A Novel Method for Determination of the Thermal Diffusivity of Thin Films Using a Modulated CO<Subscript>2</Subscript> Laser

The thermal diffusivity of thin metal films has been measured by combining a fast infrared radiation thermometer with a mercury cadmium telluride (MCT) detector and a CO₂ laser modulated at a radio frequency up to 2 MHz. The laser output beam modulated by an acousto-optic modulator (AOM) is directed to the front surface of the blackened copper thin film (10m thick, 9.5 mm in diameter). The thermal radiation from the back surface of the sample is detected. From the observed phase delay in the detected signal of 0.68 radian to the input laser beam, the thermal diffusivity is determined to be 1.11 × 10^-4m²·s^-1, which agrees well with the value of 0.99 × 10^-4m²·s^-1 calculated from literature results. The method is generally applicable for measurements of thermal properties of nano/micro materials.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22--27, 2003, Boulder, Colorado, U.S.A.     

Design modelling and measured performance of the vacuum system of the Diamond Light Source storage ring

A one-dimensional diffusion model of the Diamond Light Source storage ring vacuum system is described and its predictions are compared with actual measured static (without beam) and dynamic (with beam) pressures over more than 2000 A h of beam conditioning at 3 GeV. An average specific thermal outgassing yield of 1·10⁻¹¹ mbar l/(s cm²) during initial beam circulation is obtained, which reduces to 2·10⁻¹² mbar l/(s cm²) after an accumulated beam dose of 1000 A h and an elapsed time of 769 days. In the presence of stored electron beam, the pressure rises as expected due to photon stimulated desorption (PSD). The PSD yield reduces with beam dose according to a (−2/3) power law as was applied in the model. Predicted and measured dynamic pressures generally agree within a factor of 2 over the whole range of beam conditioning dose studied.     

Performance test of Si PIN photodiode line scanner for thermal neutron detection

Thermal neutron imaging using Si PIN photodiode line scanner and Eu-doped LiCaAlF₆ crystal scintillator has been developed. The pixel dimensions of photodiode are 1.18 mm (width)×3.8 mm (length) with 0.4 mm gap and the module has 192 channels in linear array. The emission peaks of Eu-doped LiCaAlF₆ after thermal neutron excitation are placed at 370 and 590 nm, and the corresponding photon sensitivities of photodiode are 0.04 and 0.34 A/W, respectively. Polished scintillator blocks with a size of 1.18 mm (width)×3.8 mm (length)×5.0 mm (thickness) were wrapped by several layers of Teflon tapes as a reflector and optically coupled to the photodiodes by silicone grease. JRR-3 MUSASI beam line emitting 13.5 meV thermal neutrons with the flux of 8×10⁵ n/cm² s was used for the imaging test. As a subject for imaging, a Cd plate was moved at the speed of 50 mm/s perpendicular to the thermal neutron beam. Analog integration time was set to be 416.6 μs, then signals were converted by a delta-sigma A/D converter. After the image processing, we successfully obtained moving Cd plate image under thermal neutron irradiation using PIN photodiode line scanner coupled with Eu-doped LiCaAlF₆ scintillator.     

Single-electrode monitors of parameters of intense beams of relativistic electrons

Conclusions When using the above single-electrode monitor operating in the atmosphere one can monitor the position of a beam of ultrarelativistic electrons of high density ( 5 × 10¹⁵ electrons/ (cm²·sec)) with an error of ±0.2 mm. By monitoring the displacement of the beam (or of the monitor with cuts), one can measure the true dimensions and density distribution of intense electron beams.Translated from Izmeritel'naya Tekhnika, No. 10, pp. 73–75, October, 1977.     

