Can You Solder Thermocouple Wire

Pt-Rh thermocouple is a very important temperature measuring element. It has the advantages of wide measuring range, convenient use, and high precision, so it is widely used in industry, agriculture, national defense and scientific research.

Pt-Rh thermocouples are widely used in various high-temperature furnaces and other heating equipment to detect or control temperature. They are precious metal materials. Correct use of platinum-rhodium thermocouple not only saves materials and funds for the country, but also obtains accurate temperature measurement. One of the main reasons for the errors of platinum-rhodium thermocouples is parasitic potential.

The basic structure of platinum-rhodium thermocouple:

The basic structure of Pt-Rh thermocouple for industrial temperature measurement includes thermocouple wire, insulating tube, the protective tube, and junction box.

Common platinum-rhodium thermocouple wires and their properties:

1. Platinum-rhodium 10-platinum thermocouple (scale S, also known as the single platinum-rhodium thermocouple) whose positive component is the platinum-rhodium alloy containing 10% rhodium.

Negative extremely pure platinum; its characteristics are:

(1) Thermoelectric properties are stable, anti-oxidation is strong, suitable for continuous use in the oxidizing atmosphere, long-term use temperature can reach 1300 C, over 1400 C, even in the air, pure platinum wire will recrystallize, making the grain coarse and fracture;

(2) High accuracy. It is the highest accuracy of all thermocouples and is usually used as a standard or for measuring higher temperatures.

Platinum-rhodium thermocouple is a common temperature measuring element in the temperature measuring instruments. It measures temperature directly, converts temperature signal into the thermoelectric potential signal, and converts temperature of the measured medium through the electrical instrument (secondary instrument). Installation methods of thermocouples are as follows: the main fixing methods are the threaded connection, flange connection, and welding. Their application occasions and requirements are different, mainly based on pressure, temperature and other parameters. So what aspects should we pay attention to when installing and using?

1. When using thermocouple compensating conductor, attention must be paid to the type matching, the polarity cannot be disconnected, and the temperature of the compensating conductor and the thermocouple connecting end cannot exceed 100 C.

2. When the temperature is measured or controlled by a thermocouple with a large time constant, the temperature shown by the instrument fluctuates very little, but the actual temperature of the furnace may fluctuate greatly. In order to measure temperature accurately, thermocouples with small time constants should be selected.

3. After power-on according to the instrument wiring diagram, the instrument first displays the thermocouple indexing number of the instrument, then displays the range of the instrument, and then measures the setting temperature of the digital tube display in the lower row of the instrument, and the digital tube display in the upper row of the instrument.

4. The thermal couple wire cannot be installed in the area where the measured medium seldom flows. When measuring the gas temperature in the tube with the thermocouple, the thermocouple must be installed in the opposite direction of the flow velocity and be in full contact with the gas.

5. In order to make full heat exchange between the measuring end of the thermal couple and thermal resistance and the medium under test, the measuring point should be selected reasonably and the valve should be avoided as far as possible.

6. According to the law of intermediate conductor, in the practical application of thermocouple temperature measurement, hot-end welding and cold-end open circuit is often adopted, and the cold-end is connected with the display instrument through a connecting wire to form a temperature measurement system.

Pt-Rh thermocouples are suitable for high-temperature applications in various production processes. They are widely used in powder metallurgy, sintering furnace, vacuum furnace, smelting furnace, glass, steel-making furnace, ceramics, and industrial salt bath furnace.
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Pyrolytic Boron Nitride: Layered Structure and Simple Quality Assurance

Boron nitride is an interesting advanced ceramic material. Although it’s also an electric insulator, low coefficient thermal expansion and has great corrosion resistance, the hardness is quite low (only 4, similar with graphite). Compared to more conventional alumina and zirconia, it's a machinable material due to the low hardness. Pyrolytic boron nitride is a thin (1~3mm) boron nitride material produced by CVD process.

When normal hot pressed boron nitride acts more like graphite, except for the electric conductivity, pyrolytic boron nitride has quite a lot different properties due to the structure. As the material is “grown” from chemicals around in gas phase, PBN by nature has a layered structure. The strong, thin and paralleled slices of PBN give this material some flexibility, which is abnormal for most of ceramics, including hot pressed boron nitride. Although thin and semi-transparent, PBN is not that brittle as it looks like. It is actually quite hard to break a PBN thick disc.

*It takes the quite large force to take this 3mm PBN disc apart, and it’s significantly layered before broken.



Hot Pressed Boron Nitride material is used to make Boron Nitride crucible for growing single crystals in most cases. For this application, it would be a serious issue if the slices tear apart from each other. As the crystal growth process requires accurate temperate control, the lower thermal conductivity caused by the layered structure will be harmful. During the manufacturing, it is common to build up some internal force inside the PBN material, especially on the tips, and this force is the major reason for layered defects for PBN products. That's why the VGF crucible is harder to make and cost considerably more than simple shape crucibles.

It is quite simple to examine if a PBN part is layered. As the material is usually quite thin, it is semitransparent. Under strong lights, there will be the shadow at the layered area. Though it looks normal from outside, layered products should be treated as unqualified products.

*Layered crucible tip. It can be detected with the LED on a cell phone



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What is Tungsten Alloyed With?

