ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Wed, 09 Oct 2024 23:35:43 GMT2024-10-09T23:35:43Z501151- A micromechanical plastic model for ZrO2-toughened ceramicshttps://scholarbank.nus.edu.sg/handle/10635/92650Title: A micromechanical plastic model for ZrO2-toughened ceramics
Authors: Lam, K.Y.; Zhang, J.M.; Ong, P.P.
Abstract: An increment plasticity formulation is established for the transformation plastic behavior of zirconia-toughened ceramics based on micromechanics of heterogeneous media. Mori-Tanaka estimates are established for the overall plastic strains and the overall yield stresses. The plastic flow function is determined in terms of the transformation twinning of a single zirconia particle embedded in an infinite matrix with the average matrix stresses. Numerical examples are given for the transformation yielding of ceramics with aligned zirconia particles under uniaxial loading. © 1995.
Sun, 01 Jan 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/926501995-01-01T00:00:00Z
- Interatomic potential and transport properties of Na + ions in He/elhttps://scholarbank.nus.edu.sg/handle/10635/92728Title: Interatomic potential and transport properties of Na + ions in He/el
Authors: Ong, P.P.; Tan, T.L.; Lam, K.Y.
Abstract: Hitherto unavailable experimental values of the ratio, DT/K, of the transverse diffusion coefficient to mobility for Na + ions drifting in helium gas are reported. Results were obtained with estimated accuracies of better than 3% at temperatures between 302 and 304 K, and at electric field to neutral gas density ratios, E/N, ranging from 5 to 400 Td. By an independent approach, the three transport parameters are also computed by Monte Carlo simulation using the modified Tang-Toennies potential and the Koutselos, Mason and Viehland potential as input, separately. Comparison between the simulated results and available experimental data suggests that the latter potential represents fairly well the true interaction in the long and intermediate ranges, but its short-range repulsive region is too soft. © 1993 IOP Publishing Ltd.
Sat, 28 Aug 1993 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/927281993-08-28T00:00:00Z
- Interaction between a circular inclusion and a symmetrically branched crackhttps://scholarbank.nus.edu.sg/handle/10635/51711Title: Interaction between a circular inclusion and a symmetrically branched crack
Authors: Lam, K.Y.; Ong, P.P.; Wude, N.
Abstract: The interaction problem between a circular inclusion and a symmetrically branched crack embedded in an infinite elastic-medium is solved. The branched crack is modeled as three straight cracks which intersect at a common point and each crack is treated as a continuous contribution of edge dislocations. Green's functions are used to reduce the problem into a system of singular equations consisting of the distributions of Burger's dislocation vectors as unknown functions through the superposition technique. The resulting integral equations are solved numerically by the method of Gauss-Chebychev quadrature. The proposed integral equation approach is first verified for two limiting cases against the literature. More effort is paid on the effect of inclusion on both the Mode I and Mode II stress intensity factors at the branch tips. The effect of inclusion on the branching path is also investigated. © 1998 Published by Elsevier Science Ltd. All rights reserved.
Sun, 01 Feb 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/517111998-02-01T00:00:00Z
- A new integral equation formulation for the analysis of crack-inclusion interactionshttps://scholarbank.nus.edu.sg/handle/10635/51691Title: A new integral equation formulation for the analysis of crack-inclusion interactions
Authors: Lam, K.Y.; Zhang, J.M.; Ong, P.P.
Abstract: A new integral equation method for the analysis of the interactions between cracks and elastic inclusions embedded in a two-dimensional, linearly elastic, isotropic infinite medium subjected to in-plane force is presented. By distributing dislocations along the crack lines and forces along the matrix-inclusion interfaces, a set of coupled integral equations is obtained. The discretization procedure of the integrals involved is discussed and the relations between the stress intensity factors and the values of the dislocation functions at the respective crack tips are derived. Several sample problems are presented in order to determine the versatility and the accuracy of this approach. © 1992 Springer-Verlag.
Fri, 01 May 1992 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/516911992-05-01T00:00:00Z
- Suppression of UV photoluminescence in sandwich-structured Si/C composite filmshttps://scholarbank.nus.edu.sg/handle/10635/115957Title: Suppression of UV photoluminescence in sandwich-structured Si/C composite films
Authors: Zhu, Y.; Yuan, C.L.; Liu, R.; Ong, P.P.
Abstract: Thin films of composite silicon/carbon (Si/C) were prepared by pulsed-laser ablation alternately on C and Si materials on a rotary target, followed by vacuum deposition of the ablated materials on an ultra-clean glass substrate. The film structure consisted of alternate nanolayers of Si nanocrystals and amorphous C, with a fairly sharp demarcation boundary between adjacent layers forming well-defined sandwich structures. At room temperature, this composite nanolayered structure was found to yield much lower photoluminescent (PL) emission in the ultra violet region (300-390 nm) in comparison with that obtained for pure Si or for Si/Al2O3 thin films (see ZHU Y., WANG H. and ONG P.P., J. Phys. D, 33 (2000) 1965, and ZHU Y. and ONG P.P., J. Phys. Condens. Matter, 13 (2001) L1). The suppression mechanism of the UV PL emission appeared to occur in the interfacial surfaces between adjacent crystalline Si and amorphous C layers. It provides a possible way to selectively filter out the usually undesirable UV component of the PL emission from the silicon nanoparticles.
Tue, 01 Oct 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1159572002-10-01T00:00:00Z
- Interaction potential, transport properties, and velocity distributions of Na+ ions in Nehttps://scholarbank.nus.edu.sg/handle/10635/58415Title: Interaction potential, transport properties, and velocity distributions of Na+ ions in Ne
Authors: Ong, P.P.; Hogan, M.J.; Lam, K.Y.; Viehland, L.A.
Abstract: Comparisons of experimentally measured transport properties with values determined from calculations based on three-temperature-kinetic theory, on bi-Maxwellian kinetic theory, and on Monte Carlo simulations have established that the Na+-Ne interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)] is a close approximation to the true potential. (In the bi-Maxwellian approach, the ions are modeled in first-approximation as a large fraction that behaves according to a low-temperature Maxwell distribution and a small fraction that follows one of higher temperature; the true distribution is computed from these by a weighted-residual method.) This potential was then used to compute velocity distribution functions of Na+ ions in room-temperature Ne at widely different average ion energies. The skewness of the distributions in the direction of the electric field initially increased rapidly with increasing energy, and then slowly decreased, as did the excess kurtosis both parallel and perpendicular to the electric-field direction. It was clearly established that there is correlation between the perpendicular and parallel velocity-component distribution functions and that the correlation increases with increasing average ion energy. © 1992 The American Physical Society.
