Spectral measurements of hypervelocity impact flash

We have revisited the well-known phenomena of impact flash by examining the optical spectra generated by hypervelocity impacts at velocities up to several tens of kilometers per second. This particular effort, sponsored by an LDRD from our laboratories, has looked at the flash from impacts over a range of velocities from 6 to 25 km/s, using two different experimental environments. Both two- and three-stage light-gas guns and magnetically driven flyers on the Z accelerator were used. Here, we describe the latter set of experiments. Using standard tabulations of atomic spectral data, we were able to identify strong spectral lines from the principal materials used in the various shots. We demonstrated that the impact-flash spectra were qualitatively reproducible from shot to shot, and have confirmed the feasibility and credibility of using impact-flash spectroscopy for remote sensing applications such as meteoroid-impact or missile-defense engagement analyses.     

Temperature measurements of expansion products from shock compressed materials using high-speed spectroscopy

Results from spectral radiance measurements using optical multi-channel analyzer over the visible and near infrared regime provide estimates of temperature from expansion products resulting from shocked materials. Specifically, we have made spectral radiance measurements over the wavelength regime of 300–1500 nm. Experiments conducted on aluminum, cerium, and Composition-B high explosive span a wide regime of E/E_v, where E is the internal energy increase of the shocked material, and E_v, is the specific energy required to vaporize the material. For the materials investigated, the ratio is ∼1, 3 and 5 for aluminum, cerium, and Composition-B, respectively. The basic assumption made to deduce these temperatures is that the debris cloud is radiating as a blackbody with emissivity of one and independent of the wavelength. We are also assuming that the probe is monitoring the debris, which is at a single temperature and that there is no spatial temperature gradient. Temperatures at or above the boiling point are confirmed for aluminum and cerium, while the results for Composition-B provide the time-dependent temperature expansion history for shocked Composition-B over the stress regime of 28–130 GPa. These are the first measurements of temperature obtained from the expansion products from materials that have been shocked to very high pressures.     

An evaluation of specific heat measurement methods using the laser flash technique

Different methods for adapting the laser flash technique to measure simultaneously specific heat have been proposed in the literature. Among them are the coating method, the absorbing disk method, the double-specimen method, the pulse heating-cooling method, and the cavity method. These methods are briefly reviewed, and their merits and demerits are evaluated.     

Effects of infrared detector nonlinearity on thermal diffusivity measurements using the flash method

When using an infrared detector to measure temperature changes as in the case of the flash technique, the effects of detector nonlinearity can have drastic effects on the experimental data. In the flash technique, the detector nonlinearity tends to shift the calculated half-time to larger values, resulting in underpredicted values of thermal diffusivity especially in experiments performed at room temperature. In order to predict the error in the diffusivity calculation, the nonlinear relationship between the detector signal and the temperature change was developed into a Taylor series expansion used in the flash technique's mathematical model. The nonlinear detector model proves to yield accurate correction factors for the presently calculated values of diffusivity. In order to utilize the model, it is necessary to estimate the maximum temperature rise of the back surface and the degree of detector nonlinearity.     

Accurate determination of specific heat at high temperatures using the flash diffusivity method

The flash diffusivity method can be extended, very simply, to measuring simultaneously thermal diffusivity and specific heat and thus obtaining the thermal conductivity directly. This was accomplished by determining the amount of heat absorbed by a sample with a well-known specific heat and then using this to determine the specific heat of any other sample. The key to using this technique was to have identically reproducible surfaces on the standard and the unknowns. This was achieved earlier by sputtering the surfaces of the samples with a thin layer of graphite. However, the accuracy in determining the specific heat was within ±10% and there was considerable scatter in the data. Several improvements in the technique have been made which have improved the accuracy to ±3% and increased the precision. The most important of these changes has been the introduction of a method enabling the samples to be placed in exactly the same position in front of the light source. Also, the control of the thickness and the application of the graphite coating have turned out to be very important. A comparison of specific heats obtained with this improved technique and with results obtained using other techniques has been made for two materials.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

闪光对焊技术研究现状及发展趋势

详细介绍了闪光对焊工艺和设备的研究及其应用现状,探讨了闪光对焊的机理,并分析了闪光焊接过程产生的主要缺陷,讨论了其预防措施.最后着重介绍了闪光对焊技术的发展趋势.     

