Measurement Of Specimen Temperature Research Articles

Temperature calibration of TEM specimen heating holders by isothermal sublimation of silver nanocubes

Micro electro mechanical systems (MEMS) based TEM specimen heating holders exhibit excellent thermal stability and minimal specimen drift, which allows thermally-activated processes to be studied dynamically at high spatial resolution. The advantages of MEMS-based devices arise from the very small thermal masses of the sample studied, but this poses particular challenges for the precise measurement of specimen temperature. Previously, it has been proposed that the size-dependent sublimation behavior of Ag nanoparticles could be used to measure the specimen temperature by applying the Kelvin equation, but the effects of the capping ligands used in the nanoparticle synthesis and of electron beam heating have limited the application of such approaches. Here it is shown that for an appropriate choice of experimental parameters (nanoparticle size, loading, intermediate holding temperature, and illumination conditions) the sublimation of Ag nano-cubes can be used to measure the specimen temperature to an accuracy of ±5 °C, over the range 700–850 °C. The measurements are reproducible from area to area on the same MEMS chip, and from chip to chip of the same type. The values of specimen temperature obtained are consistently lower than the calibrated MEMS heater plate temperatures, and it is shown that this cannot be explained on the basis of random errors in the experimental measurements or systematic errors in the materials parameters used for the Kelvin equation analysis. It is proposed that this is instead due to the low thermal conductivity of the electron-transparent amorphous silicon nitride support membrane on the chip. As further evidence for this, it is shown that for a thicker crystalline Si support with a higher thermal conductivity, the magnitude of the difference is smaller. This approach could be extended to other temperature ranges by using nanoparticles of other metals with different vapor pressures and sublimation temperatures.

Tribological behaviour of carbon–carbon composite

In the present study, the wear behaviour of cross ply (0/90°) C–C composite with 60 vol.-% fibres has been studied with sliding distance, applied load and sliding velocities. The measurement of specimen temperature has been carried out to study the effect of frictional heating. Furthermore, wear debris and wear track observations are correlated to understand the wear mechanism. The bulk wear increases linearly with distance after an initial running-in period. The temperature studies reveal that frictional heating is more with increase in load or sliding velocity under dry conditions, however, presence of lubrication reduces frictional heating, because exposure of surface for direct contact is reduced, and hence wear rate in all studies with lubrication is less than that under dry condition. The wear track studies show graphite powder, peeling of fibres and dislodging of the surface. At low loads, smearing of graphite powder keeps the wear rate low, but as the load increases; dislodging, delamination of surface and breaking of fibres dominate, and wear rate sharply increases, however, sliding velocity initially enhances the graphite formation reducing the wear, but as the velocity reached an optimum value, there is extensive breakage of fibres, dislodging and delamination of surface, and the wear rate increases sharply.

Hydrated Specimen Stability in the Electroscan ESEM: Specimen Surface Temperature Variations and the Limitations of Specimen Cooling

Chamber temperature and pressure are the two principal factors in preserving hydrated specimens in a high vapour pressure or environmental SEM; in this study the Electroscan ESEM. Hydrated specimen stability, is achieved by operating at or near water saturated conditions in the specimen chamber.The ambient chamber temperature (in the ESEM) has been measured and found to be ∼29°C, ∼7°C higher than the laboratory environment. Water saturation at this temperature requires a chamber pressure of ∼30t and poor image resolution results from the high degree of primary beam scattering under these conditions. A low cost Peltier effect cooling stage has been constructed to provide cooling of specimens to ∼5°C. At this temperature water saturated conditions are achievable in the chamber at ∼5t with resultant good image resolution.Usage has shown that stability of sensitive hydrated specimens still remains difficult. This is due to the cooling and measurement of specimen temperature through the stage at the base of the specimen, rather than at the surface.

Sources of error in thermogravimetry: balance inclination and specimen temperature

This paper considers two sources of error in thermogravimetry which previously have received insufficient attention, namely, instability in the inclination of microbalance supporting structures and specimen temperature measurement. For the Cahn RH microbalance, for example, which is basically a beam system dynamically balanced by an electromagnetic torque, principles of mechanics are applied to a simple model to obtain the functional variation of the microbalance output in terms of weight loads, geometric parameters and the inclination of the balance supporting structure. Together with this treatment, measurement of apparent weight changes as a function of microbalance inclination allows limits to be established for the weight errors due to changes in inclination. For the Cahn RH balance, this limit can be as large as 600 mu g mrad-1. In relation to specimen temperature measurement, uncertainties in thermogravimetrically-derived rate constants for alloy oxidation resulting from uncertainties in temperature are considered, and techniques to minimize these uncertainties are discussed.