Wed, 01 Jan 1992 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/584151992-01-01T00:00:00Z
- TOF study of pulsed-laser ablation of aluminum nitride for thin film growthhttps://scholarbank.nus.edu.sg/handle/10635/98415Title: TOF study of pulsed-laser ablation of aluminum nitride for thin film growth
Authors: Chu, C.; Ong, P.P.; Chen, H.F.; Teo, H.H.
Abstract: Laser ionization time-of-flight mass spectrometry (TOF-MS) was used to study the process of pulsed-laser deposition (PLD) of Aluminum Nitride (AlN) thin films. The components of the plume from the AlN target obviously changed under different deposition conditions. For the same impurity level, positive AlN ions were freer from impurities than negative ions. However, negative ions were more likely to form larger clusters. In particular, a novel cluster, comprising 10 AlN, was observed in negative TOF, suggesting that it is likely to yield highly oriented crystalline thin films when deposited on a substrate. © 1999 Elsevier Science B.V. All rights reserved.
Fri, 01 Jan 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984151999-01-01T00:00:00Z
- Structural properties of CdS-doped glass nanocrystallites grown by pulsed laser deposition in high vacuumhttps://scholarbank.nus.edu.sg/handle/10635/98061Title: Structural properties of CdS-doped glass nanocrystallites grown by pulsed laser deposition in high vacuum
Authors: Ong, P.P.
Abstract: High quality CdS-doped glass (CDG) nanocrystalline thin films were fabricated by pulsed laser deposition (PLD) from a solo-physically arranged CdS/SiO2 target. Both EDX and XRD analyses confirmed the presence of the new CdO composite. TEM confirmed the different crystalline structures but yielded slightly smaller values for their crystal sizes.
Mon, 01 Jan 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/980612001-01-01T00:00:00Z
- Surface modification of CdS nanocrystallites doped in SiO2 matrixhttps://scholarbank.nus.edu.sg/handle/10635/98134Title: Surface modification of CdS nanocrystallites doped in SiO2 matrix
Authors: Wang, H.; Zhu, Y.; Ong, P.P.
Abstract: Nanometer-sized CdS semiconductor particles were prepared by pulsed laser deposition (PLD) and dispersed in SiO2 matrix. Surface modification of the CdS nanocrystallites deposited with the SiO2 material on a substrate caused the generation of CdO composite, which formed an encapsulating shell layer surrounding the inner CdS crystallite. Raman and photoluminescence (PL) analyses clearly identified and characterized this component. Annealing of the film caused growth in size of the CdO shell as established from the redshift of its PL band gap emission. This growth was found to keep in unison with the corresponding inner CdS crystalline growth, and a possible mechanism is suggested. Finally, through proper selection of the substrate temperature during PLD to remove crystalline stress, it was possible to obtain a complete optimum CdS profile without the accompanying CdO formation or characteristics. © 2001 American Institute of Physics.
Thu, 05 Jul 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/981342001-07-05T00:00:00Z
- Supersensitive electrical measurements and their associated techniqueshttps://scholarbank.nus.edu.sg/handle/10635/98116Title: Supersensitive electrical measurements and their associated techniques
Authors: Ong, P.P.
Abstract: Low-level electrical measurements have undergone a technological revolution. The paper discusses the new instruments that can measure currents and voltages which are only slightly larger than the theoretical limit of sensitivity. The measurements of such feeble quantities are naturally highly vulnerable to extraneous effects, but the strenuous efforts required in dealing with them are richly rewarded with the pedagogic and heuristic benefits derivable. Accordingly such measurements should be included in an undergraduate curriculum.
Thu, 01 Jan 1987 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/981161987-01-01T00:00:00Z
- Synthesis and characterization of Y2O3:Eu3+ thin films on silicon substrate by pulsed laser ablationhttps://scholarbank.nus.edu.sg/handle/10635/98177Title: Synthesis and characterization of Y2O3:Eu3+ thin films on silicon substrate by pulsed laser ablation
Authors: Gang, G.; Ong, P.P.; Chen, C.; Roth, S.
Abstract: Y2O3:Eu3+ thin films were prepared by pulsed laser ablation on silicon (100) substrate. The relative photoluminescence intensity dependence on annealing time was studied. The microstructures of annealing thin films were studied by atomic force microscopy. It was observed that the photoluminescence intensity at 611 nm from thin film annealed at 1000°C for 4 h tended to saturate, the average crystal size of the thin film was estimated to be 71 nm by x-ray diffraction measurements.
Wed, 07 Jun 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/981772000-06-07T00:00:00Z
- The effects of Cu doping on the magnetoresistive behavior of perovskites La0.7Ca0.3MnO3https://scholarbank.nus.edu.sg/handle/10635/98263Title: The effects of Cu doping on the magnetoresistive behavior of perovskites La0.7Ca0.3MnO3
Authors: Yuan, C.L.; Zhu, Y.; Ong, P.P.
Abstract: The electronic and magnetic properties of Cu-doped perovskite La0.7Ca0.3Mn1-xCux O3 obtained by doping Cu on its Mn sites have been studied. The perovskite structure was found to remain intact up to the highest doping level of x = 0.20. At low Cu concentration (x = 0.05) the temperature-dependence of resistivity of the material exhibited up to two peaks corresponding to the magnetic transitions from the PM to the FM phase, and from the FM to the AFM phase. In general, the doping level was found to suppress the ferromagnetic ordering of the material, increase its resistivity, and produce large values of magnetoresistance near the resistivity peak. These results were explained as due to the formation of the antiferromagnetic phase. © 2001 Published by Elsevier Science Ltd.
Fri, 30 Nov 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/982632001-11-30T00:00:00Z
- The Coriolis Interaction between the v9 and v7 Fundamental Bands of Methylene Fluoridehttps://scholarbank.nus.edu.sg/handle/10635/98250Title: The Coriolis Interaction between the v9 and v7 Fundamental Bands of Methylene Fluoride
Authors: Goh, K.L.; Tan, T.L.; Ong, P.P.; Teo, H.H.
Abstract: The infrared spectrum of the v7 and v9 bands of methylene fluoride-d2 (CD2F2) has been recorded with an unapodized resolution of 0.0024 cm-1 in the frequency range of 940-1030 cm-1 using the Fourier transform technique. A weak b-type Coriolis interaction term was found to couple these two vibrational states with band centers about 42 cm-1 apart. By fitting a total of 1031 infrared transitions of both v7 and v9 with a standard deviation of 0.0011 cm-1 using a Watson's A-reduced Hamiltonian in the Ir representation with the inclusion of a b-type Coriolis resonance term, two sets of rovibrational constants for v7 = 1 and v9 = 1 states up to sextic order were derived. The v7 band is C type, while the v9 band is A type with band centers at 961.8958 ± 0.0005 and 1003.7421 ± 0.0001 cm-1, respectively. © 2000 Academic Press.