Thermal Behavior of a Multi-layered Thin Slab Carrying Periodic Signals Under the Effect of the Dual-Phase-Lag Heat Conduction Model

The thermal behavior of a two–layered thin slab carrying periodic signals under the effect of the dual-phase-lag heat conduction model is investigated. Two types of periodic signals are considered, a periodic heating source and a periodic imposed temperature at the boundary. The deviations among the predictions of the classical diffusion model, the wave mode, and the dual-phase-lag model are investigated. Analytical closed-form solutions are obtained for the temperature distribution within the slab. The effect of the angular frequency, thickness of the plate, dimensionless thermal relaxation time, dimensionless phase-lag in temperature gradient, thermal conductivity, and thermal diffusivity on the temperature distribution of the slab was studied. It is found that the deviations among the three models increase as the frequency of the signals increases and as the thickness of the plate decreases. It is found that the use of the dual-phase-lag heat conduction model is necessary when the metal film thickness is of order 10^–6 m and the angular frequency of the signals is of order 10¹²rad · s^–1.     

Gas release of the dielectric coating of an anode and its effect on the characteristics of an ion diode with electron flux insulation by a radial magnetic field

Generation of an ion beam by an ion diode is accompanied by the release of gas from the surface of the dielectric coating of the anode. The magnitude of the pulse pressure of gas in the vacuum chamber depends on the type of the dielectric used and can reach 6 × 10^–4–2 × 10^–3 mm Hg. The magnitude of the limit vacuum chamber pressure at which ion beam parameters remain constant has been found. Characteristics of the ion beam current in the vacuum chamber pressure range of 2 × 10^–4–1.3 × 10^–3 mm Hg are presented.     

Modeling of an Electron-Optical System with Converging Sheet Beam for a Traveling Wave Tube of Terahertz Frequency Range

A converging sheet electron beam with a cross section of 0.05 × 2 mm and current density of 200 A/cm², which is formed by an electron gun, is modeled using the synthesis and analysis methods at the condition of magnetic shielding of the cathode. The deformation in the cross section of the beam in the focusing magnetic field is analyzed based on a computer three-dimensional model of an electron optical system with a sheet electron beam. The current-voltage characteristic of an electron gun is studied experimentally in the pulse mode. A collector current of 200 mA is obtained with the beam thickness being 70 μm.     

Thermophysical Parameters of Optical Glass BK 7 Measured by the Pulse Transient Method

This paper is focused on the pulse transient method. The theory of the method and the measuring regime (time window) are analyzed. The results of the analysis are verified on borosilicate crown glass BK7, which is a candidate for a standard for thermal conductivity. Thermal contact and surface effects affect the length of the time window in which the evaluation procedure is applied. The one-point evaluation technique is compared with the results of the fitting procedure that uses the time window found by difference analysis. The values of the thermal conductivity, thermal diffusivity, and specific heat were found to be 1.05 W· m^–1 · K^–1, 0.548 × 10^–6m ^{– 2} · s^–1, and 767 J· kg^–1 · K^–1, respectively, using the one-point evaluation technique.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom.     

Failure of silicon carbide with local thermal loading

A study is made of the failure of disks measuring Ø22 × 4 mm made of sublimated (SiC_sub) and reaction-sintered (SiC_r.s.) silicon carbide with heating of their surface by an electron beam with diameter 3, 4, and 7 mm. Depending on time (1·10^–3–10 sec) and the level of loading (up to 10 kW/cm²) the different nature of specimen failure is established: from surface damage to separation into parts. Surface damage of SiC_sub was characterized by local development of plastic deformation, cracks, and chips due to the effect of compressive stresses, but for SiC_r.s.. it was characterized by erosion caused apparently by sublimation of free silicon.Translated from Problemy Prochnosti, No. 8, pp. 67–71, August, 1990.     

Phase and Structural Modifications in Porous Silicon Under Pulse Heating

The phase transitions in crystalline and amorphous porous silicon layers on silicon single crystal under isothermal or laser pulse nanosecond heating were modeled. The pulse heating was described as an adiabatic process by using a quasi-statistical approximation through homogeneous nucleation and growth of a new phase. The calculation of the free energy of porous silicon for cylindrical, spherical, and complex structures of the pores and its dependence on the pore radius, overall porosity, and thermoelastic stresses was made. The equilibrium free energy increased to 0.15 and 0.09 eV, with a corresponding decrease in melting temperature of 400 and 300 K for crystalline and amorphous porous silicon, respectively. The Laplace pressure retards this shift no more than 10 K. The possibility of epitaxial silicon layer formation (0.1 to 1.2 m thick) on porous silicon after pulse heating (30 ns; beam density from 2 to 10 kJ·m^–2) is shown.     