Residual temperature measurements of light flash under hypervelocity impact

Experimental and theoretical results for light flash temperatures are presented for impacts on soda-lime glass by iron projectiles. The experiments were performed with a 2 MV Van de Graaff, where iron dust particles (0.14–0.63 μm in diameter) impacted soda-lime glass at a velocity range of 5–20 km s⁻¹. Theoretical calculations were based on the assumption of the Mie–Gruneisen equation of state (EoS) with different values for the Gruneisen coefficient and hydrodynamic behaviour (no strength effects were considered). Within the scatter of experimental data, results suggest a constant value for the average light flash temperature of approximately 2600 K independent of iron dust impact velocity. Although theoretical calculations are limited by the use of the Mie–Gruneisen EoS up to the point of incipient vaporisation of the target material, relatively good agreement with experiments is observed. This agreement suggests that the observed constant temperature may be due to material phase change from incipient to complete vaporisation over the range of velocities considered.     

Characterization of polarization phenomenon in Al-Schottky CdTe detectors using a spectroscopic analysis method

CdTe radiation detectors equipped with Schottky contacts are known to show spectral response degradation over time under biasing. Nevertheless, they can be used as high-resolution spectrometers for X-rays and gamma-rays with moderate cooling and high voltage. Spectroscopic long-term measurements have been performed with Al/CdTe/Pt pixel detectors of 0.5, 1 and 2 mm thicknesses and ²⁴¹Am source from −13 to +16 °C to evaluate how long they can be operated. Experimental results are confronted to simulations using the charge accumulation model for electric field. Activation energy for collection efficiency stability and peak shift was measured at 1.0-1.2 eV although deep acceptor levels responsible for hole detrapping during polarization were evaluated by other methods at E_V +0.6-0.8 eV. The difference is probably due to a thermal effect of pre-polarization before biasing the detector.     

The effects of a finite pulse time in the flash thermal diffusivity method

After a brief review of the finite pulse time effects in flash thermal diffusivity measurements, an analytical expression for an exponential shape pulse was determined using the Green function method. The results were compared with those obtained by Larson and Koyama. It was found that, using the Larson and Koyama equation, when the dimensionless time is equal to zero, the dimensionless temperature rise V cannot reach zero, and when _p, the time characterizing the dimensionless pulse, approaches 1/n ² (n=1,2,3,...), a large error of _1/2 will result. These contradictions have been resolved by the present work. In other respects, both sets of results concurred. The results are compared with the triangular pulse and are discussed.     

钽条闪光对焊工艺参数对其组织性能的影响

为解决连续化生产中钽条焊接的技术难题,在惰性气体的保护下采用电容储能式预热闪光对焊,对轧制后的4 mm×4 mm钽条进行焊接试验研究.利用金相显微镜、扫描电镜和力学性能检测设备等分析检测手段,对不同焊接顶锻力作用下的焊接区(包括热影响区)的显微组织、晶粒和晶界微区成分、硬度以及焊接区拉伸强度等进行了测试.研究表明,采用合适的闪光对焊工艺可以实现大尺寸钽条的对接,焊缝组织致密,接头具有较高的结合强度,对于大尺寸钽条无头轧制与连续拉拔技术的发展具有一定的使用价值.     

增加液氧过冷度以降低液氧闪蒸损失

由于1450 m3/h液体空分设备液氧产品的过冷度不够大,液氧降压进入贮槽的过程中存在闪蒸损失。增设外置式液氧过冷器,增加液氧过冷度,降低了液氧损耗。简介1450 m3/h全液体空分设备的流程,分析外置式液氧过冷器的流程方案、方案的具体实施及其效果。     

Effects of carbon nanotubes additions on flash ignition characteristics of Fe and Al nanoparticles

The influences of carbon nanotubes (CNTs) additions on the flash ignition characteristics of Iron (Fe) and aluminum (Al) nanoparticles (NPs) were presented. CNTs can be used as the additive to these metal nanoparticles for improving the flash ignition and burning processes. Different mass fractions of CNTs additions were considered. The mixture of Al and CNTs could combust in air with obvious giant flame, whereas the mixture of Fe and CNTs combusted under a relative stable condition with slight red light. The temperature distributions were measured using non-contact optical method and showed that Al NPs mixed with CNTs were burning at a higher temperature level than Fe NPs. Although different mass fractions of CNTs cannot significantly change the overall flash ignition phenomenon, CNTs additions influenced the minimum ignition energy (MIE) of mixtures. The appropriate content of CNTs addition can decrease the Fe NPs MIE significantly. However, the Al NPs MIE decreased all along with the increase of CNTs content. The micro- and nano- structures of Fe and Al NPs with CNTs additions before and after ignitions were examined by scanning electron microscope and high-resolution transmission electron microscopy. It was found that the special thermal conductive characteristics of CNTs and the cross-connected features for metal particles with CNTs caused the enhancement of flash ignition.     

