Tungsten-Rhenium Thermocouples in Air Thermal Emf Stability Analysis

A Tungsten Rhenium Thermocouples Positive and Negative Comparative Stability

Tungsten- rhenium thermocouple in the air oxidation will occur after a certain time thermopower mutation to constitute the positive and negative tungsten -3% -25 % rhenium and tungsten rhenium alloy oxidation rate is different. Which can be expected, the stability of the thermocouple will they produce different effects . It can be seen from Figure 1 : Tungsten rhenium thermocouples having different stability of positive and negative . 1000 ℃, tungsten rhenium 3 pole poor stability, thermopower mutation first occurred , and tungsten rhenium 25 very good stability , a later mutates. Stability of the positive and negative differences are two kinds of tungsten and rhenium alloy oxidation behavior consistent . 800,900 ℃ , with the decrease in the content of rhenium , tungsten rhenium alloy antioxidant capacity enhancement. -3% Tungsten rhenium relatively good adhesion due to the formation of a layered structure oxide film having less than 25% of rhenium, tungsten corrosion rate ; 1000 ℃ , the rhenium content is higher, the stronger the oxidation resistance of alloys . As a result of 25% tungsten, rhenium oxide layer formed by reaction sintering of the oxide film layer structure , the outer layer is less rich tungsten rhenium oxide thin , inner layer is a dense tungsten , rhenium oxide mixed layer , relative to the base ratio, rhenium has enriched . Reaction sintering the oxide film becomes more dense , increased to 25% of tungsten rhenium at 1000 ℃ antioxidant capacity , the corrosion rate is lower than -3% of tungsten rhenium . Therefore , tungsten rhenium 3 pole less stable at 1000 ℃ , the first occurrence of thermopower mutations. Therefore, positive and negative changes in the sequence of stability is a direct reflection of the level of oxidation rate .

2 . Tungsten rhenium thermocouple thermoelectric power thermoelectric mutation causes

Positive and negative relative stability during the experiment at the same time , using two 0.8 mm nickel chromium wire 2cm long were suspended tungsten -3 % -25 % tungsten, rhenium and rhenium alloy wire of a group, working with the thermocouple placed on the same side of the temperature zone 1000 ℃ , when mutations occur when the first thermoelectric take a set of samples , the second mutation then remove another set of samples , respectively, electron microprobe analysis . The results show that : the first time thermopower mutation , -3% tungsten rhenium alloy anode has been completely oxidized, and the negative -25 % tungsten rhenium alloy wire core part still alloy phase . The second thermoelectric mutated, it is also negative alloy phase has been fully oxidized [1] . Thus , tungsten rhenium thermocouple thermoelectric power is due to the positive and negative mutations caused by oxidation .

To further confirm this phenomenon, making the overall stability test thermocouple , remove mutations moment thermocouple, will end its work made ​​metallographic sample . Based on our 1000 ℃ tungsten rhenium thermocouple potential mutations just occurred working end time by metallographic analysis of morphology , thermocouple potential mutation causes the selective oxidation of alloying elements is not caused by changes in composition , but the pole due to oxidation of a thermocouple the original tungsten rhenium alloy turned into oxides. This is a qualitative component , that is, from alloy into oxides , this phenomenon is associated with tungsten- rhenium alloy high temperature oxidation behavior consistent . Under the experimental conditions , as rhenium and tungsten oxide evaporation , resulting in the oxidation of the tungsten rhenium alloy catastrophic oxidation follows a linear law of cracking . Cracks formed during the oxidation , loose oxide layer is difficult to form the oxygen diffusion resistance, so that the oxidation reaction occurs at the metal / oxide layer interface ; since rhenium is far less than the oxygen affinity of tungsten , and their oxides generated with a more the steam thus large tungsten rhenium alloy does not occur during the oxidation of rhenium selective oxidation , the oxide layer which are made from tungsten and rhenium oxide mixture. Thus , in an oxidizing atmosphere , there is no selective oxidation caused by the diffusion of metal cations as a result of gradual change in the alloy composition . Thus, as the oxidation process proceeds, shrinking alloy wire cross-section , but the composition changed, until eventually completely oxidized into oxide , the material properties of a fundamental change , closely related thermoelectric properties also changed completely , thus tungsten- rhenium thermocouple leads thermopower mutations. Through the above experiment , the first to reveal the potential of tungsten rhenium thermocouple nature of mutations . Further, since the tungsten- rhenium alloy is generated after oxidation of rhenium oxide has a high electrical conductivity , resulting in the tungsten- rhenium thermocouple has failed circuit illusion thermopower still occurs , which would address the long outstanding problem.

3 tungsten rhenium thermocouples oxidation technology and antioxidant tungsten rhenium thermocouples

Tungsten- rhenium thermocouple thermoelectric effect in the air is the main factor unstable oxidation, and in order to replace platinum-rhodium thermocouple they can only work in an oxidizing atmosphere , so can be used as oxidation technology : ① Coating Protection Act ; ② evacuated sealed protection Act ; ③ evacuated inflatable seal protection law ; ④ filling sealing protection laws.

Coating Protection Act technically there are still some problems, and taking the time to fill or seal protection law has passed the batch test, every year at least one thousand or more antioxidant tungsten rhenium thermocouple temperature measurement for industrial furnaces . Since the protective tube for the artificial creation of tungsten- rhenium thermocouple working non-oxidizing atmosphere , resistance to oxidation of the tungsten rhenium thermocouple term stability. Author entities using patented technology to produce tungsten rhenium thermocouple type antioxidant , after sintering plant in Anshan pellet furnace ( oxidizing atmosphere ) using seven months (5000h), after removal of the test results showed that it stable performance, precision up to 1% , and can continue to use.


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