Thu, 01 Jun 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/982502000-06-01T00:00:00Z
- The ν2 bands of HD18O and D2 18O: Rovibrational constants and additional transitionshttps://scholarbank.nus.edu.sg/handle/10635/98331Title: The ν2 bands of HD18O and D2 18O: Rovibrational constants and additional transitions
Authors: Wang, W.F.; Tan, T.L.; Tan, B.L.; Ong, P.P.
Fri, 01 Mar 1996 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/983311996-03-01T00:00:00Z
- The v2 and v8 + v10 bands of CF2=CH2https://scholarbank.nus.edu.sg/handle/10635/98328Title: The v2 and v8 + v10 bands of CF2=CH2
Authors: Wang, W.F.; Tan, T.L.; Ong, P.P.
Abstract: The high-resolution FTIR spectrum of CF2=CH2 was measured and analyzed in the Fermi interacting v2 and v8 + v10 bands around 1735 cm-1. Both bands have strong infrared absorption with an A-type appearance. Watson's A-reduced Hamiltonian in the Ir representation was employed in the computation of the energy levels, taking into account the Fermi resonance contribution. A total of 2119 v2 transitions and 2056 v8 + v10 transitions were eventually assigned with a rms deviation of 0.00075 cm-1. In a nonlinear least-squares fit, accurate rovibrational constants of the upper states were determined with the band origins v2 = 1728.49829 ± 0.00005 cm-1 and v8 + v10 = 1741.50207 ± 0.00004 cm-1. Finally, a set of equilibrium rotational constants of CF2=CH2 were derived. © 1997 Academic Press, Inc.
Wed, 01 Jan 1997 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/983281997-01-01T00:00:00Z
- Thermionic oxide sources of Al+, Ga+, In+, and Tl+https://scholarbank.nus.edu.sg/handle/10635/98381Title: Thermionic oxide sources of Al+, Ga+, In+, and Tl+
Authors: Tan, T.L.; Zhou, Z.L.; Ong, P.P.
Abstract: Oxides (Al2O3, Ga2O3, In2O3, and Tl2O3) of Group IIIA metals were coated on tungsten filaments to form thermionic emitters of purely ground state Al+, Ga+, In+ and Tl+ ions respectively. Variations of total ion emission current and ion purity with source filament temperature, and with time were studied. A very pure (99.9%) source of Al+(1 S0) could be obtained from the Al2O3-coated filament when heated to 2200°C but the filament was short lived. A stable ion current and a purity between 92% and 96% of Al+ could be produced at a lower temperature of 2000°C after 100 min of heating. At 1900°C, the Ga2O3-coated filament could produce Ga+(1S0) ions of 98% purity but its ion current was unstable with time. However, in the lower range of 1600-1820°C, a more stable Ga+ ion current of about 80% purity could be obtained shortly after heating. For the In2O3-coated filament, the purity of In+(1S0) ions reached a maximum of 90% at 1580°C while the total ion current increased to a maximum of 2900 nA at 1690°C. A very stable In+ ion current could be obtained at 1580°C. The Tl2O3-coated filament using tungsten or tantalum wires was found to be unsuitable as a source of Tl+ ions. © 1995.
Fri, 31 Mar 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/983811995-03-31T00:00:00Z
- Thermionic aluminosilicate sources of Tl+ ionshttps://scholarbank.nus.edu.sg/handle/10635/98380Title: Thermionic aluminosilicate sources of Tl+ ions
Authors: Zhou, Z.L.; Tan, T.L.; Ong, P.P.
Abstract: Thermionic filament sources of Tl+ ions were made using tungsten wire coated with synthetic thallium aluminosilicates in the β-eucryptite (Tl2OAl2O32SiO2) and two other compositions (2Tl2OAl2O32SiO2 and 2Tl2OAl2O34SiO2). Three different methods for the production of aluminosilicate melts were used. The performance characteristics as thermionic emission sources of Tl+ ions based on total emission current and purity of ion emission as functions of time and filament surface temperature were studied and compared. Our results indicated that the sample of composition ratio of 2Tl2OAl2O 32SiO2, which was produced by resistance heating under atmospheric pressure in air, is an optimum Tl+ ion source at temperatures between 1150 and 1350°C after 150 h of continuous heating. © 1995 American Institute of Physics.
Sun, 01 Jan 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/983801995-01-01T00:00:00Z
- Thin films of silicon nanoparticles embedded in Al2O3 matriceshttps://scholarbank.nus.edu.sg/handle/10635/98394Title: Thin films of silicon nanoparticles embedded in Al2O3 matrices
Authors: Zhu, Y.; Ong, P.P.
Abstract: Silicon nanoparticles embedded in a host matrix of Al2O3 thin film about 260 nm thick were prepared by using the pulsed laser deposition method. The laser target consisted of a small silicon wafer glued onto the surface of a circular Al2O3 plate, which was set to rotate uniformly when laser-ablating. TEM and EDS results showed that the films consisted of silicon nanoparticles in the form of nanocrystals with diameters of less than 6 nm dispersed in the amorphous Al2O3 matrices. Strong PL from the as-prepared and the annealed samples was observed. The SIMS depth profile of the silicon content of the film indicated that the silicon nanoparticles in the Al2O3 matrices were arranged in fairly well demarcated thin layers sandwiched between layers of the host material. FTIR results evidently suggested that the encapsulating amorphous Al2O3 material forms a good host for the prevention of atmospheric oxidation of the silicon nanoparticles. However, prolonged annealing can cause the silicon nanoparticle surface interfacing the host material to be oxidized by the oxygen mostly from the Al2O3 molecules and be bonded to the Al or O-Al bonds.
Mon, 01 Jan 2001 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/983942001-01-01T00:00:00Z
- The High-Resolution Infrared Spectrum of the v5 Band of Deuterated Formic Acid (DCOOH)https://scholarbank.nus.edu.sg/handle/10635/98279Title: The High-Resolution Infrared Spectrum of the v5 Band of Deuterated Formic Acid (DCOOH)
Authors: Goh, K.L.; Ong, P.P.; Tan, T.L.; Wang, W.F.; Teo, H.H.
Abstract: The Fourier transform infrared spectrum of the v5 band of deuterated formic acid (DCOOH) has been measured with a resolution of 0.004 cm-1 in the frequency range of 1090-1180 cm-1. Using a Watson's A-reduced Hamiltonian in the Ir representation, a total of 1731 assigned unperturbed transitions have been analyzed to provide rovibrational constants for the upper state (v5 = 1) with a standard deviation of 0.000363 cm-1. The band is A type with an unperturbed band center at 1142.31075 ± 0.00002 cm-1. The band is expected to be perturbed by a nearby v4 band through a Fermi resonance term and possibly a Coriolis term. The resonance is particularly noticeable for Ka = 10, and 11, at high J values. About 215 perturbed lines were identified but they were not included in the final fit. © 1998 Academic Press.