Characterization of Platinum and Gold ions emitted from laser produced plasmas using solid state nuclear track detectors

Investigations on key characteristics of ions emitted from Laser Induced plasmas are reported. A Q-switched Nd: YAG pulsed laser at 1064 nm, 9-14 ns and 1.1 MW, is focused on 4 N pure Gold (Au) and Platinum (Pt) targets to produce plasma plume. The ions thus emitted are detected by CR-39 Solid State Nuclear Track Detectors (SSNTD’s), placed at angles 0°, 30°, 60°, and 90° with respect to the target normal. The distances between the targets and detectors are 9 cm. The experiments are performed under a vacuum ∼10⁻³ torr and the ion tracks are recorded on SSNTD’s for 300 laser shots. The etched detectors are analyzed with the help of Motic Images Optical Microscope (Motic Advance 3.1). The flux of ions exhibits anisotropic behavior where as maximum energies of these ions are not only anisotropic but also forward peaked.     

Characteristics of Schottky barrier silicon nanocluster floating gate flash memory

The silicon nanocluster floating gate memory device based on the Schottky barrier metal-oxide-semiconductor field effect transistor (SB-MOSFET) was proposed. The silicon nanoclusters were formed via the digital gas-feeding low pressure chemical vapor deposition. Erbium silicide process was used to form the Schottky junctions at the source/drain. In addition to the SB-MOSFET operation, the program/erase times of the nonvolatile memory device were determined to be 10 ms and 100 ms under the + 18 and − 18 V gate bias conditions, respectively. Maximum memory window was 5.5 V and the charge retention characteristics were maintained with a memory window of 0.5 V at 10⁶ s.     

汽车盘体零件小飞边模锻成形工艺开发及数值分析

汽车盘体零件通常采用机加工成形或者两步锻造成形(预锻+终锻),而机加工成形的效率和材料使用率较低、两步锻造成形的设备及模具成本较高,因此对该盘体零件的一步锻造成形的可行性进行了研究。根据汽车盘体零件的结构特征,设计了3种不同形式的飞边槽结构,并通过有限元分析软件Deform-3D分别进行了建模及数值分析,对比了不同飞边槽结构对汽车盘体零件填充质量、锻造成形力及锻模磨损的影响。最终得出了采用沟式飞边槽结构更优的结论,同时从等效应力分布规律和温度分布规律两方面分析了该结构下锻后零件的质量。结果表明:采用沟式飞边槽结构能得到填充完整、质量好、无瑕疵的汽车盘体零件,并通过生产实践进行了验证,提高生产率的同时有效降低了材料成本、模具成本、设备成本。     

A high-temperature system based on the laser flash method to measure the thermal diffusivity of melts

A high-temperature measuring system has been developed to undertake measurements of thennal difusivity and specific heat up to 1900 K. The overall design allows measurements on solids to be undertaken using the accepted standard techniques and analytical procedures. The specific design for molten materials and especially slags is based on the differential threelayer technique utilizing a special cell which can be accomodated in the system. In this method, the liquid specimen is sandwiched between an upper inner platinum crucible and a lower outer platinum crucible, to provide a three-layered sandwich. A laser pulse irradiates the surface of the upper platinum crucible and the temperature response of the surface of the lower platinum crucible is observed. For the purpose of accurate measurement of specimen thickness at the measuring temperature, two runs are performed in which the thicknesses arel andl+°Dl, wherel is unknown butl can be set accurately with a built-in micrometer. The thermal difusivity is obtained through a curve-fitting method by a personal computer using a three-layer analysis with a correction for the radiative component based on the transparent body assumption. Following verification of the basic performance, using solids of known properties and water and ethanol, a continuous casting mixture has been evaluated. The initial results on the fluids are in good agreement with those in the literature.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Impact toughness of Al–Si–Cu–Mg–Fe cast alloys: Effects of minor additives and aging conditions

