波形齿夹具张拉CFRP 带的力学性能试验研究及夹具体系设计

对波形齿夹具在张拉CFRP 带时的受力状态的分析,理论上提出了CFRP 带理论伸长值的修正以及波形齿夹具配套螺杆的轴力、剪力验算。针对这3 个方面的问题,共设计了两大类试验来研究张拉CFRP 带时的波形齿夹具:一类是钢梁台座上仅张拉CFRP 带的试验;另一类是张拉CFRP 带加固钢筋混凝土T 形梁的试验。试验表明:以折减率来考虑波形齿夹具张拉CFRP 带的理论伸长值的修正;在张拉时螺杆轴力与CFRP 带的预拉力值呈定值比例,在工程实际中可以通过螺杆轴力值对CFRP 带预拉力值的监控。并提出了单个螺杆的轴力值与剪力值的计算公式。     

Industrialisation of the insulation vacuum barrier for the large hadron collider (LHC) magnet cryostats

The insulation vacuum (<10⁻⁴ Pa) of the large hadron collider magnet cryostats, thermally protecting the superconducting magnets which operate at 1.9 K in superfluid helium, is divided in to 214 m long segments separated by means of insulation vacuum barriers.The insulation vacuum barrier is a leak-tight stainless steel welded structure, composed of two concentric corrugated cylinders and one internal bellows linked together by a 6 mm thick central plate. As the vacuum barrier mechanically links the cryostat vacuum vessel operating at ambient temperature and the 1.9 K superconducting magnets, it is designed to have minimum heat conductivity. Conduction heat in-leak is intercepted at 65 K by a high-purity copper ring brazed onto the stainless steel central plate and thermally linked to a cryogenic line by a copper-aluminium soldering. The thermal performance has been experimentally validated by cryogenic testing.This paper presents the results obtained after industrialisation, manufacture and testing of prototypes and series units. Qualification of leak-tight welds in thin-sheet stainless steel (thickness 0.15-1.3 mm) has been carried out. Ultrasonic testing is performed on all brazing and soldering. Helium leak testing is performed, using dedicated tooling, to ensure a leak-tightness to a rate better than 10⁻⁹ Pa m³ s⁻¹.     

隔热板冲压成形工艺参数优化

目的确定隔热板合理可行的成形方案。方法简要分析了零件的成形特点,并利用有限元软件对零件的拉延成形进行了数值模拟。设计正交实验时,以压边力、1号和3号拉延筋的完全锁模力、2号和4号拉延筋的完全锁模力及摩擦因数4个参数为自变量,以最大厚度减薄率、破裂情况以及未充分变形区大小为优化目标。结果 2号和4号拉延筋是该隔热板成形的主要影响因素,锁模力都为60 N/mm时,隔热板拉延成形效果较好。结论通过分析各因素对优化目标的影响,得出了优化的工艺参数值,为零件的实际生产提供指导。     

Thermal conductivity of rare earth fluoride crystals

Thermal resistance of LaF₃ and HoF₃ monocrystals is linearly dependent on temperature. In LaF₃ crystals a deviation from the linear law was observed at T>170°K in the direction of higher thermal-conductivity values. In doped LaF₃:Eu²⁺ in the region T>170°K thermal resistance is lower than in pure specimens.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 30, No. 1, pp. 135–138, January, 1976.     

Low temperature thermal conductivity of aluminum alloy 1200

The thermal conductivity of aluminum alloy 1200 was determined from 4.2 K to 160 K using a thermal conductivity integral method. This steady state method has been implemented in a cryostat having a cold finger cooled with liquid helium and nitrogen. These materials were considered to create thermal link for the Planck research satellite. Two samples are studied; the “as fabricated” 1200 alloy and the 1200 H19 (cold-drawn). As expected, the evolution of the thermal conductivity with temperature of both alloys follows the electronic thermal conductivity theory with a good accuracy below 60 K. At higher temperature, the thermal conductivity reaches a maximum then decreases as T^?n and finally remains constant due to the electron–phonon scattering. As expected, the thermal conductivity of the cold-drawn alloy, 1200 H19, is reduced compared to that of the 1200 alloy due to a higher concentration of defects in the metal.     