Wed, 01 Jul 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/982791998-07-01T00:00:00Z
- Thermal stability of mesoporous silica molecular sievehttps://scholarbank.nus.edu.sg/handle/10635/98368Title: Thermal stability of mesoporous silica molecular sieve
Authors: Gu, G.; Ong, P.P.; Chu, C.
Abstract: Mesoporous silica molecular sieves (MCM-41) with pore diameters from 3 to 10 nm were synthesized by a liquid-crystal template mechanism. A surface phonon mode at around 961 cm-1, connected with the mesoporous structure was observed. At the annealing temperature of 800 °C, the mesoporous structure of the molecular sieve was found to collapse completely, based on Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) measurements. Further annealing of MCM-41 results in transformation to the cristobalite phase of silica.
Thu, 01 Jul 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/983681999-07-01T00:00:00Z
- Studies of rubidium aluminosilicates as thermionic emitters of Rb+ ionshttps://scholarbank.nus.edu.sg/handle/10635/98072Title: Studies of rubidium aluminosilicates as thermionic emitters of Rb+ ions
Authors: Tan, T.L.; Ong, P.P.; Fong, T.M.; Soo, K.A.
Abstract: Filament sources of Rb+ ions were created using 90% Pt-10% Rh gauze coated with synthetic rubidium aluminosilicate compounds of various compositions. The performance characteristics of these compounds as thermionic emission sources of Rb+ ions based on total emission current and purity of ion emission as functions of time were studied and compared. In addition, these characteristics were investigated of β-eucryptite (Rb2O·Al2O3·2SiO2) and spodumene (Rb2O·Al2O3·4SiO2) samples at four different filament surface temperatures. It was concluded that the β-eucryptite composition of rubidium aluminosilicate gives the most satisfactory results as a Rb+ ion emitter at a temperature of 1100°C. © 1994.
Thu, 30 Jun 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/980721994-06-30T00:00:00Z
- v3 band of DCOOHhttps://scholarbank.nus.edu.sg/handle/10635/98541Title: v3 band of DCOOH
Authors: Goh, K.L.; Ong, P.P.; Tan, T.L.
Abstract: The high resolution Fourier transform infrared (FTIR) spectrum of the v3 band of DCOOH has been measured with a resolution of 0.004 cm-1 in the spectral range 1670-1810 cm-1. Using the Watson's A-reduced Hamiltonian in the Ir representation, 713 infrared transitions have been assigned and fitted to provide rovibrational constants for the v3 = 1 state with a standard deviation of 0.000439 cm-1. The v3 band centre was found at 1725.87497±0.00003 cm-1. The band is perturbed by the 2v8 band, situated at 1762.9 cm-1, through Fermi resonance and possibly a Coriolis term. Perturbations of v3 by v5+v7 and v5+v9 are also expected. About 280 perturbed v3 lines were identified and excluded in the final fit.
Mon, 01 Nov 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985411999-11-01T00:00:00Z
- Velocity distributions of He+, Ne+ and Ar+ in parent gaseshttps://scholarbank.nus.edu.sg/handle/10635/98552Title: Velocity distributions of He+, Ne+ and Ar+ in parent gases
Authors: Ong, P.P.; Hogan, M.J.
Abstract: Measurements are made of the velocity distributions of He+, Ne+ and Ar+ ions drifting in their respective parent gas under the action of a uniform electric field at room temperature (297-300K). Considerable improvements have been implemented in the present apparatus in comparison with older systems. The drift tube is greatly enlarged and its pressure accurately controlled by a regulating servo-valve. A new ion optical system coupled with a wider quadrupole analyser head ensures a high collection efficiency of ions transmitted through the retarding potential grids. A novel in-drift obstacle and a 90 degrees off-axis detector effectively suppress interference from UV photons and metastable species. Experimental results for E/N ranging from 60 to 320 Td are compared with both the Wannier equation for the RMS ion velocity and the corresponding Maxwellian distribution. The typical experimental curve is found to possess only a single maximum with its peak displaced towards the low velocity end and an enhanced high velocity tail in comparison with the Maxwellian.
Tue, 01 Jan 1985 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985521985-01-01T00:00:00Z
- Velocity distribution functions of Na+ ions drifting in helium, neon and argonhttps://scholarbank.nus.edu.sg/handle/10635/98551Title: Velocity distribution functions of Na+ ions drifting in helium, neon and argon
Authors: Hogan, M.J.; Ong, P.P.
Abstract: Measurements are reported of the velocity distribution functions of room-temperature Na+ ions drifting in helium, neon and argon gas under the influence of an electric field at E/n in the range 16-261 Td. The distributions were found to become wider, have larger RMS velocities and become more skewed toward higher velocities as E/n was increased. They also were shifted increasingly further from the velocity space origin with increasing E/n. The ion-to-neutral-mass ratio was found to be an important parameter in determining the properties of the distributions. The experimental results were used to evaluate the accuracy of theoretical analytic expressions for the velocity distributions.
Fri, 01 Jan 1988 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985511988-01-01T00:00:00Z
- Validity of the effective temperature scale for a dilute Li+ swarm in a Xe gashttps://scholarbank.nus.edu.sg/handle/10635/98547Title: Validity of the effective temperature scale for a dilute Li+ swarm in a Xe gas
Authors: Ong, P.P.
Abstract: The effective-temperature concept arising from the two-temperature (2T) kinetic theory for the nonreactive migration of a dilute ion swarm in a neutral gas under the action of a uniform electrostatic field has been tested over a wide temperature range by Monte Carlo simulation (MCS). Such a test over the prescribed temperature span has not been made experimentally because of the inherent difficulty in designing an apparatus which permits the temperature of the ion drift chamber to be varied so widely. The MCS test is here applied to Li+ in Xe, which offers the most favorable conditions for the 2T theory to be a valid approximation of the more sophisticated three-temperature theory. Results established that for such cases it is possible to express the mobility K0 and transverse diffusion DT, but not the longitudinal diffusion DL, in terms of a univariate effective-temperature scale instead of the separate parameters of gas temperature and the ratio of electric field to gas concentration. More remarkably, the ratio DT/K was found to bear a simple linear relation with Teff over a very wide range. © 1993 The American Physical Society.
Fri, 01 Jan 1993 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/985471993-01-01T00:00:00Z
- Transport properties of Rb+ ions in Ne and Ar gaseshttps://scholarbank.nus.edu.sg/handle/10635/98451Title: Transport properties of Rb+ ions in Ne and Ar gases
Authors: Tan, T.L.; Ong, P.P.; Zhou, Z.L.