The effects of Sr modification and aging treatment on the impact toughness of a near eutectic Al–11%Si–2.7%Cu–0.3%Mg–0.45%Fe alloy were investigated. Charpy impact tests were performed on unnotched specimens in the as-cast and heat-treated conditions. It was found that the presence of Fe- and Cu-containing phases increases the alloy brittleness which reduces impact toughness. The eutectic Si phase also plays an important role, where the size/morphology of the Si particles controls the area of α-Al matrix available which affects ductility and toughness. Increasing the Mn content leads to an increase in the volume fraction of the α-Al₁₅(Mn,Fe)₃Si₂ phase formed and to sludge formation, which facilitates crack initiation and propagation. Crack propagation occurs mainly via the Al₂Cu and/or α-Al₁₅(Fe,Mn)₃Si₂ phases. In the non-modified alloys, the Si phase also plays a considerable role in the fracture process. The impact behaviour of aged alloys is influenced by the amount, size and morphology of hardening precipitates formed in the alloy, depending on the aging conditions. Aging at 240 °C produces a significant increase in the impact energy values of the low Mn-content alloys, as a result of alloy softening. The high Mn-content alloys also show a similar increase in impact energy values, but at a steady level across the same range of aging times, due to the persistence of the α-Al₁₅(Mn,Fe)₃Si₂ phase.     

Characterization of bismuth tri-iodide single crystals for wide band-gap semiconductor radiation detectors

Bismuth tri-iodide is a wide band-gap semiconductor material that may be able to operate as a radiation detector without any cooling mechanism. This material has a higher effective atomic number than germanium and CdZnTe, and thus should have a higher gamma-ray detection efficiency, particularly for moderate and high energy gamma-rays. Unfortunately, not much is known about bismuth tri-iodide, and the general properties of the material need to be investigated. Bismuth tri-iodide does not suffer from some of the material issues, such as a solid state phase transition and dissociation in air, that mercuric iodide (another high-Z, wide band-gap semiconductor) does. Thus, bismuth tri-iodide is both easier to grow and handle than mercuric iodide. A modified vertical Bridgman growth technique is being used to grow large, single bismuth tri-iodide crystals. Zone refining is being performed to purify the starting material and increase the resistivity of the crystals. The single crystals being grown are typically several hundred mm³. The larger crystals grown are approximately 2 cm³. Initial detectors are being fabricated using both gold and palladium electrodes and palladium wire. The electron mobility measured using an alpha source was determined to be 260±50 cm²/Vs. An alpha spectrum was recorded with one of the devices; however the detector appears to suffer from polarization.     

Design and manufacture of an impact–electric current discharge joining machine

An impact–electric current discharge joining machine, which can simultaneously apply impact load and electric current, has been designed and manufactured to produce a high-strength joint between materials with little change in their appearance. The objective functions of the designed and manufactured impact–electric discharge machine are almost satisfied. Partial joining is achieved between FCD450/FCD450 samples and between FCD450/Al samples and the joints exhibit low fracture stress. Process parameters can be controlled to improve joint strength, by adjusting the setting values of the functions provided in the machine.     

Correlation between microstructures and mechanical properties of high-speed friction stir welded aluminum hollow extrusions subjected to axial forces

The AA6005A-T6 aluminum hollow extrusions were friction stir welded at a high welding speed of 2000 mm/min and various axial forces. The results show that the nugget zone (NZ) is characterized by fine equiaxed grains, in which a low density of equilibrium phase β is observed. The grains in the thermo-mechanically affected zone (TMAZ) are elongated, and the highest density of dislocations and a low density of β' precipitates can be found in grains. The heat affected zone (HAZ) only experiences a low thermal cycle, and a high density of β” precipitates and a low density of β' precipitates remain in the coarsened grains. The microhardness evolutions in the NZ, TMAZ and HAZ are governed by the grain refinement and dislocation strengthening, the dislocation and precipitation strengthening, and the precipitation and solid solution strengthening, respectively. When increasing the axial force, the changing trend of one strengthening mechanism is contrary to the other in each zone, and the microhardness increases in different zones. As a result, the tensile strength roughly increases with raising the axial force, and all joints show good tensile properties as the high welding speed inhibits the coarsening and dissolution of strengthening precipitates significantly.