Tungsten heavy alloys, also called WHAs, Virtually all commercial WHAs are two-phase materials, with the principal phase being nearly pure tungsten in association with a binder phase containing transition metals plus dissolved tungsten.
WHA provides a very high density, as is apparent when compared to other metals, as shown in the table as follows:


WHAs was once referred to as “tungsten heavy metals,” but that nomenclature has been largely abandoned so as to avoid confusion with toxic heavy metals such as Pb, Hg, and others with which WHAs have no relation. While the primary selection is made on the basis of very high density, WHA provides tungsten alloy manufacturers a unique set of associated engineering benefits which includes:
• Low toxicity, low reactivity surface character
• Ability to be custom manufactured in a wide range of sizes and shapes
• Readily recycled for economy and environmental friendliness.
• Strength comparable to many medium carbon steels
• Ability to be machined with common shop tools and techniques
• High elastic stiffness
• Low CTE in combination with relatively high thermal conductivity

Due to these characteristics, as an ideal material, WHA is widely used for new mass property applications as well as the replacement of Pb or U in existing applications.

As a consequence, WHAs display a unique property set derived from both components — their fundamental properties resulting from those of the principal tungsten phase. The selection of a WHA for a given application is typically made on the basis of very high density — whether gravimetric or radiographic.

Tungsten heavy alloys are distinctly different from related materials such as pure tungsten metal and cemented carbides (most commonly WC-Co). Only very rarely could one material be used as a substitute for another. WHA provides many of the properties of pure W, yet in a form that provides:
• Lower fabrication cost due to the reduced sintering temperature,
• A greater range of both size and shape can be manufactured due to full density attainment via liquid phase sintering (LPS) as opposed to final densification by post-sinter thermomechanical processing,
• Generally improved machinability,
• Preservation of desirable properties of pure tungsten.

Tungsten heavy alloys are not related to “tungsten (T grade) steels.” In difference to pure W however, tungsten heavy alloys are not high temperature materials. Elevated temperature properties of WHA are strongly influenced by the lower melting temperature binder phase.
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Why is Boron Nitride Slippery?

Most people want to learn more about the 'soft' and 'slippery' crystalline nature of boron nitride, because of this special property, Hot Pressed Boron Nitride is used in lubricants and cosmetic preparations.

Firstly, we must know the definition of boron nitride.
What is Boron nitride?
Boron nitride (BN) was first found in 1840's by an English chemist, W.H.Balmain, by using molten boric acid and potassium cyanide, but unfortunately, he found that this new compound was un‐stable and required many methods to obtain a stable boron nitride.

For nearly a hundred years studies on boron nitride remained in laboratory scale due to the technical difficulties of different production techniques and high cost of the material which is obtained with these synthetic methods but in 1950's Carborundum and Union Carbide companies tried to obtain high purity boron nitride powder on an industrial scale and fabricated shaped parts of boron nitride for commercial applications with sophisticated hot pressing techniques.


What are the Properties of hexagonal boron nitride?

There are the main boron nitride properties as follows:
Non toxicity,
Easily machinability- non-abrasive and lubricious,
Chemical inertness,
Non-wetting by most molten metals,
High thermal conductivity,
Low thermal expansion,
Good thermal shock resistance,
High electrical resistance,
Low dielectric constant and loss tangent,
Microwave transparency.

Hexagonal boron nitride is being widely used because of its unique combination of properties which include:
Chemical inertness (corrosion resistance against acids and molten metals),
High temperature stability (melting point near 2600ºC),
Low density (2.27 g.cm-3 theoretical density),
Stability in air up to 1000ºC (in argon gas atmosphere up to 2200ºC and in nitrogen up to 2400ºC), Stability to thermal shock,
Easy workability of hot-pressed shapes,
Excellent electrical insulating character
Very high thermal conductivity.

As a thermal conductor, BN ranks with stainless steel at cryogenic temperatures and with beryllium oxide, BeO, at elevated conditions; above 700ºC, the thermal conductivity of hexagonal boron nitride exceeds that of toxic BeO.

The particular interest are its good dielectric properties (dielectric constant is 4, i.e half of that of α-Al2O3 ), also high dielectric strength and its ability to lubricate over a wide range of temperatures.

Its small coefficient of friction is retained up to 900ºC, whereas other solid lubricants like graphite and molybdenum disulphide are burnt away at lower temperatures.

Because of its high temperature stability and inertness against carbon and carbon monoxide up to 1800ºC it is as a refractory ceramic superior to the nitride ceramics Si3N4 and AlN and the oxide ceramics magnesium oxide, CaO, zirconia. ,

Due to its non-wetting properties it is stable to attack by molten glass, molten silicon, boron, nonoxidizing slags, molten salts (borax, cryolite) and reactive metal melts (e.g Al, Fe, Cu, Zn).

Because of its poor sinterability, dense shapes of hexagonal boron nitride are obtained almost exclusively by hot-pressing.

It must be recognized that the most chemical and physical properties of axial hot-pressed BN shapes depend on the nature and the amount of additives used for densification (up to 6 wt. % of B2O3, metal borates or SiO2 ).

Further some thermal (coefficient of expansion, thermal conductivity) and mechanical (flexural strength, Young's modulus) property values vary according to the direction of hot-pressing, BN being similar to graphite in respect of anisotropy.

By hot-pressing isostatic of canned boron nitride powder, theoretically dense and pure hot pressed boron nitride shapes without texture and with improved properties can nowadays be obtained.
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