Abstract: The ratio of the transverse diffusion coefficient to mobility, DT/K, has been experimentally measured for Rb+ ions drifting in Ne and Ar gases in the presence of a uniform electric field. Results at 303 K are reported at electric field to neutral gas density ratio, E/N, values ranging from 5 to 500 Td for both Rb+Ne and Rb+Ar systems. The reduced mobility KO and the ratios of the longitudinal and transverse diffusion coefficients to mobility, DL/K and DT/K respectively, were calculated for both Rb+Ne and Rb+Ar systems with a Monte Carlo simulation (MCS) technique which used the interaction potential of Koutselos, Mason and Viehland (KMV) (J. Chem. Phys., 93 (1990) 7125) as input. The accuracy of this potential has been evaluated by comparing the DT/K results to the experimentally determined values. The KMV potential appears to be a good representation of the actual potential in the whole E/N range (5-500 Td) for Rb+Ne but only in the 5-350 Td range for Rb+Ar. Values of DL/K and DT/K were also derived using KO obtained from the present MCS calculations and from experimental data available in the literature, employing the generalised Einstein relations (GER) based on the three-temperature theory. In the E/N range studied for both Rb+Ne and Rb+Ar systems, the accuracy of the experimental measurements of DT/K is estimated to be ±3% and that of the MCS calculations to be ±2.5%. © 1995.
Fri, 15 Dec 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984511995-12-15T00:00:00Z
- Transport properties of Na+ ions in Kr gashttps://scholarbank.nus.edu.sg/handle/10635/98450Title: Transport properties of Na+ ions in Kr gas
Authors: Tan, T.L.; Ong, P.P.; Hogan, M.J.
Abstract: The transverse-diffusion coefficients for Na+ ions drifting in Kr gas have been experimentally measured with estimated accuracies of better than 3% at E/N values ranging from 5 to 500 Td (1 Td=10-21 V m2) at about 303 K. In addition, elaborate calculations of the transport properties for the Na+-Kr system have been made by using Monte Carlo simulations (MCSs) and the generalized Einstein-relation techniques. The experimental results are compared with values obtained from MCSs using the interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)] as input. Agreement within 3% for the entire range of E/N is observed in the comparisons. MCS-calculated values of longitudinal-diffusion coefficients are also compared with those determined experimentally by Thackston et al. [J. Chem. Phys. 73, 2012 (1980)]. © 1993 The American Physical Society.
Fri, 01 Jan 1993 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984501993-01-01T00:00:00Z
- Transverse diffusion and interaction potential for K + drifting in Arhttps://scholarbank.nus.edu.sg/handle/10635/98452Title: Transverse diffusion and interaction potential for K + drifting in Ar
Authors: Ong, P.P.; Hogan, M.J.
Abstract: Experimentally measured values of the ratio, D T/K, of the transverse diffusion coefficient to the mobility for K + ions drifting in argon gas are reported. Results were obtained with estimated accuracies of ±3% at E/N values ranging from 10-600 Td (1 Td=10 -21 V m 2) and have been adjusted to 298 K. These experimental values are compared with values obtained from Monte Carlo simulations using several proposed interaction potentials as input. The comparisons suggest that none of the potentials examined is a totally accurate representation of the K +-Ar interaction potential. © 1991 IOP Publishing Ltd.
Thu, 14 Feb 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984521991-02-14T00:00:00Z
- Aluminosilicate sources of Cs+ ionshttps://scholarbank.nus.edu.sg/handle/10635/95748Title: Aluminosilicate sources of Cs+ ions
Authors: Ong, P.P.; Tan, T.L.
Abstract: Synthetic cesium aluminosilicates in the form of β-eucryptite (Cs 2O·Al2O3·2SiO2) and spodumene (Cs2O·Al2O3·4SiO 2) were used to construct thermionic emitters of Cs+ ions. Total ion emission current and purity of ion emission of Cs+ ions for these two forms of aluminosilicates were studied and compared at four different material surface temperatures over 50 h of continuous heating. In addition, the ion emission characteristics were investigated for three other compositions of cesium aluminosilicates. It was revealed that as a Cs + ion source, β-eucryptite gives the optimum ion emission at the temperature of 1100°C. © 1994 American Institute of Physics.
Sat, 01 Jan 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/957481994-01-01T00:00:00Z
- An apparatus for measuring DT/K without in-vacuum mechanical motionhttps://scholarbank.nus.edu.sg/handle/10635/95756Title: An apparatus for measuring DT/K without in-vacuum mechanical motion
Authors: Ong, P.P.; Hogan, M.J.
Abstract: A new drift-tube apparatus has been developed for measuring D T/K, the ratio of the transverse diffusion coefficient to the mobility of ions in a neutral gas under the influence of an electric field. It harnesses existing technology in mechanical, electronic, materials, and computer engineering. The main features are the ion current detector, which comprises 65 parallel rods, and an external mechanical multiplexer, which was designed to have extremely low (fA order) leakage currents and noise levels. Unlike with other earlier systems, the need for precise mechanical motion or adjustments of experimental conditions inside the drift tube during data collection has been circumvented. Its design therefore simplifies construction and makes operation less error prone. Data collection is computer controlled. A value for D T/K can now typically be obtained in less than 20 min, which greatly increases the rate at which results can be obtained as well as reduces the chance of unwanted variations in experimental conditions occurring during data collection. Results are estimated to be accurate to within 3%.
Tue, 01 Jan 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/957561991-01-01T00:00:00Z
- Analysis of the coriolis interaction of the ν12 band with 2ν10 of cis-d2-ethylene by high-resolution Fourier transform infrared spectroscopyhttps://scholarbank.nus.edu.sg/handle/10635/95777Title: Analysis of the coriolis interaction of the ν12 band with 2ν10 of cis-d2-ethylene by high-resolution Fourier transform infrared spectroscopy
Authors: Goh, K.L.; Tan, T.L.; Ong, P.P.; Teo, H.H.
Abstract: The Fourier transform infrared spectrum of the ν12 band of cis-d2-ethylene (cis-C2H2D2) has been recorded with an unapodized resolution of 0.0024 cm-1 in the frequency range of 1280-1400 cm-1. This band was found to be mutually coupled by Coriolis interaction with the unobserved 2ν10 band situated approximately 10 cm-1 below ν12. By fitting a total of 771 infrared transitions of ν12 with a standard deviation of 0.00075 cm-1 using the Watson's Hamiltonian with the inclusion of a c-type Coriolis resonance term, a set of accurate rovibrational constants for V12=1 state was derived. The ν12 band is A type with a band centre at 1341.1512±0.0001 cm-1. Accurate rovibrational constants for the V10=2 state were also derived.
Fri, 04 Aug 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/957772000-08-04T00:00:00Z
- An appraisal of the three-temperature theory and the interaction potentials of K +-He and K +-Kr by Monte Carlo Simulationhttps://scholarbank.nus.edu.sg/handle/10635/95757Title: An appraisal of the three-temperature theory and the interaction potentials of K +-He and K +-Kr by Monte Carlo Simulation
Authors: Ong, P.P.; Hogan, M.J.
