Front Surface Of Sample Research Articles

Considerations on measurement of bidirectional transmittance distribution function of thick samples over a wide range of viewing zenith angles

Abstract We delve into theoretical and experimental considerations for determining the spectral bidirectional transmittance distribution function (BTDF) of thick samples across a broad viewing zenith angle range. Nominally, BTDF is defined as the ratio of transmitted radiance to incident irradiance measured from the same plane. However, when employing thick samples for BTDF measurements, the viewing plane of the transmitted beam may shift from the front to the rear surface of the sample, altering the measurement geometry compared to using the sample front surface as the reference plane. Consequently, the viewing zenith angle from the sample rear surface increases relative to the sample front surface, and the sample-to-detector-aperture distance decreases by an amount corresponding to the sample thickness. We introduce a method for determining the BTDF of thick samples, considering the transformation of practical measurement results to a scenario where the measurements are conducted at a very large distance from the sample. To validate the method, we utilize a BTDF facility equipped with two instruments that significantly differ in their sample-to-detector-aperture distances. We evaluate the impact of a 2 mm sample thickness on the BTDF by assessing the ratio of transmitted and incident radiant fluxes as a function of viewing zenith angle relative to the sample rear surface. The evaluation is conducted in the wavelength range from 550 nm to 1450 nm in 300 nm steps, and in the viewing zenith angle range from −70° to 70° in 5° steps. Measurements are performed in-plane at an incident zenith angle of 0°. It is concluded that consistent determination of BTDF of a thick sample is possible by converting the experimental parameters of the real measurements at relatively short distances from the sample to correspond to those that would be obtained from measurements at very large distances from the sample.

A multispectral radiometry method for measuring the normal spectral emissivity and temperature

A multispectral radiometry method was developed to simultaneously determine normal spectral emissivity of continuum spectra and temperature of materials. Linear dependence assumption would be a constraint to associate the spectral emissivity to the temperature when spectral radiance for various spectrum was measured within small temperature differences. The methodology was evaluated for different parameter constraints, and validated by seven emissivity hypothesis models. To verify the reliability of the experimental device and the multispectral radiometry method, the spectral emissivities of graphite at high temperatures were verified that the deviations between the official experimental data and the results in this work are all less than 5 %. Besides, the spectral emissivity and temperature of the graphite-carbon fiber composite in the spectral range of 0.3–2.5 μm at 1473–1873 K were measured. The temperature uniformity of the sample front surface was evaluated, and the relative error of the temperature distribution is less than 2 %. The uncertainties of the spectral emissivity and temperature in the studied spectrum are less than5.0 % and 6.0 %, respectively. The method is useful for accurately measuring the sample spectral emissivity without direct temperature measurements at high temperatures.

Generation and Control of Terahertz Spin Currents in Topology‐Induced 2D Ferromagnetic Fe3GeTe2|Bi2Te3 Heterostructures (Adv. Mater. 9/2022)

Topological Insulators In article number 2106172, Tianxiao Nie, Jimin Zhao, Xiaojun Wu, and co-workers report the successful realization of room-temperature 2D ferromagnetism in topological-insulator-induced van der Waals Fe3GeTe2, evidenced by ultrafast terahertz emission spectroscopy results. Furthermore, the direction of the external magnetic field is rotated and the sample's front surface reversed to probe the terahertz radiation symmetry.

Thermal damage and ablation behavior of plasma sprayed Ba2SmTaO6 coatings irradiated by high power continuous laser

Ba2SmTaO6 laser protection coatings of ≈200 ​μm thickness were deposited onto stainless steel surfaces by air plasma spraying, and the laser irradiation resistance of the coatings was investigated. For laser irradiation with a laser power density less than 1000 ​W/cm2, the coatings kept intact. For a laser power density exceeding 1500 ​W/cm2, the Ba2SmTaO6 coatings underwent recrystallization, grain growth occurred, and certain spray morphology features disappeared by melting. In the case of a laser power density of 2000 ​W/cm2 applied for 10s, the incident laser parameter was beyond the coatings protection threshold, and the coating peeled off. The samples back surface temperature kept unchanged within the first 1s of laser irradiation, indicating that Ba2SmTaO6 coatings have excellent laser protection capability and can limit the rise of the substrate temperature. However, the low thermal conductivity of Ba2SmTaO6 leads to a detrimental laser energy concentration at the beginning of the laser irradiation period on the sample front surface, resulting in a rapid increase of the surface temperature up to the melting point.

