1. How do you manufacture (extract) high temperature MgO?
The manufacture of MgO, from raw materialsto finished products, is included in the presentation that I have prepared.
2. If the purity of the MgO increases, does the leakage current become less?
No, often the leakage current will increaseas the purity of MgO increases, depending on the particular chemistry of the MgO. For an example, see the results shown in question 3.
3.What is the
insulation resistance of 99.9% MgO between 750C and 1000C?
I have some data for 99.5% MgO, which shows low Mohms compared to typically 22SR (chart included in presentation).
4. What MgO should be
used for heating elements that operate at 1150C?
Both 22SR and PG can be used in this
application, with 22SR providing the lowest leakage and greatest element life.
5.What are the mA
leakage and Mohms at 300C to 1150C?
The actual MgO leakage and resistance is
dependant on the particular heating element design. UCM measures leakage against Watt density,
not temperature. However, the ASTM
Specific Impedance test does compare Mohm-cm and temperature (Specific
Impedance and Watt-Density charts in presentation).
6.What high temperature MgO should be used to reduce noise caused by leakage current and lengthens heating element life?
The best high temperature MgO to reduce leakage and extend elementlife is 22SR.
7.Does higher F Density
result in extended heating element life?
Not necessarily. High F Density MgO can improve heating
element life compared to extremely low F Density MgO. Since optimum compaction density in the
heating element, which optimizes heat transfer from heating coil to sheath, is
required to extend heating element life, UCM low F Density or high F Density
MgO powders are designed such that the optimum compaction can be achieved.
8.Does filling (tapping)
to the maximum fill density damage the heating element?
The UCM MgO products are designed such that
the maximum fill density achieved during filling will not damage the heating
element during normal roll reduction or pressing. Yes, it is possible to do harm to the heating
element by over compaction the MgO powder and crushing the MgO to a very fine
powder, thus reducing the thermal conductivity of the MgO. Over compaction can also result in damage to
the wire, which may result in hot spots in the element and higher leakage.
9.What causes heat-treated elements to show higher leakage than elements that are not heat-treated? If heat-treating in air means annealing in an oxygen atmosphere, the the problem could be excessive oxidation of the pins and sheath leading to higher leakage. If Silicone Treated (ST) MgO is used, the annealing process could be decomposing the silicone, thus resulting in higher leakage.
10.What causes blacking
of MgO in elements operated 12 hours at 1150C?
One cause of blackening of MgO is the operation of an MgO filled heating element over 700C when the resistance wire is made of a FeCrAl alloy. This blackening is a result of the movement of the Fe into the MgO. MgO can also take on a black color using NiCr wire if the sheath or wire is not cleaned properly. The black color is the result of the decomposition of forming oil on the sheath or winding oils on the wire.
11.What component of
the MgO reacts with NiCr wire resulting in reduced heating element life?
The MgO does not react with the NiCr wire. In a heating element operated at high temperatures, the Cr in the NiCr wire starts to move from the wire into the MgO. This is the normal failure mode of heating elements. If oxygen is readily available, such in an unsealed heating element, the NiCr wire forms a Cr2O3 layer on the surface of the wire, reducing the movement of the Cr into the MgO, thus increasing the heating element life.
12.Does UCM guarantee “super insulation resistance” as does ASAHI for GX-D?
Yes, 22SR has a very high insulation resistance, considered betterthan any of our competitors.
13.How can the
insulation resistance of MgO in high (1150C) and medium (750C) temperature
heating elements be sustained?
First, in both cases, NiCr wire should be used. Second, do not hermetically seal the heating, but allow it to “breath” to allow for the formation of Cr2O3 on the wire, thus extending the heating element life. In hermetically sealed heating elements, the Cr from the NiCr wire rapidly migrates into the MgO, thus reducing the wire diameter and increasing resistance, which causes the heating element to operate at a higher temperature. This higher operating temperature accelerates the migration of the Cr away from the wire into the MgO, resulting in failure of the heating element prematurely.
14.Is it possible for PbO end seals to react with the MgO to cause poor insulation resistance?
I am not familiar with PbO end seals, but since the ends of the terminals are normally relatively cool, it is not likely that the PbO will react with the MgO. My only concern would be the electrical conductivity of the PbO used.