Abstract: Monte Carlo simulation calculations on a supercomputer have been made of the mobility of K + ions drifting in pure He and Kr gases respectively under the action of a uniform electric field. Typical errors of 1% or less are achieved. By making direct comparisons with other theoretical and experimental data, these calculations have helped to verify both the three-temperature theory used to derive the theoretical mobility as well as the validity of various interaction potentials proposed. Results of these comparisons together with an earlier one for the K +-Ar system provide evidence that the three-temperature theory is generally applicable for the three different ion-neutral mass ratios studied, but yields a slight overestimation of the mobility around the peak mobility region. © 1991 IOP Publishing Ltd.
Sun, 28 Jul 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/957571991-07-28T00:00:00Z
- A study of K+-He and K+-Kr atomic interaction potentials based on transverse diffusion measurementshttps://scholarbank.nus.edu.sg/handle/10635/95689Title: A study of K+-He and K+-Kr atomic interaction potentials based on transverse diffusion measurements
Authors: Hogan, M.J.; Ong, P.P.
Abstract: The ratio of the transverse diffusion coefficient to the mobility, D T/K, has been measured for K+ ions drifting in helium and krypton in the presence of a uniform electric field. Results at 298 K are reported at E/N values ranging from 10 to 220 Td for K+-He and 5 to 420 Td for K+-Kr. Values of DT/K for these two systems were also calculated with a Monte Carlo simulation method which used a number of proposed potentials as input. The accuracy of these potentials has been evaluated by comparing the results to the experimentally determined values. For K+-He none of the available potentials seems to be completely accurate, but for K+-krypton the potential of Koutselos, Mason, and Viehland appears to be a very good representation of the actual potential. Within the ranges studied, the estimated accuracy of the experimental measurements is ± 2.5% and the accuracies for the Monte Carlo calculations are ± 1% for K+-He and ± 0.5% for K +-Kr. © 1991 American Institute of Physics.
Tue, 01 Jan 1991 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/956891991-01-01T00:00:00Z
- A Monte Carlo technique to determine the effectiveness of roadside trees for containing traffic noisehttps://scholarbank.nus.edu.sg/handle/10635/95645Title: A Monte Carlo technique to determine the effectiveness of roadside trees for containing traffic noise
Authors: Tang, S.H.; Ong, P.P.
Abstract: A computer-generated simulation of noise attenuation by foliage of trees lining urban roads has been carried out. For the simulation of foliage the program uses experimentally measured leaf parameters as input variables. A geometrical reflection model is assumed for the noise trapped by the tall building facade parallel to the road. The results show that belts of trees do not significantly attenuate traffic noise at ground level but, for elevated levels, effective attenuation of the traffic noise occurs, especially at the higher-frequency end.
Fri, 01 Jan 1988 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/956451988-01-01T00:00:00Z
- Strong photoluminescence with fine structures from sandwich-structured films of silicon nanoparticles embedded in Al2O3 matriceshttps://scholarbank.nus.edu.sg/handle/10635/98043Title: Strong photoluminescence with fine structures from sandwich-structured films of silicon nanoparticles embedded in Al2O3 matrices
Authors: Ong, P.P.; Zhu, Y.
Abstract: Sandwich-structured thin films of silicon nanoparticles embedded in Al2O3 matrices were prepared by pulsed laser ablation on a slowly rotating target which alternately exposed the Si and Al2O3 materials to the laser beam. SIMS profile studies showed that the sharpness of the demarcation boundary between the Si and Al2O3 layers depended strongly on their annealing conditions. Correspondingly, the photoluminescence (PL) of the films was likewise found to depend strongly on annealing conditions. Very high-intensity and stable PL was achievable under the most appropriate annealing conditions. In addition, the PL spectra also contained multiple fine structures which did not depend on the annealing conditions. Annealing caused a significant blueshift of the main peak, but no corresponding blueshift of the fine structures was observed. Our results can be interpreted in relation to the quantum confinement model of the observed PL. The fine structures are attributed to the different interfacial states between the silicon nanocrystals and aluminum oxides. The intensity of the PL was strongly affected by the conditions of the Si-Al2O3 interfacial states in the films. © 2002 Elsevier Science B.V. All rights reserved.
Fri, 01 Nov 2002 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/980432002-11-01T00:00:00Z
- Strong and stable photoluminescence from sputtered silicon nanoparticleshttps://scholarbank.nus.edu.sg/handle/10635/98038Title: Strong and stable photoluminescence from sputtered silicon nanoparticles
Authors: Zhu, Y.; Wang, H.; Ong, P.P.
Abstract: Silicon nanoparticles have been prepared by means of direct dc sputtering of the silicon material onto the liquid nitrogen-cooled surface of the stainless-steel trap. By periodically harvesting the deposits followed by ultrasonic agitation in 2-propanol it was possible to produce nanometre-size silicon crystals of less than 10 nm in diameter, and in which the silicon particle surfaces were barely oxidized. XPS measurements of the samples so prepared revealed that the oxidation states of their surface layers were changed in different significant ways when the as-prepared sample was annealed in air or in a vacuum. However, all these chemical changes have very little effect on the photoluminescence level of the samples. Its intensity remains strong and stable in the region of 300-550 nm, both before and after annealing either in the atmosphere or in an ultra-high vacuum at up to 500°C and for up to 6 h. Moreover, the photoluminescence intensity stays constant even after the samples were aged in the atmosphere at room temperature (300 K) for 22 days. The photoluminescence stability of our silicon nanoparticles, regardless of changes in their surface chemical structure, enhances their conduciveness for commercial applications.
Mon, 21 Aug 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/980382000-08-21T00:00:00Z
- SODIUM ION PRODUCTION IN A SIMPLE MICROWAVE DISCHARGE ION SOURCE.https://scholarbank.nus.edu.sg/handle/10635/97957Title: SODIUM ION PRODUCTION IN A SIMPLE MICROWAVE DISCHARGE ION SOURCE.
Authors: Hogan, M.J.; Ong, P.P.
Abstract: Microwave discharge ion sources have commonly been used to produce ions from gases. In this paper we report the extension of such a source to the production of ions from a non-gaseous material without the complication of a magnetic field. The source is simple in design and easily constructed. Discharges which were initiated in argon produce Na** plus ions from both sodium iodide and pure sodium. The absolute and relative rates of production of Na** plus ions were found to vary greatly with the discharge pressure and incident microwave power.
Mon, 01 Sep 1986 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/979571986-09-01T00:00:00Z
- Transport properties and interaction potential for Na+ ions in Xe gashttps://scholarbank.nus.edu.sg/handle/10635/98446Title: Transport properties and interaction potential for Na+ ions in Xe gas
Authors: Tan, T.L.; Ong, P.P.; Hogan, M.J.