Исследование влагопоглощения авиационных углепластиков в условиях теплого влажного климата

Long-term moisture absorption of aeronautical carbon fiber reinforced plastic based on epoxide in conditions of warm humid climate is studied (city of Sochi). The phenomenon of cyclical stochastic fluctuation of moisture absorption of seasonal nature in samples, placed in open atmospheric stand of the climate platform, is established. Thermal-moisture characteristics of the air layer near the sample surface during insolation are examined. They determine the front surface overheating of the sample and the humidity gradient change of the near-surface air layer relative to meteorological data. The layer size, which is 6-8 mm from the sample surface, is found by direct measurements. It depends not only on the intensity of the insolation, but also on the convection and wind heat exchange with the surrounding atmosphere. During summer insolation, the maximum temperature of the sample front surface (Т) in open atmospheric stand reached 54,0oC, and the relative humidity (RH) in the near-surface layer did not exceed 12, 9 %. Regarding the atmospheric meteorological parameters, this indicates an overheating of the surface by 24,7o C and a decrease in the near-surface RH by 46, 1%. The obtained data exceed daily fluctuations of atmospheric parameters for T by 4 times and for RH by 3 times. The importance of the obtained results is determined by the fact that cyclic changes in the sample surface temperature result in aging of aeronautical carbon fiber- reinforced plastic. They contribute to the development of physical and chemical processes in the material and during the operation of products (25-30 years) they can significantly reduce their strength properties.

Filamentary damage of fused silica irradiated by a 532 nm nanosecond laser

In this study, we use a time-resolved pump-probe shadowgraph technique to investigate the evolution characteristics of filamentary damage in bulk fused silica induced by a nanosecond pulse at 532 nm. The pump laser focuses on the front surface of sample and filamentary damage appears independently in the middle of sample. The whole damage process can be divided into single filament (SF), double filaments (DFs), and long filament (LF) successively. At the same time, the improved moving focus model is proposed by taking into account the temporal shape of the laser pulse and the laser is blocked and reflected by plasma at the critical density. It is in good agreement with the experimental result of filamentary damage process and helps to explore the mechanism of laser-induced filamentary damage in nanosecond regime.

Continuous laser illumination for in situ investigation of tungsten erosion under transient thermal loads

Two types of in situ optical diagnostics of erosion processes on tungsten surface caused by transient heat loads were developed and applied on the BETA (Beam of Electrons for materials Test Applications) facility at Budker Institute of Nuclear Physics. Tungsten plates were exposed to an electron beam with a duration of up to 350 μs and with an absorbed heat flux factor (HFF) of 20-50 MJ/m2s0.5, which is below the melting threshold. The distribution of thermal radiation on the sample front surface was imaged by a CCD camera. In addition, the illumination of the target surface with a continuous wave laser was used to record various types of material modification: roughening, cracking, bending, and local melting. The tile bending is associated with thermal expansion and irreversible plastic deformation of the heated thin layer. The lift of edges of the cracks during their formation is detected by recording the intensity of reflected and scattered radiation, and the resulting structure of the cracks is visualized with a CCD camera. The latter system can also identify the appearance of molten grains on the irradiated surface due to their low thermal bond to the material bulk. The described optical systems are able to detect first wall damaging during a plasma discharge in devices with magnetic confinement.

Analysis of nanosecond and femtosecond laser ablation of rear dielectrics of silicon wafer solar cells

Laser ablation of rear dielectrics of PERC (Passivated Emitter and Rear Cell) type silicon solar cells is by far the most cost-effective method to open a variety of patterns for local contact formation. In this paper, rear dielectric ablation of p-type PERC solar cells with an implanted phosphorus emitter is investigated using two laser sources with green wavelength and with pulse durations in the nanosecond (ns) and femtosecond (fs) range. The ns laser source emits 38 ns pulses and the ablation behaviour is linear for the entire range of available pulse fluences. The fs laser source emits 480 fs pulses and has two operating regimes, gentle and strong, based on the incoming pulse fluence. To evaluate the impact of laser induced damages on PERC solar cells, four different laser conditions are considered: fs gentle regime, fs strong regime, ns low fluence and ns high fluence. The solar cells’ one-sun I–V parameters show that the cells processed in the fs strong regime have a severe reduction in the open-circuit voltage by almost 40 mV, fill factor loss by ~7% and efficiency loss by 3–4%. Fill factor loss analysis shows that a loss of almost 7% absolute comes entirely from J02 recombination, revealing a possibility of severe damage to the front side emitter. The solar cells processed with the other three laser conditions have comparable I–V characteristics. SEM analysis of the front surface of samples with laser treated rear surface shows that the fs strong regime produces severe damage to the front-surface texture resulting in lower Jsc, whereas this effect is not observed for the other laser conditions.

