Use of Thermocouples in Industrial Ceramics Applications

Measurement of high temperatures, greater than 1000 C, using ceramic based thermocouples is a well established process. In order to do this successfully the resistance wires within the thermocouple need to be insulated and protected.

The use of Ceramic materials to protect the platinum based wire is a method that has been used for many years, due to the exceptional heat resistant properties of these materials. The thermocouple is the most commonly used heat measurement implement and is used for measuring the temperature inside of a furnace, typically being used for the melting or heat treatment of metal or indeed for manufacturing ceramic products.

The ceramic components in a typical thermocouple device are the outer protection tube that is exposed to the furnace. In some cases multiple tubes are used inside each other to provide the level of insulation and protection required.

Ceramic insulators (Tubes with 2 or more core holes through them) are also used to isolate the two or more wires that go to the bi-metallic joint at the head which is the point from which the temperature is measured.

The typical material used for the tube is a 99.7% alumina material, impervious tube, which is attached typically to a metal thermacouple head, containing a terminal block which is used to connect to a standard wire to the instrumentation. In a less demanding heat atmosphere a mullite tube may be used as a cheaper alternative, which will work sufficiently up to 1600 'oc. The mullite tube is also an impervious tube and has 60% alumina content.

Ceramic materials are ideal for many high temperature applications but are also ideally suited to aggressive wear and chemical situations.

Alumina and Zirconia crucibles

The link between metals and ceramics is a long one. Archaeologists have found evidence linking the use of ceramics to contain molten metal as far back as 6000BC. Ceramic containers (Crucibles) continue to be used to this day.

Whereas the basic principals of casting have remained the same over the years, the technology has changed significantly. Methods of melting have changed with the advent of mains power although believe it or not there are still a small number of foundries in the UK melting on an open fire. The ceramics used are also much more technically advanced.

As more stringent requirements for castings such as jet engine components and other high-tec applications have developed then the ceramics used for these materials and the methods to produce the ceramics have continued to evolve.

Alumina and Zirconia are the materials of choice for these high end applications. Crucibles made from these materials can be formed in a variety of ways giving differing performance characteristics. It is often the case that high end zirconia crucibles are used as a matter of course as this is the safest option to prevent contamination. The problem with this is that in a lot of cases this results in a crucible spend considerably higher than necessary.

As with all applications involving ceramics, the selection process for the material and production method is critical if you are to achieve the most cost effective solution.

Design thinking for industrial ceramics

With the current state of the industrial economy around the world this is an ideal time to consider the use of alternative materials for your applications and processes. Industrial ceramic materials, for example, offer a vast array of compositions and performance characteristics and can be a cost effective alternative in many harsh environments such as high temperature, electrical resistance, wear applications and chemical contact.

One thing however should be considered carefully in the design of ceramic alternatives. The tolerance of a dimension on a metal part may only have minor effect on cost. The same can not be said about ceramic and the cost difference for a slightly tighter tolerance can be significant.
Very tight tolerancing can be achieved on ceramic components. The question you should always ask yourself is “Do we need it?”. This should also be the case for any standard tolerances listed on the drawing. If you need precision this can be achieved, but where you don’t need it, money can be saved.

As the accuracy of formed ceramics can vary greatly, it is important to chose the correct forming method. Careful selection of the process used to make the part or indeed to form the base part for later machining will keep the costs to a minimum. The use of a slightly more expensive forming process can at times save a considerable amount of machining.
For improved tolerance the parts can be machined “green” (before firing) but for very tight tolerances then the parts must be machined after firing. Finish machining is often difficult and slow. This can add significant cost so should only be used when necessary.

The removal of unnecessary tolerancing and features such as chamfers can often result in massive savings. It is not uncommon for a part with excessive tolerance requirements to cost several times as much as a part correctly designed to be made in ceramic and to suit the application.

In conclusion, it is worth fully exploring alternatives materials such as industrial ceramics , to determine what cost savings, or product life and performance enhancements can be achieved. Remember however, alternate materials may necessitate a change of design thinking in order to maximise the advantages they can offer.

Thermal shock in industrial ceramics

There are many reasons for thermal shock failure in industrial applications of ceramics. On analysis they usually come down to one or more of the following factors.

Material selection
Material processing
Design of component

Application/use of the product

It is often possible to improve the performance by changing one or more of these but as with all ceramic applications thermal shock is only part of the equation and changes must be looked at in context of all the performance requirements.

When designing any product in ceramic it is necessary to look at the overall requirement and often then to find the best compromise that will work.

In high temperature applications, thermal shock is often the main cause of failure. It is comprised of a combination of thermal expansion, thermal conductivity and strength. Rapid changes in temperature both up and down cause temperature differentials within the part, not unlike a crack occurring by putting an ice cube against a hot glass. Movement through differing expansion/contraction leads to cracking and failure.

There are no simple answers to the thermal shock issue however the following guidelines do tend to be beneficial.

Select a material grade that has some inherent thermal shock characteristics but meets the needs of the application. Silicon carbides and silicates are excellent. Alumina based products are less good but can be improved with the right design.
Porous products are generally better than impervious and will take larger changes in temperature.
Thin walled products perform better than thick wall. Also avoid large transitions in thickness throughout the part. Sectional parts may be better as this provides less mass and offers a Pre cracked design alleviating stress raisers.
Minimise the use of sharp corners as these provide ideal starting points for cracks.
Avoid tension loading of the ceramic. Parts can be pre stressed through design to help alleviate this problem.
Where possible look at the application process to see if it is possible to provide a more gentle change in temperature. Pre heating the ceramic or reducing the rate of temperature change.

The above points will help alleviate thermal shock problems but it is always best to discuss the situation with experts in the field.

Industrial Ceramics Articles

Use of Thermocouples in Industrial Ceramics Applications

Alumina and Zirconia crucibles

Design thinking for industrial ceramics

Thermal shock in industrial ceramics