15.What is the electrical character of MgO?
MgO is a natural electrical insulation. What makes if better than other materials is that it also has a very good thermal conductivity, thus allowing the heat to move rapidly from the coil to the sheath. So, the combination of high electrical resistance along with high thermal conductivity makes MgO the best material for the production of heating elements. Also, the hardness of MgO is similar to the resistance wire, so MgO minimizes the damage to the wire during compaction, Particular insulation resistances of UCM MgO will be covered in the presentation.
16.What is the
relationship between Mesh and Tap Density?
The sieve analysis or meshes depict the particle size distribution while the Tap Density depicts the typical fill density of the MgO in a heating element. By increasing the coarse (+60 mesh) and fine (-325 mesh) fractions of the particle size distribution, the Tap Density can be increased. However, there are limitation to the amount of +60 mesh and –325 mesh that can be used in a filling machine. An excess of +60 mesh may cause bridging in the element and/or result in the inside of the resistance coil not being filled. Excessive –325 mesh can result in poor flow properties, which could result in poor fill density and/or incomplete fill.
17.What are the merits
of the various shaped particles of MgO?
Since MgO naturally has a cubic crystal structure, and cubes do not flow well or pack well, UCM attempts to “polish” the particles to achieve as round a particle as possible to enhance flow and promote particle packing in the filling machine.
18.What temperature is necessary to decompose magnesium hydroxide and remove the water from the MgO? The decomposition temperature of magnesium hydroxide is about 270C.
19.Is carbon decomposed
at 750C?
The decomposition temperature of carbon is dependant on the form the carbon is in and whether oxygen is present. Carbon contamination in heating elements is usually in the form of an organic compound, such as oil. In this case, the carbon compound will react with oxygen to form gaseous carbon monoxide or carbon dioxide, which can then exit the heating element. This can occur at temperatures lower than 750C, depending on carbon compound and the availability of the oxygen.
20.What is the leakage
current of MSM MgO products?
The actual insulation resistance and leakage current of an MgO in a heating element is dependant on the dimensions of that heating element, so these values vary from one element to another (chart of the typical leakage of the UCM products in the test UCM test element included in the presentation).
21.Is 22SR better for high temperature heating element than Tateho H?
Tateho produces numerous grades of MgO, the designations changing from one customer to another. I am not familiar with H, but I can say that the MSM 22SR is considered a superior product when compared to our competitors. I will be happy to test a sample of Tateho H and report our findings.
22.Is the UCM leakage current and insulation resistance better than Tateho?
Competitive product testing and customer feed back have bothconfirmed that the UCM products are better than the Tateho products.
23.What is the status of the UCM silicone treated products?
UCMtypically adds 0.35% silicone oil to MgO products to provide a moisture barrier throughout the life of the heating element. These ST MgO powders are limited in applications where the operating temperature of the heating element is not more than about 400C. For high temperature MgO powders to be used in heating elements that operate above 400C, UCM adds a light silicone treatment, LST that improves the shelf life of the MgO by reducing moisture absorbance of the MgO. A discussion of ST MgO will be included in the presentation.
24.What causes the
insulation resistance of the heating element made with 22SR to drop during
operation, sometimes recovering and sometimes not?
There are several things that would account for such failures, such as, carbon contamination (dirty sheath or wire), or moisture in the MgO after annealing. I would need more details, and possibly a failed element, to better analyze the failure mode.
25.Why is the heat resistance temperature the same for 33 ST and 33 LST? Can I assume that “heat resistance temperature” means application temperature?
Of course, 33 LST can beused in application at higher temperatures than can be the 33 ST, solely based of the decomposition temperature, about 450C, of the silicone fluid. All UCM recommended application temperatures for MgO products is based on the non-silicone treated product, since the silicone can reduce the application temperature of any product.
26.Does LST MgO from
the bottom of the can contain more silicone fluid, causing blackening of the
MgO?
The amount of silicone in the LST products does not vary within the container. Furthermore, the amount of silicone used in LST products will not cause blackening of the MgO in a heating element. There just is not enough carbon present in the LST MgO to cause blackening. The blackening is probably caused by contamination in the sheath or on the wire, or the result of using Fe based wire, and just happened to occur in elements made from MgO in the bottom of the container.