Helium ion irradiation-induced microstructure evolution on the surfaces of thin nickel foils

Abstract Polycrystalline nickel foil (sample I) with thickness of ∼950 nm was irradiated with 0.5–1.2 MeV helium ions at room temperature. Another piece (sample II) with same thickness was mounted behind to receive the irradiation of transmitted helium ions (∼0.026–0.537 MeV). Morphology evolutions on irradiated surfaces were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results show that the open cracks, which were located mainly at grain boundaries, occurred on the surfaces of both irradiated samples. Interestingly, ripple patterns were observed to be regularly arranged on the front surface of sample I. The compressive stress resulting in the sliding on close-packed (1 1 1) planes was regarded as the origin of the ripple formation. Moreover, protrusion islands and its surrounding microstructures were observed on the front surface of sample II. The mass transport driven by the lateral stress generated in the helium ion irradiation were discussed as possible reasons.

Emissivity Model of Aluminum 7075 During the Growth of Oxide Layer Over the Temperature Range from 800 to 910 K at a Wavelength of 1.5 μm

The emissivity models of aluminum 7075 were studied during the growth of oxide layer over the temperature range from 800 to 910 K at a wavelength of 1.5 μm. In the experiment, the samples were heated to a certain temperature and kept at that temperature for approximately 6 h. To determine the accurate emissivity, the surface temperature of specimens was measured by the two thermocouples, which were symmetrically welded onto the front surface of samples. The average of their readings was regarded as the true temperature. Eleven models were employed to evaluate the variation of emissivity with temperature or growth of oxide film on the specimen surface at a certain condition. The effect of the number of parameters used in the models on the fitting quality was studied at a certain temperature. The conclusion was obtained as more the number of parameters used in the models, the better the fitting quality became on the whole. The variation of emissivity with temperature was investigated at a certain thickness of oxide layer. The three approximate models of variation in emissivity with temperature and thickness of oxide film were proposed. The strong oscillations of emissivity were observed during the initial heating period, which were affirmed to arise from the interference effect between the two radiations stemming from the oxide layer and coming from the substrate.

Sample width and thickness effects on upward flame spread over PMMA surface

Upward flame spread has the same propagating direction with air flow and buoyancy, and features as the most hazardous fire case in all flame spread configurations. It has been a long time for fire researchers to find a simple and effective method to evaluate upward flame spread behaviors, especially for different materials and sample sizes. The aim of this work was motivated by the research of sample width and thickness effects on upward flame spread behavior, including flame spread rate during acceleration propagation for different sample thickness and width, theoretical global mass loss prediction based on Emmons’s hypothesis, and dimensionless flame height scaling with dimensionless heat release rate for steady stage burning. Four kinds of sample thicknesses were selected, including 1.7, 3.5, 5, and 7mm. For each kind of thickness, six sample widths ranging from 40 to 90mm were prepared. To eliminate the side flame spreading effects, one set of contrast experiments with sample sides sealed was also performed, by which way flame could only spread along sample front surface and flame propagation was inhibited along both sides. Based on Emmons’s hypothesis, a method for calculation of global mass loss rate was developed. Theoretical global mass loss rate over pyrolysis surface of upward flame spread configurations was calculated and could fit the experimental data well. Finally, a dimensionless heat-release rate for wall flames of different sample sizes was used to scale the dimensionless flame height with a power-law exponent 0.58. The results of this study have implications concerning designs for high-rise building fire safety problems and can help to get better understandings of upward flame spread mechanism from aspects of heat and mass transfer.

Detection and Quantification of Defects in Composite Material by Using Thermal Wave Method

This paper explored the results of experimental investigation on carbon fiber reinforced polymer (CFRP) composite sample with thermal wave technique. The thermal wave technique combines the advantages of both conventional thermal wave measurement and thermography using a commercial Infrared camera. The sample comprises the artificial inclusions of foreign material to simulate defects of different shape and size at different depths. Lock-in thermography is employed for the detection of defects. The temperature field of the front surface of sample was observed and analysed at several excitation frequencies ranging from 0.562 ㎐ down to 0.032 ㎐. Four-point methodology was applied to extract the amplitude and phase of thermal wave’s harmonic component. The phase images are analyzed to find qualitative and quantitative information about the defects.

