Sensitive elements of temperature converters based on HfNi1-xCuxSn thermometrical material

Abstract

The results of experimental studies of sensitive elements of temperature transducers based on semiconductor thermometric material HfNi1-xCuxSn are presented. Thermometric materials HfNi1-xCuxSn, x=0.01–0.10, were produced by fusing a charge of components in an electric arc furnace with a tungsten electrode (cathode) in an atmosphere of purified argon under a pressure of 0.1 kPa on a copper water-cooled base (anode). Heat treatment of the alloys consisted of homogenizing annealing at a temperature of 1073 K. The samples were annealed for 720 hours. in quartz glass ampoules vacuumed to 1.0 Pa in muffle electric furnaces with temperature control with an accuracy of ±10 K. Diffraction data arrays were obtained on a STOE STADI-P powder diffractometer (Cu Kα1 radiation), and the structural characteristics of HfNi1-xCuxSn were calculated using the Fullprof program. The chemical and phase compositions of the samples were monitored using metallographic analysis (scanning electron microscope Tescan Vega 3 LMU). The thermoelectric pair platinum-thermometric material Pt-HfNi0.99Cu0.01Sn was the basis of the thermoelectric converter. Modeling of thermometric characteristics of sensitive elements of thermotransducers in the temperature range of 4.2– 1000 K was carried out by the full potential linearized plane wave method (Full Potential Linearized Augmented Plane Waves, Elk software package). The results of experimental measurements served as reference currents for modeling characteristics. X-ray phase analysis showed the absence of traces of extraneous phases in the diffractograms of the studied samples of HfNi1-xCuxSn thermometric materials, and the microprobe analysis of the concentration of atoms on their surface established the correspondence to the original composition of the charge. Refinement of the crystal structure of HfNi1-xCuxSn showed that the introduction of Cu atoms orders the structure, which makes it stable, and the kinetic characteristics are reproducible during thermocycling at temperatures T=4.2–1000 K. Ordering the structure of the thermometric material HfNi1-xCuxSn leads to changes in the electronic structure. At the same time, the number of donors decreases – Ni leaves the Hf position, and the substitution of Ni atoms for Cu leads to the generation of structural defects of the donor nature (Cu atoms contain more 3d-electrons), and another donor band εD Cu will appear in the band gap εg. For the sensitive elements of thermoconverters at Cu impurity concentrations x=0.005 and x=0.01, the temperature dependences of the specific electrical resistance ln(ρ(1/T)) contain activation areas, which is consistent with the results of electronic structure modeling. This indicates the location of the Fermi level εF in the band gap εg, and the negative value of the thermopower coefficient α(T) at these temperatures specifies its position – near the conduction band εC. The value of the activation energy from the Fermi level εF to the bottom of the conduction band εC was calculated. For the base semiconductor n-HfNiSn, the Fermi level εF lies at a distance of εF=81 meV from the co εC conduction band εC, and in the sensitive elements of thermoconverters with concentrations of HfNi0.995Cu0.005Sn and HfNi0.99Cu0.01Sn – at distances of εF=1 meV and εF=0.3 meV respectively. Therefore, an increase in the concentration of the Cu donor impurity leads to a rapid movement of the Fermi level εF to the bottom of the conduction band at a rate of ΔεF/Δx≈81 meV/%Cu. The impurity concentration x=0.01 is sufficient for the metallization of the conductivity of sensitive elements of HfNi1-xCuxSn converters at low temperatures. This is possible if the Fermi energy εF is close to the conduction band εC (εF=0.3 meV), which simplifies the thermal ionization of donors and the appearance of a significant number of free electrons. However, this impurity donor zone still does not intersect with the bottom of the conduction band εC. At concentrations of the Cu donor impurity in HfNi1-xCuxSn, x=0.2–0.07, the high-temperature activation regions disappear on the temperature dependences of the resistivity ln(ρ(1/T, x)), which indicates the movement of the Fermi level εF from the band gap εg to the conductivity εC. At the same time, the values of specific electrical resistance ρ(T, x) increase monotonically with increasing temperature), and the scattering of electrons by phonons determines the conductivity of sensitive elements of thermotransducers based on the thermometric material HfNi1-xCuxSn. The metallization of the electrical conductivity of the thermometric material HfNi1-xCuxSn at concentrations x>0.01 is accompanied by a rapid decrease in the values of the thermopower coefficient α(x, T). Thus, if in n-HfNiSn at a temperature of T=80 K, the value of the thermal erst coefficient was αx=0=–178 μV/K, then in the HfNi0.93Cu0.07Sn material αx=0.07=–24 μV/K. The results of the kinetic properties of HfNi1-xCuxSn are consistent with the conclusions of structural and energetic studies. The simulation of the conversion functions of the sensitive elements of the resistance thermometer and the thermoelectric converter in the temperature range of 4.2–1000 K was carried out. As an example, the conversion functions of the thermoelectric pair Pt-HfNi0.99Cu0.01Sn are given. The ratio of change of thermo-EMF values to the range of temperature measurements in thermocouples is greater than all known industrial thermocouples. However, due to the metallization of the conductivity of the thermometric material HfNi1-xCuxSn, x>0.01, the temperature coefficient of resistance (TCR) of the obtained resistance thermometers is greater than the TCR of metals, but is inferior to the value of TCR of sensitive elements made of semiconductor materials.