Detectors on the Basis of High-Purity Epitaxial GaAs Layers for Spectrometry of X and Gamma Rays

The characteristics of detectors of soft-X and γ rays based on high-purity epitaxial GaAs layers are discussed. The characteristics of detectors with different rectifying contacts are compared, that is, those with a Schottky barrier and a p–n junction. The spectral characteristics of the manufactured detectors that were obtained under the irradiation by ⁵⁷Co and ²⁴¹Am sources at different bias voltages and in a photovoltaic mode and the simulation results using the Geant 3.21 software package are presented.     

A device for testing a fast nanographite photodetector at high temperatures

The results of tests of a fast photodetector based on a nanographite film grown on a silicon substrate are presented. The temperature dependence of the optoelectric signal from the photodetector in the range 300–800 K has been studied under the vacuum conditions. It has been shown experimentally that, when the temperature increases from 300 to 625 K, the photodetector’s sensitivity decreases by approximately 30%. When the temperature increases to higher values, the optoelectric-signal amplitude drops linearly and is halved at T = 740 K as compared to room temperature. Extrapolation of the experimental data shows that the optoelectric response of the photodetector disappears at T > 1000 K.     

Segmented high-purity germanium detectors

Segmented planar high-purity germanium detectors were developed. Segmentation of p-n junction was performed by implantation of boron (for the n-type Ge) and mechanical grooving of the Li diffusion layer (for the p-type Ge). The electric and spectrometric characteristics of each individual segment and interactions between segments were investigated. The reliability and stability of the detectors were improved by using special passivation of the crystal surfaces.     

Spectroscopy of Single AlInAs Quantum Dots

A system of quantum dots based on Al _x In_1?xAs/Al _y Ga_1?yAs solid solutions is investigated. The use of Al _x In_1?xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al _x In_1?xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al _x In_1?xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al _x In_1?xAs quantum dots.     

An a-C/<Emphasis Type="Italic">n</Emphasis>-Si heterostructure as an ionizing radiation detector

The use of a vacuum pulsed-deposited heterostructure—amorphous carbon films on n-type silicon (a-C/n-Si)—as an ionizing radiation detector has been investigated.     

Nucleation of two-dimensional Si islands near a monatomic step on an atomically clean Si(111)-(7×7) surface

The process of nucleation of 2D islands near a monatomic step at the initial stage of growing of a silicon film on the Si(111)-(7 × 7) surface is studied by means of in situ ultrahighvacuum reflection electron microscopy. The dependence of the depletion region width W near the step, where no islands are formed, on the deposition rate R is described by the expression W² ∝ R^-χ with the exponent χ = 1.18 and χ = 0.63 at temperatures of 650 and 680 °C, respectively. It is demonstrated that the change in χ is associated with the step structure, which provides the transformation from the growth kinetics limited by attachment of adatoms to the step to that limited by diffusion of adatoms. A competition of the processes of nucleation and attachment to the step leads to an increase in the critical size of the island nucleus from i = 1 far from the step to i = 3–5 near the step and to i = 6–8 on the terrace of critical width for 2D nucleation.     

Temperature at discrete contacts in slipping friction with lubricant

The temperature at individual contacts of rough surfaces, i.e., the flash point T _fl in slipping friction with lubricant, is determined numerically. It is shown that the appearance of T _fl during thermocycling is a high-frequency wave process. The temperature over the depth of the contacting surfaces is determined, in the form of damping temperature waves. The surface temperature in friction with lubricant may be determined using two parameters: the conventional heating thickness of a uniform body; and the heat-assimilation coefficient.     

