Materials and Chemicals

Hexoloy Silicon Carbide Tubes for Protecting Your Temperature Sensor

13 June 2018

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Temperature sensors provide essential data for many critical systems. For many applications, these sensors are exposed to harsh environments and therefore must be protected to guarantee the safe and normal operation of the system. Ideally, the temperature sensor’s protection from the harsh environment will not significantly alter the sensor’s operating parameters. Precise and accurate measurements are usually imperative for optimal system performance. In higher temperature environments with abrasive particulates, Hexoloy tubes provide unmatched protection without compromising sensor accuracy.

The old paradigm of traditional ceramics and ductile metals as protective sheaths for thermocouples is being challenged by a new class of silicon carbide (SiC). Hexoloy alpha silicon carbide (Hexoloy) is a single-phase silicon carbide product that is extremely pure and uniform. Silicon carbide has been mass produced since 1893 because of its hardness, but purity and porosity have always been limiting factors. Saint-Gobain Ceramic Materials has been a major contributor to the Silicon Carbide thermocouple protection tube industry for over 20 years. Improvements in the sintering process have led to unprecedented levels of purity, which have improved upon the material’s performance. In the harshest, most corrosive and abrasive environments, Hexoloy will outperform traditional commercially available ceramics and metal alloys.Figure 1: Hexoloy silicon carbide tubes for protecting temperature sensors. Source: Saint-Gobain CeramicsFigure 1: Hexoloy silicon carbide tubes for protecting temperature sensors. Source: Saint-Gobain Ceramics

Hexoloy is produced by pressure-less sintering of submicron silicon carbide powder in a proprietary extruding process. The sintering process results in a self-bonded, fine grained (less than 10 μm) SiC product. Densities of fired parts are consistently in excess of 95 percent of the theoretical density of silicon carbide (3.21 g/cm3). Extremely hard and lightweight, Hexoloy has a high resistance to corrosion, high temperatures and thermal shock while also having a high thermal conductivity. This makes Hexoloy an ideal material for temperature sensor tubes in a variety of traditionally challenging environments such as incinerators, chemical processing and furnaces.

Due to its high density, low porosity and chemical inertness, Hexoloy has universal corrosion resistance, meaning that it is suitable for almost all chemically corrosive environments. It has superior oxidation resistance due to a silicon dioxide film that forms after sintering. It can be used in the presence of hot liquids or gases, strong acids, bases or oxidizers. Since it is made without binders, Hexoloy thermocouple protection tubes will not contaminate process media making it suitable for high purity processing applications.

A high thermal conductivity, five times that of alumina, means that small temperature changes can be accurately captured with high sensing speed and no hysteresis. Hexoloy also has a low coefficient of thermal expansion enabling its use in extremely high temperatures up to 1,950 degrees Celsius (dependent on application). It can withstand sudden temperature changes with a resistance to thermal shock more effectively than tungsten carbide, aluminum oxide and Reaction Bonded silicon nitride. In general, these thermal properties make Hexoloy a promising alternative to ductile metals in high temperature applications.

In addition to its excellent chemical and thermal properties, Hexoloy has exceptional hardness of 2,800 kg/mm2. This makes it 50 percent harder than tungsten carbide. With a specified wear rate (pin on disc) of SiC versus SiC 1x10-9 mm2/kg, coefficient of friction (pin on disc) SiC versus SiC 0.2, Hexoloy is excellent for applications involving high abrasion or sliding wear. Even at temperatures as high as 1,650 degrees Celsius in air, or up to 1,950 degrees Celsius in argon/nitrogen, Hexoloy can withstand abrasion, making it ideal for waste incinerators, gas, oil, coal or biomass fired chambers. Yet, Hexoloy is also light, weighing 40 percent as much as steel and about the same as aluminum.

Hexoloy has an as-fired surface finish of 32-64 micro-inches. The excellent surface finish of Hexoloy combined with tight dimensional control leads to parts that require little to no machining or finish grinding. Mass production of thermocouple tubes with complex shapes by extrusion and pressure forming are available. Consolidation by sintering at temperatures above 2,000 degrees Celsius results in a single-phase, fine-grain silicon carbide product that’s extremely pure and uniform, with virtually no porosity. Hexoloy tubes for temperature sensors are available in a variety of diameters and lengths specific to the parameters and constraints of the application.

In summary, the combination of Hexoloy’s thermal, chemical and physical properties makes it uniquely qualified for protective temperature sensor tube applications in challenging environments. These properties, as well as Hexoloy’s high thermal conductivity and low thermal expansion make it the perfect material for protecting a temperature sensor in a tight space while still providing a quick and accurate thermal response.

As technology advances and chemical processors, incinerators and other systems become more precise and efficient, materials involved with those systems must necessarily evolve in performance as well. Hexoloy is a next-generation material designed to perform in a variety of environments, at high temperatures, with precision. It provides the performance and protection thermocouples need in next-generation systems.

For more on Hexoloy, visit Saint Gobain.

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