Atom Transport Under Varying Pressure * *

Abstract Study of the effect of hydrostatic pressure on the rate of diffusion-limited processes gives in­ formation regarding the intrinsic volume changes associated with formation and motion of defects. While there are basic problems associated with interpretation of the motional volume, such effects may be measured directly in some cases and subtracted to give unambiguous results for the defect formation volume. Experimentally, precise measurement of specimen temperature within a high-pressure environment poses the most severe practical constraint. Results are given for measurements to pressures of 7 kilobars of ionic conductivity in a series of pure and doped alkali halides: NaCl, KCl, NaBr, and KBr, all of which are predominantly cationic vacancy conductors. The activation volumes are found to be pressure dependent in the intrinsic regions. In all cases the Schottky pair vacancy formation volume is found to be larger than the molar volume, the ratio being largest for NaCl and smallest for KBr. Results are also given for the total activation volume for diffusion of Ti44 in beta-Ti, a case of "anomalous" bulk diffusion. The tracer penetration plots do not follow a perfect gaussian profile. However, if the penetration plots are fit to a single gaussian, the activation volume derived is consistent with that expected for a vacancy mechansm in bcc metals

Measurement of energy dissipated in superconductors during flux jumps

The energy dissipated, during field cycling and during flux jumps, has been calculated from measurements of specimen temperature on samples of Pb-Bi eutectic alloy. The dissipation thus calculated is compared with that obtained directly from the area of the magnetization curves. The agreement is well within experimental error.The energy dissipated ΔW during three types of flux jump has been measured. For complete flux jumps with no flux penetration of the sample between jumps, ΔW is found to be equal to ½ΔBΔH. For complete flux jumps with flux penetration between successive jumps, ΔW is found to be equal to ¾(ΔB)2. For partial flux jumps ΔW is equal to between (H−B) ΔB and 2·5(H−B) ΔB, showing that much of the extra flux introduced during the jump is concentrated near the centre of the sample. These results are entirely consistent with the description of flux jumps in terms of the critical state model of irreversible type II superconductors.

A technique for accurate specimen temperature measurement in a thermobalance: E J Chatfield,J Sci Instrum, 44 (8), Aug 1967, 649–651

A technique for accurate specimen temperature measurement in a thermobalance: E J Chatfield,J Sci Instrum, 44 (8), Aug 1967, 649–651

A technique for accurate specimen temperature measurement in a thermobalance

A null method is described by which specimen temperatures in a thermobalance may be accurately recorded. Split planes and knife edges of stainless steel are used on the balance beam assembly to provide a conducting path from a noble metal thermocouple in contact with the specimen to a potentiometric recorder. The technique has been tested at temperatures up to 650°C by comparison against standard melting points and an accuracy to ±1 degC was obtained over the whole range.

試作高温疲労試験機と低炭素鋼の高温疲労

Rotary bending test is the easiest to carry out among various sorts of testing method of fatigue, and is often practised for determining fatigue characteristics of metallic materials. For fatigue tests at elevated temperatures, rotating load type machine is more preferable to rotating specimen type in order to achieve accurate measurement of specimen temperature. A rotary bending fatigue testing machine of rotating load type for tests at elevated temperatures was designed and manufactured. A brief description on the machine is rendered in this paper.The original intention of the present study is to discuss the influence of frequency of stress cycles on fatigue life of heat resistant alloys at elevated temperatures. As a preliminary study for it, fatigue of low carbon steel at elevated temperature of 450°C was investigated by applying three sorts of stress cycles, 170, 750 and 3000per min., using the new testing machine. Each test was carried out under the condition of constant strain amplitude.The results of the tests are summerized as follows; as far as the strain amplitude applied is large, the number of stress cycles till fracture is not affected by the frequency of applied cyclic stress and so, under the action of the definite magnitude of strain amplitude, specimens fracture at definite number of stress cycles. When the strain amplitude is small, however, specimens are likely to fracture at definite time notwithstanding different frequency of stress cycles imposed. The intermediate feature of fracture is apparent in the test under strain amplitude between both extremes which lead to the results contradictory to each other.The mechanism of fatigue at elevated temperatures is discussed by taking inelastic deformation in every stress cycles, which depends on frequency of stress cycles, as operative factor on fatigue. The qualitative interpretation of the experimental results is given.

The measurement of specimen temperature in a high temperature X-ray powder camera

Errors of specimen temperature measurement by means of thermocouples placed near the specimen are examined theoretically and experimentally for the case of an evacuated Unicam camera, where the furnace arrangements can be regarded as an imperfect radiation bath. Errors which increase monotonically with temperature arise because the radiation temperature of the furnace is not everywhere the same. Errors which are of opposite sign, and which increase to a maximum at about 200° C and then decrease with increasing temperature, arise because of conduction down the thermocouple leads. Design requirements to minimize the errors and keep them constant from one specimen to another are given.