Numerical Analysis and Verification of the Gas Jet from Aircraft Engines Impacting a Jet Blast Deflector

The process of the gas jet from aircraft engines impacting a jet blast deflector is not only a complex fluid–solid coupling problem that is not easy to compute, but also a safety issue that seriously interferes with flight deck environment. The computational fluid dynamics (CFD) method is used to simulate numerically the impact effect of gas jet from aircraft engines on a jet blast deflector by using the Reynolds-averaged Navier-Stokes (RANS) equations and turbulence models. First of all, during the pre-processing of numerical computation, a sub-domains hybrid meshing scheme is adopted to reduce mesh number and improve mesh quality. Then, four different turbulence models including shear-stress transport (SST) k-w, standard k-w, standard k-ε and Reynolds stress model (RSM) are used to compare and verify the correctness of numerical methods for gas jet from a single aircraft engine. The predicted values are in good agreement with the experimental data, and the distribution and regularity of shock wave, velocity, pressure and temperature of a single aircraft engine are got. The results show that SST k-w turbulence model is more suitable for the numerical simulation of compressible viscous gas jet with high prediction accuracy. Finally, the impact effect of gas jet from two aircraft engines on a jet blast deflector is analyzed based on the above numerical method, not only the flow parameters of gas jet and the interaction regularity between gas jet and the jet blast deflector are got, but also the thermal shock properties and dynamic impact characteristics of gas jet impacting the jet blast deflector are got. So the dangerous activity area of crew and equipments on the flight deck can be predicted qualitatively and quantitatively. The proposed research explores out a correct numerical method for the fluid–solid interaction during the impact process of supersonic gas jet, which provides an effective technical support for design, thermal ablation and structural damage analysis of a new jet blast deflector.     

An automatic setup for measuring low-field temperature dependencies of magnetoresistances of film samples

A simple method for magnetoresistance measurements in weak fields based on using a low-frequency alternating magnetic field is proposed. The special feature of this method is the direct measurement of the difference in resistance ΔR of the sample under study in the field and in the absence of the field, which offers a chance to significantly increase measurement sensitivity. The automatic setup for measuring temperature dependencies of magnetoresistance of film metallic samples with a ΔR sensitivity of up to 10^?7 Ω is described. The temperature dependencies of magnetoresistance of thin-film nanocrystalline Co-Cu alloy samples in weak magnetic fields and in a temperature range of 77–300 K are studied.     

Experimental investigation of the preferred Strouhal number used in self-resonating pulsed waterjet

Self-resonating pulsed waterjet (SRPW) is superior to plain waterjet in many ways and is being employed in numerous applications. To further improve the performance of SRPW, the optimal value of the preferred Strouhal number (S_d), which is used to determine the chamber length of a self-resonating nozzle, was experimentally studied at inlet pressures of 10 MPa and 20 MPa. The axial pressure oscillation peak and amplitude were used to evaluate the performance of SRPW, in order to find the optimum S_d value. Results show that S_d value determines the self-resonance behavior of an organ-pipe nozzle and greatly affects the intensity of the axial pressure oscillation. Under the experimental conditions, the optimum S_d values are 0.315 and 0.278 respectively, corresponding to inlet pressures of 10 MPa and 20 MPa. Compared with the default value of 0.3 obtained from air jet experiment, the optimum S_d value at inlet pressure of 10 MPa is a little larger and oppositely a bit smaller at inlet pressure of 20 MPa. Thus, if the inlet pressure is not considered, S_d value of 0.3 is reasonable for determining the chamber length of a self-resonating nozzle for generating effective SRPW.     

A Technique for Detecting Subpicosecond Reflection or Transmission Kinetics

A double-modulation technique was developed and tested experimentally for the detection of the kinetics of reflection of subpicosecond pulses of probe near-IR radiation from a sample (or transmission through it) after the action of exciting radiation pulses. A probe signal is detected at a frequency that is equal to the sum of stabilized unequal frequencies of interrupting exciting and probe radiations, thus permitting the elimination of the contribution of scattered exciting radiation from the valid signal. When detecting the semiconductor- sample reflectance kinetics with a time resolution of 0.13 ps, the reflection sensitivity ΔR/R = 5 × 10^–6 was reached.     

