【作者】 尹亮；

【导师】 祁欣；

【作者基本信息】 北京化工大学 ， 凝聚态物理， 2011， 硕士

【摘要】 温标由温度固定点、内插仪器和内插公式构成,温度固定点是温标的重要组成部分。温度固定点具有数值准确、复现性高、随时间变化小等优点,固定点研究在温标制定和实际计量检测校准中具有重要的地位。1990年国际温标(ITS-90)在13.8K至1234K温区就是由一系列定义固定点和标准铂电阻温度计内插完成国际温标复现的。一般来说,电阻温度计具有使用温区宽、一致性好、使用方便、测量仪器配备方便等特点,在接触测温领域得到广泛应用,常见的标准温度计和工作用温度计多为电阻温度计。但由于电阻温度计的阻值会受到杂质污染和应力的影响,而且电阻温度计的电阻～温度特性会随时间产生漂移,所以电阻温度计需要定期校准以保证其准确度。相对来说,温度固定点的复现性和稳定性要远高于电阻温度计,因此电阻温度计的稳定性考查和使用中的校准,也尽量在固定点上进行。ITS-90在13.8K至1234K温区规定了17个定义固定点及其温度值,最低一个固定点是平衡氢三相点(13.8033K),其热力学温度的不确定度是0.5mK,最佳复现性是0.1mK,13.8K以下没有定义的固定点。因此,在13K以下温区寻找、发展性能良好的温度固定点,是温标研究、温度计性能研究和测量校准工作的重要基础。氦原子量为4,正常沸点4.224K,当使用真空泵对液氦进行减压降温时,液氦不会像其他液体那样在三相点温度下固化,而是在超流转变温度Tλ=2.1768K处发生相变,由正常相液氦(HeⅠ)转变为超流相液氦(HelⅡ)。正常相液氦热导有限,一旦温度降到超流转变温度以下,超流相液氦热导率突然增大几个数量级。同时,液氦在超流转变温度附近的比热呈现很窄的尖峰状,因此常将液氦超流相变称为液氦的λ转变。物质的熔点、沸点、三相点、凝固点和冷凝点是一级相变,有相变潜热,可用绝热法获得温坪进行复现。而液氦超流转变是二级相变,没有相变潜热,只有比热、热导和磁化率等参数的变化,无法用常规的绝热法获得相变温坪,只能用扫描变温法由相关参数的变化来确定相变温度。ITS-90文本只给出了饱和蒸汽压时的液氦超流转变温度为2.1768K,但未有实际应用的液氦超流转变温度复现方法。本文建立一套液氦超流转变温度复现系统,采用带毛细管结构的小型密封瓶,利用液氦超流转变时超流氦与正常氦热导率突变的性质,让一可控小热流通过毛细管,实现了毛细管中HeI/HeII两相共存和动态平衡。毛细管中HeⅠ液柱高度会随通过毛细管热流的变化自动调节,使HeI/HeII界面停留在毛细管中,获得了稳定、平坦的液氦超流转变温坪。由于通过毛细管的热流对液氦超流转变温度有下压效应,采用做不同热流的多个温坪,利用外推方法得到零热流下真实的Tλ值。采用铑铁电阻温度计进行24次液氦超流转变温度复现实验,标准偏差为0.022mmK,证明了液氦超流转变温度具有非常好的稳定性和复现性,推荐将液氦超流转变温度作为国际温标的固定点使用。本文同时建立了704型4He低温比对恒温器和703型3He低温绝热比对恒温器实验系统,对参加液氦超流转变温度复现实验的标准铑铁电阻温度计进行42个温度点的比对测量,确保液氦超流转变温度复现用温度计的准确性和稳定性。更多还原

【Abstract】 Temperature scale consists of temperature fixed points, interpolating instruments and formulas, and the temperature fixed points are an important part of the Temperature scale. Temperature fixed point plays an important role in establishing a temperature scale because of the advantages of accurate value, perfect reproducibility and small temperature drift over time. The International Temperature Scale of 1990(ITS-90) was established by interpolating a series of defined temperature fixed points with standard platinum resistance thermometer in a temperature range between 13.8K and 1234K. Resistance thermometer has been widely used as standard thermometer and working thermometer in contact temperature measurement area because of the the advantages of wide useful temperature range, good agreement, easy to use and easy with measuring instrument. However, the resistance of the resistance thermometer will be changed due to the impact of pollution by impurities and the stress and the resistance-temperature characteristics of the resistance thermometer will drift over time, so the resistance thermometer need to be calibrated regularly to ensure its accuracy. In contrast, the reproducibility and stability of the temperature fixed point is much better than the resistance thermometers, so the stability test and the calibration of the resistance thermometer is also uses on the fixed point. ITS-90 prescribe 17 defined fixed points and there temperatures in the temperature range between 13.8K and 1234K and the lowest temperature fixed point is the hydrogen triple point temperature,13.8033K, whose thermodynamic temperature uncertainty is 0.5mK and the best practical reproducibility is 0.1 mK. There is no defined temperature fixed point under 13.8033K. Therefore, to find and develop good temperature fixed point below 13K is an important basis of the temperature scale study, thermometer study and measurement.The atomic weight of helium is 4 and the normal boiling point is 4.224K. When pumping the liquid helium with vacuum to cool it down, the normalfluid helium(HeI) will change to the superfluid (HeII) at the superfluid transition temperature Tλ=2.176K, which is not curing at the triple point temperature like the normal liquid. The thermal conductivity of normal phase liquid helium (HeI) is limited. Once the temperature of the helium dropped below the superfluid transition temperature, the thermal conductivity of the superfluid phase liquid helium (Hell) suddenly increases of several orders of magnitude. Meanwhile, the specific heat of liquid helium show a narrow peak-like near the superfluid transition temperature, so the superfluid phase transition of liquid helium is called as theλtransition of liquid helium. When substance occurs phase transition at the triple point, melting point, freezing point, condensation point and the boiling point, the phase transition is a "first phase transition", so there will be a latent heat with the phase transition, and the reproduction can be obtained by adiabatic. The superfluid transition of liquid helium is the second phase transition, and there is no latent heat with the transition but the specific heat, the thermal conductivity, the magnetic susceptibility and other parameters. So the phase change temperature plateau can not be obtained with conventional adiabatic method, and the phase transition temperature can be obtained only by scanning temperature method with the changes of the related parameters. ITS-90 text provides that the liquid helium superfluid transition temperature is 2.1768K at the saturated vapor pressure, but there is no practical application method to reproduce the liquid helium superfluid transition temperature.Owing to the dramatic change in the thermal conductivity of 4He when its temperature crosses the transition of superfluid (HeI) and normalfluid (HeII), a sealed-cell with a capillary is used to realize the lambda transition temperature, Tλ. A small heat flow is controlled through the capillary of the sealed-cell so as to realize the coexistence of HeI and HeII and maintain the stay of Hel/Hell interface in the capillary. A stable and flat lambda transition temperature "plateau" is obtained. Because there is a depression effect of Tλcaused by the heat flow through the capillary, a series of heat flows and several temperature plateaus are made and an extrapolation is applied to determine Tλwith zero heat flow. A rhodium-iron resistance thermometer with series number A34 (RIRT A34) has been used in 24 Tλ-realization experiments to derive Tλwith the standard deviation of 0.022mK, which proves the stability and reproducibility of Tλ. A reference standard facility of rhodium-iron resistance thermometer (RIRT) was established. A new cryostat was designed to measure the five RIRTs which used in the realization.42 basis temperature points were measured using this set of reference standard facility which proved the accurate of the RIRTs.更多还原