Hot-Zone Design Flexibility Handles Specific Processing Needs
Thermal processing plays an important role in many manufacturing and research programs and has a significant impact on the success of an organization. Vital to good thermal processing is the selection and design of a vacuum furnace hot zone. There are many factors to consider in selecting a vacuum furnace that will handle specific process conditions. Advances in furnace technology, including materials and fabrication methods for heating elements, shields and retorts, have enabled greater flexibility in designing vacuum furnaces for many uses. Temperature is important, but it is just part of the overall picture because furnaces span a wide range of applications from vacuum brazing to hot pressing, from surface modification to sintering and heat treatment. Some of the many issues that should be reviewed before specifying a furnace are presented.
The thermal process must be specified completely, which may require prototyping new applications to determine actual working parameters including the thermal, physical and chemical environment to which the workload will be subjected. Furnace temperatures, including operating temperature, maximum temperature, temperature ramp-up rate(s) and thermal uniformity, are essential to arrive at the optimum furnace design. Operating temperature affects heating element, shield and retort materials of construction, and also impacts power requirements and control-system design. Hot zone size and shape affects the efficiency and effectiveness of a furnace, whether a small laboratory furnace or a large production system.
Other considerations include determining whether to use a horizontal hot-zone configuration (Fig. 1), which can offer better workflow, or a vertical configuration for bottom loading (Fig. 2). A retort may be required to isolate gases from hot zone components in applications where special gases are used or where a gas will carry a substance away from the product.
A vertical cylindrical hot zone lends itself to a bell jar chamber configuration, and the heating element can hang freely, being supported only at the top of the bell jar. Upper and lower trim heaters may be added to compensate for heat loss, thus optimizing temperature uniformity. A cylindrical hot zone offers maximum temperature uniformity.
As furnace temperatures increase and pressures decrease, the choices of materials and construction become more limited. Materials for the heating elements and shields typically are refractory metals, such as tungsten, tantalum and molybdenum, and/or high-purity graphite. Metal heating elements are fabricated from strip, sheet, wire and rod depending upon the configuration of the hot zone. Heat shields are necessary to contain the heat and provide thermal stability.
Hot zone material choices generally focus on the refractory metals tungsten, tantalum and molybdenum because of their high-temperature characteristics.
However, reaction between workload material and hot zone can be significant, resulting in alloying and subsequent reduction of hot zone life. For example, silicon and nickel from the workload material can evaporate and deposit on hot-zone components resulting in component degradation. Also, transfer of material from the heating element to the workload by means of physical or gaseous deposition can affect product quality. Table 1 shows the working temperature corresponding to a given rate of evaporation of 0.1 mg/cm2 h. (Note that tungsten evaporates at a rate of 60 mg/ cm2 h at 3000 C (5430 F).)
At higher temperature ranges (2400 to 3000 C, or 4350 to 5430 F, for example), creep strength of work support and heating elements also is a major engineering concern.
The heating element, which operates at a higher temperature than any other component in the system, is the most critical part of the system in terms of material choice. Heating element design takes into account thermal capability, material interactions, mechanical stability, configuration and service life. Figure 3 shows heating elements in different configurations.
Tungsten is used in the form of a BreWeave(tm) or mesh design. Weave radiation panel elements offer superior mechanical formability, strength, creep properties and ability to hold their shape for long service life. Tantalum and molybdenum usually are fabricated from sheet and strip for lower temperature service, but can be fabricated from BreWeave or mesh for longer life.
Furnace operation above 1600 C (2910 F) limits the use of ceramic material within the hot zone for suspending elements. Therefore, heating elements generally are suspended by their terminations and/or by hooks, which penetrate through the insulating shields with clearances to avoid electrical shorts (but which give additional heat loss). The hooks are secured outside the hot zone where lower temperatures allow use of ceramics.