Thermal conductivity of Al–SiC composites with monomodal and bimodal particle size distribution

The thermal conductivity of aluminum matrix composites having a high volume fraction of SiC particles is investigated by comparing data for composites fabricated by infiltrating liquid aluminum into preforms made either from a single particle size, or by mixing and packing SiC particles of two largely different average sizes (170 and 16 μm). For composites based on powders with a monomodal size distribution, the thermal conductivity increases steadily from 151 W/m K for particles of average diameter 8 μm to 216 W/m K for 170 μm particles. For the bimodal particle mixtures the thermal conductivity increases with increasing volume fraction of coarse particles and reaches a roughly constant value of 220 W/m K for mixtures with 40 or more vol.% of coarse particles. It is shown that all present data can be accounted for by the differential effective medium (DEM) scheme taking into account a finite interfacial thermal resistance.     

Thermal conductivity,heat capacity,and thermal diffusivity of selected commercial AlN substrates

The thermal transport properties of four commercially available AlN substrates have been investigated using a combination of steady-state and transient techniques. Measurements of thermal conductivity using a guarded longitudinal heat flow apparatus are in good agreement with published room temperature data (in the range 130–170 W · m^–1 · K^–1). Laser flash diffusivity measurements combined with heat capacity data yielded anomalously low results. This was determined to be an experimental effect for which a method of correction is presented. Low-temperature measurements of thermal conductivity and heat capacity are used to probe the mechanisms that limit the thermal conductivity in AlN.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Structural phase transitions and negative thermal expansion in Sc2(MoO4)3

The thermal expansion and phase transitions of the framework material Sc₂(MoO₄)₃ have been investigated from 4 to 300 K by powder neutron diffraction, and from 300 to 1053 K by dilatometry. Below 178 K Sc₂(MoO₄)₃ has a monoclinic structure, which has been determined using Rietveld refinement of X-ray and neutron powder diffraction data collected at 50 K; space group P2₁/a with a=16.22715(9), b=9.58051(6), c=18.9208(1) Å, β=125.3988(4)°. Monoclinic Sc₂(MoO₄)₃ has a positive coefficient of thermal expansion, α_V=+2.19×10⁻⁵ K⁻¹ between 4 and 170 K. There is significant anisotropy in thermal expansion with the monoclinic b-axis having a negative expansion coefficient between 4 and 86 K. Above 180 K Sc₂(MoO₄)₃ has the orthorhombic Sc₂(WO₄)₃ structure and has a negative coefficient of thermal expansion with α_V=−6.3×10⁻⁶ K⁻¹ between 180 and 300 K. The structure has been determined between 4 and 300 K using a parametric approach to Rietveld refinement. Structural changes at the monoclinic to orthorhombic phase transition are shown to be intimately related to the contraction of the orthorhombic temperature phase. Dilatometry measurements show that negative thermal expansion continues up to 1053 K.     

Superconducting transition temperature of ion bombarded niobium carbide surface layers

The influence of Ge and C ion irradiation on the critical temperature of NbC layers obtained by the diffusion of C onto Nb-Ge-Cu substrate was studied. As a result of the irradiation with Ge ions of 150 keV and fluxes of 2.10¹⁵ and 2.10¹⁶ ion cm^–2, a slight decrease of the critical temperature, of 0.3 K and 0.6 K respectively, was observed. In both cases the critical temperature reached the pre-irradiation values after a thermal treatment for 24 h at 600° C. The irradiation with C ions of 170–180 keV and fluxes up to 2.10¹⁶ ions cm^–2 led to an increase by 30% of the critical temperature at low energies and to its degradation after the thermal treatment.     

Ultrasensitive thermal sensors based on whispering gallery modes in a polymer core optical ring resonator

This study proposes a thermal sensor based on whispering gallery modes (WGMs) in a polymer core optical ring resonator (PCORR). The thermal sensitivity and detection limit (i.e., the temperature resolution) for WGMs of various orders and polarizations are theoretically studied as a function of the ring wall thickness. The results show that the temperature detection limits can be as low as 4×10(-5) and 6×10(-6) K for laser linewidths of 2 and 0.3 MHz, respectively. The ultrahigh temperature resolution makes the PCORR a very promising platform for temperature measurement. The analysis also shows that the WGM of a lower order has better thermal sensing performance and a thinner optimal thickness of the ring resonator.     