Effect of stress concentration on the corrosion-fatigue strength of iron and steel

The effect of stress raisers (0.6 mm diameter holes) on the corrosion-fatigue strength of armco iron and steel 50 plate in 0.01 N and 3% NaCl solutions was studied by tests on a base of 0.5·10⁶ cycles. A sharp reduction in the fatigue strength due to stress concentration was observed in the case of specimens tested in air, the effect of stress raisers in the case of specimens tested in corrosive media being less pronounced.     

Thermoelastic stresses arising in laser media

The article examines the thermal model of a laser diode with a heat source of finite thickness and presents the calculation of thermoelastic stress waves arising at the time of the current pulse. It is shown that the decay of the thermoelastic tensile wave in the active region does not exceed 1 nsec; beyond the limits of the active region a compressive wave originates, and the time of its passage is equal to the time of passage of the wave through the active region. The amplitude of the wave attains 4.1·10⁴ Pa with a plane heat source, and 1750 Pa when the active region is 10^–7 m thick.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 5, pp. 858–862, November, 1990.     

Effect of distance from the support on the penetration mechanism of clamped circular polycarbonate armor plates

A 1.91-mm thick circular polycarbonate plate of 115 mm diameter was impacted by a spherical steel projectile of 6.98 mm diameter at its center. Subsequent impacts were made at 10, 20, 30, 40, and 50 mm radii of the plate. Dent dimensions for the damaged plate were measured using optical microscope. For a constant projectile velocity of 138 m s⁻¹ which was below the perforation limit of the plate under investigation, a maximum thickness reduction close to the edge support was observed. The experimental work was modeled into explicit finite-element analysis program LSDYNA for simulations. LSDYNA was able to predict the dent depth and reduction in plate thickness at impact points precisely. In this research, the effect of the impact location distance from the supports on the damage mechanism of circular polycarbonate armor plates is investigated. The target plate was subjected to constant velocity projectile impacts starting at the plate midpoint and varying the impact distance from midpoint towards the clamped edge. Failure of plate is predicted close to the constrained boundary under uniform conditions.     

Improvement of semiconductor detector characteristics during supercurrent proton irradiation

Volt-ampère characteristics of Si p-type detectors are established to improve after pulse supercurrent irradiation with low energy protons. Appreciable improvement of energy resolution is observed for detectors irradiatied with the supercurrent proton beam within a dose interval from 10¹¹ to 5×10¹² proton cm⁻². Radiational resource of operation increases from 2.3 × 10¹⁰ to 3.7×10 ¹¹ proton cm⁻² for detectors which were pre-irradiated by the supercurrent proton beam.     

Characteristics of external heat transfer in a fluidization bed of coarse particles

Results are shown of an experimental study concerning the heat transfer in a fluidization bed (2–13 mm) of particles, with a cylindrical thermoprobe used for measurements. A predominance of the convective component in the heat-transfer coefficient has been confirmed in the case of coarse particles. Acriterial formula is proposed for calculating the coefficient of external heat transfer in a fluidization bed with the Archimedes number Ar=1.4·10⁵–3.0·10⁸.Translated from Inzhenemo-Fizicheskii Zhurnal, Vol. 24, No. 4, pp. 589–593, April, 1973.     

Study of the velocity distribution in an intense pulsed hydrogen beam

We present the results of measurements of the velocity distributions of particles in a pulsed hydrogen beam obtained from a dissociator with a radio frequency discharge (duration 1.0 ms, repetition rate 1 Hz). It is shown that the hydrogen inside the dissociator is heated up to ～2800 K, so the thermal dissociation of hydrogen molecules is essential. In order to cool the atoms, the gas was let through a pyrex channel 5 mm in diameter. The cooling channel walls being at room temperature and the channel having a length of 50 mm, we have obtained a supersonic beam of hydrogen atoms with a Mach number M_| = 2.7±0.25. When the channel walls were cooled by the flowing liquid nitrogen and the channel was 70 mm long we obtained a beam of cooled atoms with a Mach number M_| = 4.14±0.35. The velocity distribution of atoms depends on the power of the rf discharge inside the dissociator and on the gas consumption per pulse, and varies during the discharge pulse. For a temperature of the cooling channel walls T_ch = 77 K, a gas consumption N = 3.3×10¹⁷ molecules per pulse and a discharge power of 0.23 kW cm^?3, we have obtained an atomic beam with intensity I(0) = (2.8±0.8)×10²⁰ atoms sr^?1 s^?1 and a most probable velocity ν_MP = (1.97±0.07)×10⁵ cm s^?1.     