Investigation of autoignition of propane and<Emphasis Type="Italic">n</Emphasis>-butane blends using a rapid compression machine

The effects of pressure and temperature on the autoignition of propane andn-butane blends were investigated using a rapid compression machine (RCM), which is widely used to examine the autoignition characteristics. The RCM was designed to be capable of varying the compression ratio between 5 and 20 and minimize the vortex formation on the cylinder wall using a wedge-shaped crevice. The initial temperature and pressure of the compressed gas were varied in range of 720-900 K and 1.6-1.8 MPa, respectively, by adjusting the ratio of the specific heat of the mixture by altering the ratio of the non-reactive components (N₂, Ar) under a constant effective equivalence ratio (ø_f=l.0). The gas temperature after the compression stroke could be obtained from the measured time-pressure record. The results showed a two-stage ignition delay and a Negative Temperature Coefficient (NTC) behavior which were the unique characteristic of the alkane series fuels. As the propane concentration in the blend were increased from 20% and 40% propane, the autoignition delay time increased by approximately 41% and 55% at 750 K. Numerical reduced kinetic modeling was performed using the Shell model, which introduced some important chemical ideas, represented by the generic species. Several rate coefficients were calibrated based on the experimental results to establish an autoignition model of the propane andn-butane blends. These coefficients can be used to predict the autoignition characteristics in LPG fueled SI engines.     

Effect of Oxygen on the Self-formation of Carbonaceous Tribo-layer with Carbon Nitride Coatings under a Nitrogen Atmosphere

The ball-on-disk friction and wear tests of CN _X coatings (CN _X /CN _X ) were conducted under a nitrogen atmosphere with controlled relative humidity (RH) (3.4–40.0%RH) and oxygen concentration (100–21 × 10⁴ ppm) in this study. We found that the specific wear rate of CN _X coating on ball (W _b), which could give stable and low friction coefficient (<0.05), was below 3.0 × 10^?8 mm³/Nm. Average friction coefficients (µ _a) and W _b of CN _X /CN _X increased (µ _a: 0.02–0.33, W _b: 1.6 × 10^?8–2.4 × 10^?7 mm³/Nm) with increasing oxygen concentration (230–211,000 ppm) as well as RH (4.7–21.1%RH) under a nitrogen atmosphere. However, the W _b remained low value below 2.3 × 10^?8 mm³/Nm regardless of oxygen concentration (100–207,000 ppm) of a nitrogen atmosphere (3.4–3.9%RH) when CN _X -coated balls were slid against a hydrogenated CN _X (CN _X :H) coatings (CN _X /CN _X :H). Besides, the CN _X /CN _X :H achieved low and stable friction coefficient below 0.05 under a nitrogen atmosphere (10,000 ppmO₂) regardless of increasing RH up to 20%RH. Raman analysis indicated that the structure of carbon on the top surface of CN _X coating was changed from as-deposited CN _X coating in the case of low friction coefficient (<0.05). Furthermore, TOF-SIMS analysis provided the evidence that the carbon derived from CN _X -coated disk was considered to diffuse into the ball surface, and it mixed with the carbon derived from CN _X -coated ball on the wear scar, which formed the chemically bonded carbon tribo-layer. Low friction coefficient (<0.05) with CN _X coatings under a nitrogen atmosphere was achieved due to self-formation of the carbon tribo-layer.     

Fast-response thermistors made of synthetic single-crystal diamonds

Miniature thermistors are produced from boron-doped synthetic single-crystal diamonds grown under pressure using the thermal gradient method. It is shown that heavily doped diamonds with a boron concentration of 10¹⁹ cm^?3 or higher are most suitable for this purpose. In the temperature range of 300–700 K, coefficient β = ln(R ₁/R ₂)/(1/T ₁ ? 1/T ₂) is 2500 K. The characteristic response time of temperature-sensitive elements based on crystals with dimensions of 1 × 1 × 0.3 mm is ～100 µs; i.e., they can be used in monitoring systems with a response speed of up to 10 kHz.     