Residual stresses in plastics,rapidly cooled from the melt,and their relief by sectioning

A theory for the residual stresses in tempered glass plates has been adapted for the cooling of plastics, which have temperature dependent thermal properties. The theory was checked against experimental residual stress distributions found in quenched polycarbonate sheet, and against the analytical solution for temperature independent properties. The heat transfer coefficient for quenching polycarbonate from 170° C into iced water was found to lie between 1000 and 4000 W m^–2 K. It is known that the cutting of thin sections from a sheet relieves the residual stresses, and this is used for transparent plastics to distinguish between orientation and stress bi-refringence. An elastic stress analysis of the sectioning process showed that the section width must be less than 20% of the sheet thickness for the residual stresses to be reduced to 5% of their original values.     

Thermal conductivity of CVD diamond films

Diamond films 60 and 170 µm in thickness were grown by PACVD (plasma-assisted chemical vapor deposition) under similar conditions. The thermal diffusivity of these freestanding films was measured between 100 and 300 K using AC calorimetry. Radiation heat loss from the surface was estimated by analyzing both the amplitude and the phase shift of a lock-in amplifier signal. Thermal conductivity was calculated using the specific heat data of natural diamond. At room temperature, the thermal conductivity of the 60 and 170 m films is 9 and 16 W-cm^–1. K^–1 respectively, which is 40–70% that of natural diamond, The temperature dependence of thermal conductivity of the CVD diamond films is similar to that of natural diamond, Phonon scattering processes are considered using the Debye model, The microsize of the grain boundary has a significant effect on the mean free path of phonons at low temperatures. The grain in CVD diamond film is grown as a columnar structure, Thus, the thicker film has the larger mean grain size and the higher thermal conductivity. Scanning electron microscopy (SEM) and Raman spectroscopy were used to study the microstructure of the CVD diamond films. In this experiment, we evaluated the quality of CVD diamond film of the whole sample by measuring the thermal conductivity.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Temperature variation of thermoelectric power of vacuum deposited PbSe thin films and its thickness dependence

PbSe thin films of thicknesses in the range 20 to about 170 nm have been prepared on glass substrates held at room temperature by thermal evaporation of the bulk alloy at a pressure of 5×10^–5 torr. The thermoelectric power of these films has been evaluated as a function of temperature in the range 300 to 500 K from the thermal e.m.f. data. It was found that the thermoelectric power of all the films initially increases with increasing temperature, then reaches a maximum, and, with a further increase in temperature, decreases rapidly and also changes sign. The possible reasons for this peculiar behaviour have been given. It is also found that there is no systematic variation of thermoelectric power with thickness of the films. This is probably due to completely specular scattering at the external and internal surfaces of PbSe thin films or to changes in stoichiometry of different films.     

Variable temperature FIR-optical cryostat with unique features

This paper describes a convenient cryostat for optical and far infrared transmission and photoconductivity studies at temperatures in the range 5 K to 300 K. Up to four samples can be loaded in a cell and inserted into the cryostat from above without disturbing large metal seals or optical tails. Thermal contact between the heat sink and the cell is made automatically during the cooling process. The thermal link between the heat sink and liquid cryogen reservoir is varied by means of a modified thermosyphon. Even samples which are poor thermal conductors can be controlled in temperature within ± 10 mK by an exchange gas in the sample cell.     

Dielectric relaxations in SrTiO3 doped with La2O3 and MnO2 at low temperatures

In addition to the dielectric relaxation around 170 K in the cubic structure of Sr_1–3x/2La _x TiO₃, a similar relaxation was observed at about 70 K in the tetragonal structure with an activation energy in the range 0.13–0.16 eV which increases as x in Sr_1–3x/2La _x TiO₃ varies from 0.60–3.00 at%. This relaxation is explained by Skanavi's model and is discussed in terms of thermal motions of Ti⁴⁺ between potential minima produced by lattice distortions in the tetragonal structure. In order to provide a direct evidence for the suggestion that the dielectric relaxation around 170 K in the cubic is due to thermal motions of Ti⁴⁺, dielectric properties on manganese-doped specimens, Sr_1–3x/2La _x Mn _y Ti_1–y ),O₃ with x=1.40×10^–2 and y=0.1×10^–2, were investigated because Mn⁴⁺ substitutes for Ti⁴⁺. As well as the relaxation due to Ti⁴⁺, this specimen exhibits another peak due to Mn⁴⁺ with an activation energy somewhat smaller than that of Ti⁴⁺. The activation energies and the relative intensity of these relaxation processes are explained by the difference in ionic radii of Mn⁴⁺ and Ti⁴⁺ and the difference in formation energies of a strontium vacancy adjacent to Mn⁴⁺ and that to Ti⁴⁺, which were calculated theoretically using a shell model.     