Fracture of metals samples under conditions of fast heating by intensive X-ray radiation

Results on studying the fracture of metals samples in the form of thin disks under fast heating by the X-ray pulse with the complete spectrum are presented in the paper. The samples of such metals as iron, copper, AMg6 aluminum, VT14 titanium, molybdenum, tungsten, cadmium, lead and zinc were tested. The samples were fixed in the special cartridges equipped with the gauges of a mechanical recoil momentum. The cartridges with samples were placed at such distances from the X-ray irradiator where the energy fluxes were 1.38, 0.90 and 0.29kJ/cm2. The irradiating X-ray pulse was about 2 ns in duration. After testing, the depth of material ablation from a sample front surface and the degree and character of its spall damage were determined. The method of metallographic analysis was used for these purposes. Numerical calculations of loading conditions were made with the use of an equation of state taking into account the process of evaporation. The calculated value of maximum negative pressure in the sample at the coordinate corresponding to the formation of spallation zones or spall cracks was conventionally accepted as the material resistance to spall fracture. The comparison of obtained results with the data on the fracture of examined materials in the conditions of fast heating by the X-ray pulse with the hard spectrum and a high-current electron beam was conducted.

Experiment assessment of the ballistic response of composite pyramidal lattice truss structures

Sandwich panels with lattice cores have attracted significant interest as multifunctional structures. The lattices consist of 3D repeating unit cells constructed from plates or trusses oriented to efficiently support applied stresses. These systems show promise for supporting structural loads and mitigating the blast effects of explosions. Here, a preliminary study has been conducted to investigate the ballistic behavior of a model lattice and to explore the effect of filling the lattices void spaces with polymers and ceramics. A sheet folding and brazing method has been used to fabricate pyramidal lattice truss structures from 304 stainless steel. The impact response of the various panels was assessed after impact by spherical, 12 mm diameter, 6.9 g projectiles with an incident, zero obliquity velocity of ∼600 m/s. Empty lattice sandwich panels with an areal density of 27.7 kg m −2 do not prevent the perforation of the sandwich panel. The impact with proximal face sheet reduced the projectile velocity to ∼450 m/s (by about 25%). Interactions with the lattice trusses and the distal face sheet further slowed the projectile resulting in an exit velocity at the distal face sheet of ∼360 m/s. The projectiles energy was dissipated by face sheet plastic dishing and fracture by petaling, and by truss plastic deformation. Infiltration of the lattice with polyurethane elastomers further reduced the projectile exit velocity. The strength of the effect depended upon the modulus of the polymer (and therefore its glass transition temperature, T g). Only high modulus (high T g) elastomers fully arrested the projectile. The energy of the projectile in this case was dissipated by a combination of face sheet stretching and polymer deformation and fracture. Low modulus elastomers reduced the projectile exit velocity by about 45% (to ∼250 m/s) and also resulted in resealing of the projectile path within the sandwich panel core. The incorporation of ballistic fabric within the polymer infiltrated systems had little effect on the ballistic resistance. A hybrid sample containing metal encased Al 2O 3 prism inserts provided the greatest resistance to penetration. In this case the projectiles were arrested within a sphere diameter of the sample front surface. Several of these hybrid systems offer promise as multifunctional, ballistic resistant, load-bearing structures.

Thermal conductivity of hydrogenated amorphous silicon

The thermal conductivity of amorphous silicon thin films is measured in one dimension steady state condition. The experimental method is based on heating the sample front surface and monitoring the temperatures at its two sides. The experiments were carried out in conditions ensuring one-direction heat flow from top to bottom throughout the sample thickness. Sputtered a-Si:H films prepared with different conditions are used in order to investigate the dependence of thermal conductivity on material properties (i.e. hydrogen content and microstructure). The results show that, firstly, amorphous silicon is a very bad thermal conductor material. Secondly, the disorder in the film network plays an important role in thermal conduction. The highly disordered film exhibits the lowest thermal conductivity.

Mechanisms and Quantification of Spalling Failure in Laminated Composites under Shock Loading

The free surface spalling phenomenon in 2D carbon/carbon (C/C) and carbon-polyimide (C/P) laminates was studied by subjecting them to laser-produced nano-second rise-time compressive stress pulses. The compression pulse travels through the laminate normal to the plies and, upon reflection into a tension pulse from the sample's front surface, leads to fiber/matrix debondings either in the bundles within a ply or at the interply interface. The stress pulse amplitude responsible for initiating such a damage was determined by recording the free surface velocity by an optical interferometer, and on the continuum scale, this is referred to as the effective interply strength. Since by employing a special specimen geometry we were able to determine the onset of such delaminations, its characterization in terms of an effective interply strength should be acceptable. Additional information on the attenuation and the dispersion characteristics associated with the propagation of such stress pulses was also obtained for both types of composite micro-structures. Using the interply strength as the local failure criterion, the information on the dispersion characteristics allows for a complete design of a composite component under impact conditions.