INTRO: Alumina crushable insulators are used in place of MgO insulators for specific applications, normally in Aerospace or Nuclear industries. The endgame is to achieve a compacted powder insulation which retains it’s insulation properties even when the cable is bent into complex configurations.
Crushable insulators allow the production of Mineral Insulated Metal Sheathed (MIMS) cable (in long lengths) or probes (typically shorter complete units). The conductor wires may be thermocouple pairs, resistance wire for heating, or simply extension wiring for extreme environments. Crushable insulators are not suited for stand-alone applications and must be enclosed in a protective sheath.
Description: Crushable insulators are rigid tubes, rods, and multi-hole configurations available at lengths up to about 200 mm and generally round (not required) configurations of constant cross-section (required). This product is analogous to it’s MgO cousins, but is made from high quality Aluminum Oxide powders. TATEHO OZARK can extrude a variety of Alumina powders. For High Density extrusions, please see HLA.
Advantages
There are circumstances where MgO is not the best choice for a thermocouple or sensor application. The most common situation is in Nuclear applications in the presence of Radioactivity. Another exception might be because of a risk of moisture.
Bummers
The downside to Alumina as a substitute is that the powder grains are much more abrasive than MgO, and quickly can damage the conductor and sheath in a drawing or elongation operation.
The electrical properties are not quite as good as MgO, and the inertness to metallic elements is not as good for several metals. It is more difficult to fabricate and may require more dimensional tolerances. TATEHO OZARK maintains very few stock sizes on hand and requires longer lead times for production and higher minimum order quantities.
Grades
TATEHO OZARK offers two fundamental varieties and each has special concerns.
CCA grade is made from a vapor phase Alumina feed which is 99.9% pure. The high purity is obtained by synthesizing the product from pure elements instead of naturally occurring feedstocks. This process results in a small particle size and high surface area which makes the powder thermally reactive. This limits it’s use for simple one or two pass compactions, just enough to tighten the components in the sensor. It is not recommended for progressive drawing and annealing cycles which are typically performed to remove work hardening from the wire and sheath.
The high surface reactivity also complicates fabrication by TATEHO OZARK and requires that OD sizes greater than 5 mm require dimensional tolerance concessions.
CUA and CHA grades are fused Alpha Alumina products which are typically about 99.65% pure. The CUA grade is intended to meet ASTM type 1 specifications, but availability is unpredictable.
These products are much less reactive and are typically fired above 1300 C. Therefore, they will tolerate the thermal cycling required for serial drawing and annealing operations. However, it is still functionally an abrasive grit, and you can expect reduced coil lengths compared to MgO.
Material/Purity
See specifications for purity analysis of standard magnesium oxide and aluminum oxide grades. Other materials are available on request or by custom order.
The following are typical analyses or limits for powders used to produce extruded insulators. All raw materials are high quality fused grain powders which are processed in-house to obtain optimum particle size distributions for end use as crushable or swageable thermocouple insulators.
| GRADE | CUA | CCA | CLA |
|---|---|---|---|
| Al2O3 | 99.65% | 99.90% | 99.75 |
| Na2O | 0.01% | 0.0015% | 0.08% |
| SiO2 | 0.07% | 0.02% | 0.01% |
| MgO | 0.05% | 0.001% | 0.04% |
| CaO | 0.04% | 0.0015% | 0.04% |
| Fe2O3 | 0.03% | 0.017% | 0.04% |
| B | <10 ppm | --- | --- |
| S* | <5 ppm* | --- | --- |
| C* | 80 ppm* | --- | --- |
| Fired Density Range | typ. 2.2-2.4 | typ. 2.2-2.3 | typ. 2.2-2.3 |
| (% of Theoretical) | typ. 55%-60% | typ. 55%-60% | typ. 55%-60% |
* Powder analysis reported for reference, but Sulfur and Carbon fully volatalize in TATEHO OZARK processing and are not present in fired insulators.
STRENGTH
Insulators are fired to uniform mechanical strengths which have been established for optimum handling and swaging characteristics for different applications. A true mechanical test does not exist to characterize “crushability”. A Modulus of Rupture measurement is normally provided for historical tracking purposes. This test is a modified three point bending test performed according to ASTM standard E1652 Appendix 4 and has many limitations.
TATEHO OZARK offers the following guidelines for MOR spec ranges for Alumina:
| MOR rangen(psi) | strength | process firing shrinkage | nominal dimension range | process and elongation |
|---|---|---|---|---|
| 1500-4000 | soft | 0-0.5% | 3 mm - 10 mm | multi-pass operations, elongation 5-20X |
| 2500 - 6000 | medium soft to medium | 0.5-1.0% | 1.5 mm - 4 mm | multi-pass operations elongation 1.5-10X |
| 4000-10000 | medium to medium hard | 1.0-2.0% | 1.5 mm - 10 mm | limited elongation 1.5-5X |