Adatom concentration distribution on an extrawide Si(111) terrace during sublimation

The formation of an adsorption layer on the Si(111) surface during sublimation at temperatures of 1000–1100 °C and subsequent quenching at T = 750 °C is studied by methods of in situ ultrahigh-vacuum reflection electron microscopy and ex situ atomic force microscopy. The adatom concentration distribution on an extrawide (~60 μm) atomically flat terrace is determined for the first time, and the diffusion length x_s = 31±2 μm at T = 1000 °C is obtained. The analysis of the temperature dependence of the equilibrium concentration of adatoms near a monatomic step allows pioneering measurements of the energy necessary for adatom detachment from the step and attachment to the terrace E _ad ≈ 0.68 eV. Based on these results, the energy parameters for some atomic processes on the Si(111) surface are estimated.     

A pulsed two-channel Nd:YAG laser with an improved spatial structure of IR radiation for prompt recording and development of photothermoplastic holograms

A two-channel Nd:YAG laser for high-speed recording and development of photothermoplastic holograms has been created. In two independent channels, the laser generates monopulse radiation of nanosecond duration (30 ns) at the second harmonic frequency (λ ₂ = 0.532 μm) and IR radiation (λ ₁ = 1.064 μm) produced by quasi-stationary free-running lasing of millisecond duration (4 ms). The radiations with wavelengths λ ₂ and λ ₁ are used, respectively, for exposure and development of photothermoplastic holograms. A method based on the use of a passive resonator is proposed to improve the spatial structure of IR radiation during lasing.     

In situ cleavage mechanism for semiconductor single crystals for ultrahigh-vacuum scanning tunneling microscope

A tecnique for cleaving semiconductor single crystals under ultrahigh-vacuum conditions is proposed. A system for in situ cleavage of samples for ultrahigh-vacuum scanning tunneling microscope (STM) has been developed. STM studies of the surfaces of InAs single crystals with n and p-type bulk conduction have been performed on an Omicron ultrahigh-vacuum facility.     

Measurements of the solidification point of the GKGh-136 silicone liquid

Two methods for measuring solidification point T _S of the GKGh-136 silicone liquid are described. T _S ≌ 125 K is determined by the first method from specific features of the temperature dependence of the resistance of an organic quasi-2D conductor in a GKGh-136 droplet with a size of ～1 mm. T _S ≌ 130 K is assessed by the second method from the occurrence of a specific feature in the temperature dependence of the resistance of the GKGh-136 and fine-dispersed graphite mixture, which is caused by desorption of helium in the sample volume during warming-up >T _S .     

Effect of Quenching and Tempering on the Coercive Force of Fe-5 at. % C Powders Sintered after Mechanical Alloying

The dependence of the coercive force of Fe-5 at. % C powders sintered after mechanical alloying on the tempering temperature and on the structural state of the alloy is studied. It is shown that a transition of cementite Fe₃C from a state with a distorted crystal lattice and a small value of H _c to a state with an equilibrium lattice and a high value of H _c of this phase may be one of the reasons for an increase in the coercive force in the range of tempering 300–500°C.     

Quantitative proximate X-ray analysis of loose raw materials based on detection of diffraction and characteristic radiation

A method of rapid X-ray analysis is proposed. The content of the method is that the ratio I _d/I _i ^A is measured in two channels of a γ spectrometer, one of which is configured for the diffraction maximum of the determined phase (I _d) and the other measures the intensity of the spectral line of secondary element A (I _i ^A ), the atomic number of which is the same as that of the material of the X-ray tube anode. Results of the X-ray analysis of chromite and molybdenum are presented. The test rate was 7 min per test. The maximum deviation from the content of MoS₂ was 0.4% in standard specimens with concentrations of 24–29% and that of Fe and Cr₂O₃ was 0.3% for concentrations of 14–19%.