Abstract: The ratio, DT/K, of the transverse diffusion coefficient to mobility for Na+ ions drifting in Xe gas has been experimentally measured. Inevitable longitudinal end effects present in the drift tube were duly corrected for. Results at ∼303 K are reported at electric field to neutral gas density ratios, E/N, ranging from 5 to 440 Td. Furthermore, elaborate calculations of the reduced mobility, K0, the ratio, D L/K, of the longitudinal diffusion coefficient to mobility, and DT/K were calculated with a Monte Carlo simulation (MCS) method which used the interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)] as input. The accuracy of this potential has been evaluated by comparing the results with the experimental data and those calculated using the generalized Einstein relations technique. Agreement within 3% between the experimental values of DT/K and those from MCS calculations for the whole range of E/N suggests that the interaction potential represents fairly well the true potential. The error of the experimental measurements of DT/K is estimated to be better than 3.5% and that of the MCS calculations is better than 1% for K0 and 2.5% for D T,L/K. © 1994 American Institute of Physics.
Sat, 01 Jan 1994 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984461994-01-01T00:00:00Z
- Transverse diffusion measurements and Monte Carlo simulation studies of Rb+ ions in Kr and Xehttps://scholarbank.nus.edu.sg/handle/10635/98454Title: Transverse diffusion measurements and Monte Carlo simulation studies of Rb+ ions in Kr and Xe
Authors: Tan, T.L.; Ong, P.P.
Abstract: The ratio of the transverse diffusion coefficient to the mobility (D T/K) for Rb+ ions drifting in Kr and Xe gases at about 303 K has been measured at electric field to neutral gas density ratio (E/N) values ranging from 5 to 500 Td. Calculations of the reduced mobility (K0) and the ratios of the longitudinal and transverse diffusion coefficients to mobility (DL/K and DT/K, respectively) for both Rb +-Kr and Rb+-Xe systems were made using a Monte Carlo simulation (MCS) technique and the interaction potential of Koutselos, Mason, and Viehland [J. Chem. Phys. 93, 7125 (1990)]. Furthermore, DL/K and DT/K values were derived from K0 values obtained from the present MCS calculations and from experimental results reported in the literature, using the generalized Einstein relations which are based on the three-temperature theory. For the Rb+-Kr system, the experimental values of DT/K were found to be 2%-4% higher than those from MCS calculations for EIN values above 120 Td. For the Rb+-Xe system, the experimental DT/K values were similarly higher than the MCS values in the range 90-220 Td. Given that the standard error of the experimental measurements of DT/K is estimated to be better than 3% and that of the MCS calculations is better than 2.5%, the agreement between the present experimental DT/K data and those from MCS calculations can be considered to be fairly good. The interaction potential of Koutselos, Mason, and Viehland therefore appears to represent well the actual potential for both Rb+-Kr and Rb+-Xe systems. © 1995 American Institute of Physics.
Sun, 01 Jan 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984541995-01-01T00:00:00Z
- Transverse diffusion measurements and comparisons with Monte Carlo simulation and Gram-Charlier calculations for Cs+ ions drifting in He gashttps://scholarbank.nus.edu.sg/handle/10635/98453Title: Transverse diffusion measurements and comparisons with Monte Carlo simulation and Gram-Charlier calculations for Cs+ ions drifting in He gas
Authors: Li, M.M.; Ong, P.P.; Zhou, Z.L.
Abstract: The ratio of the transverse diffusion coefficient to the mobility, DT/K, has been experimentally determined for Cs+ ion drifting in He gas at E/N (electric field to neutral gas number density ratio) values ranging from 5 to 140 Td. Results at 303 K are reported with an overall accuracy of ± 4%. The DT/K results were effectively corrected for longitudinal end effects existing in the drift tube. In addition, Monte Carlo simulation (MCS) calculations were made for the mobility and transport coefficients using the Koutselos, Mason, and Viehland (KMV) interaction potential as input. Comparisons of MCS results and available kinetic theory's Gram-Charlier results with the present and previous experimental data give an indication of the validity and accuracy of the present potential. The theoretical results did not reproduce the experimental curve very well, suggesting that the proposed potential for the Cs+ -He pair should be modified. Further comparisons of the transverse and longitudinal diffusion coefficients indicate that the Gram-Charlier approach of kinetic theory is quite accurate even for the large ion-neutral mass ratio. © 1997 American Institute of Physics.
Sun, 15 Jun 1997 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984531997-06-15T00:00:00Z
- Transverse diffusion of Li + ion swarms in Xe gashttps://scholarbank.nus.edu.sg/handle/10635/98455Title: Transverse diffusion of Li + ion swarms in Xe gas
Authors: Tan, T.L.; Ong, P.P.
Abstract: In our transverse diffusion measurements of Li+ ions in Xe gas, appreciable longitudinal end effects were found throughout the whole range of E/N (electric field to neutral gas number density ratio) studied, i.e. 5-150 Td. By applying a known linear relationship of the measured variance (x 2) of the transverse ion current density profile with drift length z, the magnitude a 2 of the end effects can be numerically derived. The ratio of the transverse diffusion coefficient to mobility, D T/K, at 309 K for the Li+-Xe system were eventually corrected for end effects by an analysis which requires measured (x 2) values and a 2. The value of a 2 is assumed to vary linearly with E/N. The corrected D T/K values are determined with an overall accuracy of +or-4%. The self-consistency and the good agreement of the results with those calculated by Monte Carlo simulations (MCS) bears evidence of the accuracy and reliability of the D T/K results. The accuracy of the MCS calculations is estimated to be +or-2.5%. The D T/K values for Li + in Xe were also derived using reduced mobility K 0 data obtained from the MCS calculations and from experimental data available in the literature, employing the generalized Einstein relations (GER) based on the three-temperature theory. © 1995 IOP Publishing Ltd.
Mon, 14 Aug 1995 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984551995-08-14T00:00:00Z
- Traffic noise survey and analysis in Singaporehttps://scholarbank.nus.edu.sg/handle/10635/98429Title: Traffic noise survey and analysis in Singapore
Authors: Sy, H.K.; Ong, P.P.; Tang, S.H.; Tan, K.L.
Abstract: A comprehensive survey and statistical analysis of daytime traffic noise in Singapore was carried out. The results are presented in terms of average L10, L50 and L90 for four different classes of sites. By clearly distinguishing between temporal and spatial noise fluctuations, it is possible, on the basis of the Gaussian noise distributions obtained, to verify that the overall noise fluctuation can also be derivable from the respective temporal and spatial noise fluctuations. The traffic noise index (TNI) and the noise pollution index (LNP) are determined and a correlation is established between the traffic noise levels and the corresponding volume of traffic. © 1985.