Thermophysical properties of MPG-6 graphite

The thermal diffusivity and heat capacity of four MPG-6 graphite samples (density from 1664 up to 1825 kg/m³) are measured within the temperature range from 293 K up to 1650 K by the following methods: the laser flash, the differential scanning calorimetry, and the adiabatic calorimeter of linear heating. The uncertainties of the data on the thermal diffusivity, heat capacity, and density were (2–4)%, (3–5)%, and 0.5%, respectively. On the basis of the measurement results, the temperature dependence of the MPG-6 thermal conductivity is calculated and a generalizing dependence is obtained which allows one to estimate the thermal conductivity of graphite of various porosity for a wide temperature range using only the data on the macroscopic density of the samples. Reference data tables have been developed for the thermal conductivity of MPG-6 graphite of various densities.     

Heat capacity and thermal expansion of samarium titanate

Samarium titanate (Sm₂TiO₅) was prepared by solid-state synthesis and characterized by X-ray diffraction (XRD). Heat capacity measurements were carried out by differential scanning calorimeter (DSC) in the temperature range 298-800 K. Thermal expansion characteristics have been studied by high temperature X-ray diffraction technique (HTXRD) in the temperature range 298-1573 K. The heat capacity value at 298 K is 170 J K^− 1 mol^− 1. The percentage linear thermal expansion in the temperature range 298-1573 K along a, b and c axes are 0.96, 0.89 and 1.07 respectively. The average coefficient of thermal expansion value obtained in the present study for samarium titanate up to 1573 K is 10.8 × 10^− 6 K^− 1.     

Ultrahigh Thermal Conductive yet Superflexible Graphene Films

Electrical devices generate heat at work. The heat should be transferred away immediately by a thermal manager to keep proper functions, especially for high‐frequency apparatuses. Besides high thermal conductivity (K ), the thermal manager material requires good foldability for the next generation flexible electronics. Unfortunately, metals have satisfactory ductility but inferior K (≤429 W m^?1 K^?1), and highly thermal‐conductive nonmetallic materials are generally brittle. Therefore, fabricating a foldable macroscopic material with a prominent K is still under challenge. This study solves the problem by folding atomic thin graphene into microfolds. The debris‐free giant graphene sheets endow graphene film (GF) with a high K of 1940 ± 113 W m^?1 K^?1. Simultaneously, the microfolds render GF superflexible with a high fracture elongation up to 16%, enabling it more than 6000 cycles of ultimate folding. The large‐area multifunctional GFs can be easily integrated into high‐power flexible devices for highly efficient thermal management.     

Nucleate boiling heat transfer coefficients of flammable refrigerants

Nucleate boiling heat transfer coefficients (HTCs) of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) on a horizontal smooth tube of 19.0 mm outside diameter have been measured. The experimental apparatus was specially designed to accommodate high vapor pressure refrigerants such as propylene and propane with a sight glass. A cartridge heater was used to generate uniform heat flux on the tube. Data were taken in the order of decreasing heat flux from 80 kW m⁻² to 10 kW m⁻² with an interval of 10 kW m⁻² in the pool temperature of 7 °C. Test results exhibited a typical trend that HTCs of flammable refrigerants increase with increasing vapor pressure. Existing nucleate boiling heat transfer correlations showed up to 80% deviation as compared to the present data. Hence a new correlation was developed through a regression analysis taking into account dimensionless variables affecting nucleate boiling heat transfer. The new correlation showed a good agreement with data for flammable refrigerants as well as halogenated refrigerants with a deviation of 5.3%.     

The thermal conductivity of carbon - carbon fibre composites below 80 K

The thermal conductivities of some 2 dimensional carbon fibre in carbon (CCF) composite materials were measured between 1.5 K and 80 K. Measurements upon one sample were made down to 0.15 K. An analysis of the results is made and a simple model of the system was used to predict the conductivities to within ～10%.The material is a very good thermal insulator (K < 0.01 Wm^?1K^?1 at 4.2 K) with very low thermal expansion and is easy to machine. As such it may be useful as a low temperature constructional material, though the readiness with which the material adsorbs gas may limit its applications.