Integrated laser based pre-tempering at laser welding of AISI 1045 steel by using 3D-scanner optics

Laser-based pre-heating of laser beam welding with a 3D scanning optics, applied to AISI 1045 steel, is studied. Laser beam welding of heat-treatable steel is challenging due to martensitic hardening in combination with defects. Pre-tempering aims the reduction of the cooling rates and martensitic microstructure within the weld seam. An oscillating defocused laser beam was guided over the surface for pre-heating by means of a 3D scanner optics. During pre-heating, the laser power, the scanning speed and the number of cycles were varied. Welding with 4000 W and 2 m/min with a focused laser beam was executed. Thus the resulting temperature profile behind the ongoing laser beam and cooling time T_8|5 between 800 °C–500 °C was significantly extended. Two parameter combinations (15 cycles|600 W|50 mm/s(²) and 10 cycles|800 W|50 mm/s²) succeeded in a microstructure of bainite and martensite. By extending the cooling time T_8|5 to 3.11 s(²) and 4.17 s². Thus, average hardness for laser based pre-tempering of 487 HV 0.5(²) and 455 HV 0.5² was achieved. As a reference, global pre-heating at 400 °C using a heating plate can reduce the average hardness of the weld zone from 729 HV 0.5 at room temperature to 304 HV 0.5 at a cooling time T_8|5 of 5.63 s.     

Faraday cup for measuring the electron beams of TWT guns

A compact Faraday cup reported in this paper is designed to investigate the current distributions of the electron beams of Traveling Wave Tube’s guns. It consists of a 0.06 mm thick molybdenum aperture plate and a copper shield with a graphite collector inside. There are plates with different laser-cut aperture holes that were 100 μm, 50 μm, 20 μm and 10 μm in diameter for measuring electron beams with different sizes. The thermal analysis of the Faraday cup with pulse beam heating was performed and discussed in this paper. The pulse test shows that this device has fast response and small dissipation. A 0.58 beam perveance (μP) electron gun with expected minimum beam radius 1.0 mm was measured with the Faraday cup and the three-dimensional current density distribution and beam envelope were presented. The experiment results show that the design is reasonable for measuring the electron beam with a high resolution.     

Preparation of large-area high-quality YBCO thin films by pulsed laser deposition with Si heater and composite scanning of laser and target

We report on the successful preparation of large-area high-quality YBa₂Cu₃O₇ superconducting thin films by pulsed excimer laser ablation with a Si heater and composite scanning of laser beam and target. The Si heater, composite scanning of laser beam and target, and experiment results are described. The temperature variation of the Si heater was < ±0.5% in a 45×40 mm² area of 900–1000 °C. Films were deposited on LaAlO₃ substrates 35 mm in diameter. The thin films exhibited a thickness variation of ±2.5%; the superconducting properties wereT _c0=91×0.5 K andJ _c= (3.3±0.7)×10⁶ A/cm² at 77 K and zero magnetic field.     

Phonon Absorption by D − Band Tails in Ge:Sb

Uniaxial stress dependence of acoustical phonon absorption in intermediately doped Ge:Sb has been studied using heat pulse technique. Abruptly decreasing LA and FTA phonon scattering in stress interval of 3–7 · 10 ⁸ dyn/cm ² with further saturation up to 1.9 · 10 ⁹ dyn/cm ² was observed. Results obtained from phonon measurements correlate with conductivity activation energy behavior on stress. Acoustical transparency stress dependence is assumed to be connected with phonon assisted electron transitions from impurity ground state D ⁰ to D^– band.