Tue, 01 Jan 1985 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/984291985-01-01T00:00:00Z
- Changes in structure and electronic states of fullerenes incorporated in silica aerogelhttps://scholarbank.nus.edu.sg/handle/10635/95943Title: Changes in structure and electronic states of fullerenes incorporated in silica aerogel
Authors: Zhu, L.; Ong, P.P.; Shen, J.; Wang, J.
Abstract: We have developed a method to synthesize a novel C60-SiO2 aerogel composite with C60 concentration up to 0.2mol% by a sol-gel process accompanied by a supercritical drying technique. The structure and electronic states of the fullerenes in the new material were studied by laser desorption time-of-flight mass spectrometry, Fourier transform-infrared spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet and visible absorption spectroscopy, and photoluminescence, all of which show distinctly different features from a previously reported C60-silica aerogel material in which the structure of C60 was found to be retained in the fabrication process. Detailed analyses of the diverse data from the different experimental techniques yield ample evidence to suggest that the structure and electronic states of the incorporated fullerene have been considerably changed, and that the C60 in the composite is passivated by surface species (most probably OH) remaining during the hydrolysis of the aerogel precursor. © 1998 Elsevier Science Ltd. All rights reserved.
Fri, 01 May 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/959431998-05-01T00:00:00Z
- High-Resolution FTIR Spectrum of the ν5 Band of HCOODhttps://scholarbank.nus.edu.sg/handle/10635/96821Title: High-Resolution FTIR Spectrum of the ν5 Band of HCOOD
Authors: Goh, K.L.; Ong, P.P.; Tan, T.L.; Teo, H.H.; Wang, W.F.
Abstract: The high-resolution Fourier transform infrared spectrum of HCOOD has been measured in the ν5 region between 1120 and 1220 cm-1 with a resolution of 0.004 cm-1. As expected for an in-plane vibrational fundamental mode, the ν5 band is a hybrid band consisting of both a-type and b-type transitions. Using the Watson's A-reduced Hamiltonian in the Ir representation, 1943 infrared transitions have been assigned and fitted to give 12 rovibrational constants for the ν5 = 1 state. The ν5 band is primarily A type with a band center at 1177.09378 ± 0.00002 cm-1. It is found that ν5 is slightly perturbed by the nearby 2ν7. About 90 perturbed transitions were identified. © 1998 Academic Press.
Thu, 01 Oct 1998 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968211998-10-01T00:00:00Z
- High-resolution Fourier transform infrared spectrum of the ν12 fundamental band of ethylene (C2H4)https://scholarbank.nus.edu.sg/handle/10635/96816Title: High-resolution Fourier transform infrared spectrum of the ν12 fundamental band of ethylene (C2H4)
Authors: Tan, T.L.; Lau, S.Y.; Ong, P.P.; Goh, K.L.; Teo, H.H.
Abstract: The infrared spectrum of the ν12 fundamental band of ethylene (C2H4) has been measured with an unapodized resolution of 0.004 cm-1 in the frequency range of 1380-1500 cm-1 using the Fourier transform technique. By assigning and fitting a total of 1387 infrared transitions using a Watson's A-reduced Hamiltonian in the I′ representation, rovibrational constants for the upper state (ν12 = 1) up to five quartic and three sextic centrifugal distortions terms were derived. They represent the most accurate constants for the band so far. The rms deviation of the fit was 0.00033 cm-1. The A-type ν12 band with a band center at 1442.44299±0.00003 cm-1 was found to be relatively free from local frequency perturbations. The inertial defect Δ12 was found to be 0.24201±0.00002 u angstrom2.
Sun, 01 Oct 2000 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968162000-10-01T00:00:00Z
- High-resolution Fourier transform infrared spectroscopy and analysis of the ν12 fundamental band of ethylene-d4https://scholarbank.nus.edu.sg/handle/10635/96815Title: High-resolution Fourier transform infrared spectroscopy and analysis of the ν12 fundamental band of ethylene-d4
Authors: Tan, T.L.; Goh, K.L.; Ong, P.P.; Teo, H.H.
Abstract: The Fourier transform infrared (IR) spectrum of the ν12 fundamental band of ethylene-d4 (C2D4) has been measured with an unapodized resolution of 0.004 cm-1 in the frequency range of 1030-1130 cm-1. A total of 1340 assigned transitions have been analyzed and fitted using a Watson's A-reduced Hamiltonian in the Ir representation to derive rovibrational constants for the upper state (v12=1) up to five quartic terms with a standard deviation of 0.00042 cm-1. They represent the most accurate constants for the band thus far. The ground state rovibrational constants were also further improved by a fit of combination-differences from the IR measurements. The relatively unperturbed band was found to be basically A-type with a band centre at 1076.98492±0.00003 cm-1.
Fri, 17 Dec 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968151999-12-17T00:00:00Z
- High-Resolution Measurement of the v2 Band of H15NO3https://scholarbank.nus.edu.sg/handle/10635/96822Title: High-Resolution Measurement of the v2 Band of H15NO3
Authors: Wang, W.F.; Ong, P.P.; Chen, H.F.; Teo, H.H.
Mon, 01 Sep 1997 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968221997-09-01T00:00:00Z
- Impurity ion production in alkali ion sourceshttps://scholarbank.nus.edu.sg/handle/10635/96888Title: Impurity ion production in alkali ion sources
Authors: Hogan, M.J.; Ong, P.P.
Abstract: It has long been known that new alkali ion sources utilizing thermionic emission from alkali aluminosilicates can achieve high percentages of the desired ion by a sufficiently long "burn-in." Observations are reported here of instances where increased impurity ion emission can reoccur and lead to possible measurement errors in experimental systems lacking mass spectrometers. The ratio of impurity K+ ions to Na+ ions from a burned-in Na2O·Al2O3· 2SiO2 source was measured to be 0.55.
Mon, 01 Jan 1990 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968881990-01-01T00:00:00Z
- Improved spectroscopic constants for the ν7 and ν8 bands of HNO3https://scholarbank.nus.edu.sg/handle/10635/96886Title: Improved spectroscopic constants for the ν7 and ν8 bands of HNO3
Authors: Looi, E.C.; Lan, T.L.; Wang, W.F.; Ong, P.P.
Fri, 01 Mar 1996 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968861996-03-01T00:00:00Z
- Improved Rovibrational Constants for the ν3 Infrared Band of HCOODhttps://scholarbank.nus.edu.sg/handle/10635/96884Title: Improved Rovibrational Constants for the ν3 Infrared Band of HCOOD
Authors: Goh, K.L.; Ong, P.P.; Teo, H.H.; Tan, T.L.
Fri, 01 Oct 1999 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/968841999-10-01T00:00:00Z