System design examples
The Northwest Institute of Nonferrous Metal Research (NIN), National Rare Metal Material Processing Engineering Research Center in Xi'an, China required a furnace to sinter tungsten and other refractory metal parts and to conduct rare-metal processing research. With a controlled hydrogen atmosphere and process temperatures above 2500 C (4530 F), it represented a unique application challenge for one of the largest controlled-atmosphere furnaces of this type ever built.
TTI and NIN worked together to develop a furnace specification to satisfy NIN's needs and budget. A key requirement to the company's process quality is temperature uniformity of +/-10?C (+/-18?F) at temperatures above 2000 C (3630 F). This high level of temperature uniformity was accomplished by means of a special design that uses chandelier trim heaters. The top and bottom chandelier heaters were specifically designed for the large hot zone requiring tight temperature uniformity. The heaters enable tighter control of hot zone temperatures over long process periods by compensating for the normal radiation heat losses and providing additional control elements, which can be fine tuned. The chandelier trim heaters are of a tungsten-mesh construction, suspended for expansion/contraction within an end-bell structure, which provides thermal shielding with ease of access.
The bottom-loading batch furnace (Fig. 4) incorporates a large cylindrical tungsten heating element, which measures 20 in. in diameter and 36 in. tall (508 and 915 mm). The tungsten element allows operation at a temperature of 2500 C in hydrogen atmosphere for periods in excess of 24 hours per run. TTI manufactured the refractory metal heating elements using Breweave technology, which allows greater structural strength and longer life of the element. By fabricating the optimal type of element for each application, overall processing capabilities are enhanced, as well as long heater life.
According to NIN's Zhao TieFu, "The furnace is designed in an excellent manner, is easy to operate and maintain, and runs in a reliable manner. The furnace system is most suitable for production."
Tantalum Pellet Company Inc. (TPC), a division of TPC Hilton Capacitor, in Phoenix, Ariz., manufactures high quality anodes and cathodes for capacitors and electrodes for flashtubes. Tantalum foil capacitors are key electronic components in computers, consumer electronics and cellular phones. Within the capacitors are porous contacts made of tantalum powder, which have to be sintered under high vacuum.
The company required a controlled-atmosphere furnace to process the contacts. Typical sintering furnaces for this application normally would use one cylindrical heater, with the resultant hot zone being uniform for only 50% of the available volume. The challenge was to create a larger uniform hot zone to increase productive capacity for each run.
The hot zone uses tantalum heating elements and radiation shields for the sintering of tantalum powder parts to maintain process purity. Tantalum becomes more thermally conductive at higher temperatures (unlike molybdenum or tungsten), so you have more inherent heat loss with a tantalum element than with a molybdenum or tungsten element. That is, heat is conducted away from the element section through the termination points (the power leads and support pins). Therefore, heat loss is even more of a challenge when using a tantalum hot-zone furnace compared with that of a molybdenum or tungsten hot-zone furnace.
This required a new approach to furnace design to solve the problem. Designing top and bottom heating elements (Fig. 5) into the furnace enabled excellent temperature uniformity. Using a hanging cylindrical main heating element (Fig. 6) and top and bottom trim heating elements enabled controlling temperature within +/-5?C (+/-9?F) over the vertical hot zone. Thus, the full volume of the furnace (7 in. in diameter by 8 in. high, or 178 by 203 mm) is usable for processing. The design was not only able to satisfy process parameters in the sintering of the capacitor anodes, but also doubled the working capacity (Fig. 7).
The cylindrical design together with cylindrical work racks has distinct benefits compared with cuboid and rectangular hot zone designs. Heat is more evenly directed to the work and corner loss is reduced with a more even heat-loss system. The addition of the trim heaters further ensures even temperature control by having adjustable top and bottom heating. The furnace features automated controls having all the essential failsafe interlocks. This design permits completely unattended operation with excellent control of process parameters.
Temperature control of the elements is achieved with three separate PID closed-loop control systems. This arrangement provides a temperature-tuned hot zone having very low temperature gradients. A high vacuum level of 10-6 torr is obtained quickly in 15 minutes using a Varian VHS10 diffusion pump backed with a 54 cfm Leybold rotary vane mechanical vacuum pump.