that of bulk SiC. The increased C/Si composition ratio reduces the lattice constant and the effective mass of the SixC1−x film and enhances its nonlinear refractive index effectively. In comparison with the sp3-orbital C–C bond, the sp2-orbital C–C bond in SixC1−x films dominates the enhancement of nonlinear refractive index. By contrast, the nonlinear absorption coefficient changes sign and decreases magnitude when transferring the SixC1−x film from Si-rich to C-rich conditions. This sign reversal is primarily attributed to the conversion in the absorption mechanism from two-photon absorption to nonlinear saturable absorption. Unlike Si–Si bonds which favor two-photon absorption, the sp2-orbital C–C bonds with delocalized π-electrons essentially enhance the saturable absorbance. Hence, a higher nonlinear refractive index and a higher saturable absorption coefficient can be obtained by enriching the sp2-orbital C–C bonds in C-rich SixC1−x films.