Truth about Zirconia Products

While you are using Zirconia products, you should know how to take care of it. There are some fact that should be known to you about the metal if you are handling it. The products made of Zirconia should be stored in a dry place. Never use a wet crucible, always pay attention to dry it naturally before using it. If you are drying the wet Zirconia crucible then ensure that the process should be slow enough.

Therefore it is significant that you should pay a little attention when you use a Zirconia product. The necessary tips if followed well can enhance the life of your Zirconia products. It has been seen that Zirconia coil products are most fragile in nature. So careful handling is necessary. Always avoid a collision during the process of unpacking, handling, transportation as well as cleaning. Pay attention to identify if any micro-cracks present before using zirconia products. Any product with a micro-crack is not advised to be used. Only put that much material at one time in your zirconia crucibles which can get proper heating. Overloading the crucibles will only result in uneven heating. To minimize the thermal shock of the vessel it is mandatory to maintain a slow ramp rate as well as a sluggish cooling rate.

There may be a possibility that if the crucible is taken immediately out of the direct furnace right away from a high temperature it may cause a crack. So take proper care while you are handling the crucible in a higher temperature. Make sure that the vessel can only be removed from the heat while the temperature is below 100°C.

Uneven heating of the Zirconia crucibles can cause a crack. So do not use it for heating through torch or furnaces where you cannot control the temperature change rate. It is suggested that if you are using an induction furnace to heat the crucible then it should be surrounded through zirconia powder.

However, all the above tips and information is only a guide. The main intention of the topic is to warn you about the behavior of the metal. The metal may behave in a different way depending on its operating technique. The novice users may get benefitted with the help of the tips and advice, while the experienced users, can become more efficient if they follow it seriously. However, this guide is not the final end-all of information.
For more information about Zirconia and other advanced materials, please visit http://www.samaterials.com/

Pyrolytic Boron Nitride: Layered Structure and Simple Quality Assurance

Boron nitride is an interesting advanced ceramic material. Although it’s also an electric insulator, low coefficient thermal expansion and has great corrosion resistance, the hardness is quite low (only 4, similar with graphite). Compared to more conventional alumina and zirconia, it's a machinable material due to the low hardness. Pyrolytic boron nitride is a thin (1~3mm) boron nitride material produced by CVD process.

When normal hot pressed boron nitride acts more like graphite, except for the electric conductivity, pyrolytic boron nitride has quite a lot different properties due to the structure. As the material is “grown” from chemicals around in gas phase, PBN by nature has a layered structure. The strong, thin and paralleled slices of PBN give this material some flexibility, which is abnormal for most of ceramics, including hot pressed boron nitride. Although thin and semi-transparent, PBN is not that brittle as it looks like. It is actually quite hard to break a PBN thick disc.

*It takes the quite large force to take this 3mm PBN disc apart, and it’s significantly layered before broken.



Hot Pressed Boron Nitride material is used to make Boron Nitride crucible for growing single crystals in most cases. For this application, it would be a serious issue if the slices tear apart from each other. As the crystal growth process requires accurate temperate control, the lower thermal conductivity caused by the layered structure will be harmful. During the manufacturing, it is common to build up some internal force inside the PBN material, especially on the tips, and this force is the major reason for layered defects for PBN products. That's why the VGF crucible is harder to make and cost considerably more than simple shape crucibles.

It is quite simple to examine if a PBN part is layered. As the material is usually quite thin, it is semitransparent. Under strong lights, there will be the shadow at the layered area. Though it looks normal from outside, layered products should be treated as unqualified products.

*Layered crucible tip. It can be detected with the LED on a cell phone



For more information, please visit http://www.samaterials.com/

Why is Boron Nitride Slippery?

Most people want to learn more about the 'soft' and 'slippery' crystalline nature of boron nitride, because of this special property, Hot Pressed Boron Nitride is used in lubricants and cosmetic preparations.

Firstly, we must know the definition of boron nitride.
What is Boron nitride?
Boron nitride (BN) was first found in 1840's by an English chemist, W.H.Balmain, by using molten boric acid and potassium cyanide, but unfortunately, he found that this new compound was un‐stable and required many methods to obtain a stable boron nitride.

For nearly a hundred years studies on boron nitride remained in laboratory scale due to the technical difficulties of different production techniques and high cost of the material which is obtained with these synthetic methods but in 1950's Carborundum and Union Carbide companies tried to obtain high purity boron nitride powder on an industrial scale and fabricated shaped parts of boron nitride for commercial applications with sophisticated hot pressing techniques.


What are the Properties of hexagonal boron nitride?

There are the main boron nitride properties as follows:
Non toxicity,
Easily machinability- non-abrasive and lubricious,
Chemical inertness,
Non-wetting by most molten metals,
High thermal conductivity,
Low thermal expansion,
Good thermal shock resistance,
High electrical resistance,
Low dielectric constant and loss tangent,
Microwave transparency.

Hexagonal boron nitride is being widely used because of its unique combination of properties which include:
Chemical inertness (corrosion resistance against acids and molten metals),
High temperature stability (melting point near 2600ºC),
Low density (2.27 g.cm-3 theoretical density),
Stability in air up to 1000ºC (in argon gas atmosphere up to 2200ºC and in nitrogen up to 2400ºC), Stability to thermal shock,
Easy workability of hot-pressed shapes,
Excellent electrical insulating character
Very high thermal conductivity.

As a thermal conductor, BN ranks with stainless steel at cryogenic temperatures and with beryllium oxide, BeO, at elevated conditions; above 700ºC, the thermal conductivity of hexagonal boron nitride exceeds that of toxic BeO.

The particular interest are its good dielectric properties (dielectric constant is 4, i.e half of that of α-Al2O3 ), also high dielectric strength and its ability to lubricate over a wide range of temperatures.

Its small coefficient of friction is retained up to 900ºC, whereas other solid lubricants like graphite and molybdenum disulphide are burnt away at lower temperatures.

Because of its high temperature stability and inertness against carbon and carbon monoxide up to 1800ºC it is as a refractory ceramic superior to the nitride ceramics Si3N4 and AlN and the oxide ceramics magnesium oxide, CaO, zirconia. ,

Due to its non-wetting properties it is stable to attack by molten glass, molten silicon, boron, nonoxidizing slags, molten salts (borax, cryolite) and reactive metal melts (e.g Al, Fe, Cu, Zn).

Because of its poor sinterability, dense shapes of hexagonal boron nitride are obtained almost exclusively by hot-pressing.

It must be recognized that the most chemical and physical properties of axial hot-pressed BN shapes depend on the nature and the amount of additives used for densification (up to 6 wt. % of B2O3, metal borates or SiO2 ).

Further some thermal (coefficient of expansion, thermal conductivity) and mechanical (flexural strength, Young's modulus) property values vary according to the direction of hot-pressing, BN being similar to graphite in respect of anisotropy.

By hot-pressing isostatic of canned boron nitride powder, theoretically dense and pure hot pressed boron nitride shapes without texture and with improved properties can nowadays be obtained.
For more information, please visit http://www.samaterials.com/