Key Questions to Ask When Ordering Aluminium Ceramic Filter Plates

Alumina ceramics are advanced ceramic materials basically composed of aluminum and oxygen. Aluminum oxide ceramics are favored among many manufacturers for a variety of factors, such as high thermal conductivity, their insulating capability, resistance to corrosion, high melting point, and extreme hardness. For these reasons, they are preferred for several manufacturing processes. Here, we’ll discuss the properties, production, and application of alumina ceramics, as well as all other useful pieces of information about alumina ceramic materials. Let’s dive in already!

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What is Alumina Ceramic?

Alumina ceramic, also known as alumina or aluminum oxide (Al2O3), is an industrial oxide ceramic known for its extreme hardness and high thermal conductivity. The properties of alumina ceramics make them one of the most widely used ceramics for structural, wear, and corrosive environments. Alumina ceramics are typically manufactured from bauxite and can be shaped using injection molding, die pressing, isostatic pressing, slip casting, diamond machining, and extrusion. Like aluminum nitride and other important engineering materials, alumina can be produced by dry press and sinter or by hot pressing using appropriate sintering aids. Alumina-based ceramics are one of the most studied and characterized advanced ceramic materials known today. Due to the important combination of properties, their behavior has been thoroughly researched. Alumina is characterized by high hardness, excellent resistance to corrosion, thermal stability, good dielectric properties (for converting from DC to GHz frequencies), low loss tangent, and stiffness. See the next section for more details on the properties of alumina ceramic materials.

Properties of Alumina Ceramics

1. Depending on the content of Al2O3 and additives used, there are different series of aluminum oxide mateials.

• Based on the Al2O3 content, alumina can be categorized as 75% alumina, 85% alumina, 95% alumina, 99% alumina, and so on.
• Based on the difference in crystal phase, there are mullite, corundum-mullite, and corundum;
• Based on the variety of additives used, there is the chromium corundum, titanium corundum, etc.

2. Alumina is an excellent electrical insulator that is resistant to extremely high currents, and its resistance to electricity increases with its purity. The more pure alumina is, the higher the resistance would be.
3. Alumina is also known to have a very high melting point and strong mechanical strength. However, its mechanical strength decreases when the temperature goes above degrees Celsius. Due to the vast difference in its coefficient of thermal expansion, its resistance to thermal shock is less effective when exposed to very high temperatures.
4. The excellent chemical stability of alumina is the major contributor to its high resistance to corrosion.
5. Alumina is also slightly soluble in strong acid (such as hot sulfuric acid and the hot HCl and HF also have some corrosive effect.) and alkaline solutions, but does not dissolve in water. Pure alumina can resist chemical corrosion, making pure alumina the main choice for engineering components in several industrial applications. Its resistance to corrosion by chemicals has been shown to be due to its low solubility in those chemicals.
6. The introduction of additives can enhance the physical and chemical properties of alumina. These additives, combined with different production processes, can help in the production of alumina ceramics of varying sizes and shapes.
7. Alumina ceramic materials have the lowest vapor and decomposition pressures.
8. As you already know, alumina ceramics are similar to aluminum nitride based on their good thermal conductivity and good electrical insulation properties.

Application of Alumina Ceramics

1. Because of their excellent chemical stability, alumina ceramics are used widely in acid-resistant pump impellers, pump bodies, acid-carrying pipe linings and valves.
2. Aluminum oxide ceramics are used in the manufacture of textile wear parts and knives. This is due to their extreme hardness and wear resistance.
3. Aluminum oxide is the most commonly used abrasive grain in the woodwork and metalwork industry. This is because it does a better job than silicon carbide on several materials, including bare wood, painted surfaces, and metal.
4. Alumina ceramic materials are used in making spark plugs, which are useful in a wide variety of engines.
5. Transparent alumina ceramics are used in making high-pressure sodium lamps and infrared detection window material.
6. With an alumina content higher than 95%, alumina ceramics can be used as excellent electrical insulators. They also have a low dielectric loss, with a wide range of applications in electronics and electrical appliances.
7. Transparent aluminum oxide shows good permeability to visible light and infrared rays.
8. Other essential applications of alumina ceramics can be seen in seal rings, medical prostheses, laser tubes, thermocouple tubes, electronic substrates, ballistic armor, electrical insulators, grinding media, and wear components.

Further Reading: What is the Transparent Ceramic Material

Conclusion

Alumina ceramics are composed of aluminum oxide. The high thermal conductivity, high resistance to heat, excellent electrical insulation property, and high resistance to chemical corrosion of aluminum oxide mateials are some of the factors why they are a choice material for many industrial processes. Thank you for reading our article and we hope it can help you to have a better understanding of alumina ceramics. For more information, please visit https://www.preciseceramic.com/.

Q. What if my membrane is slightly discolored?

A. Although the silver metal membrane is 99.97% pure silver, the formation of extraneous compounds is possible over time. For example, silver can become tarnished, especially when the environment contains certain emissions as described below. To minimize contamination of the membrane, leave it in sealed packs. Silver compounds may form on the surface which are primarily cosmetic imperfections and do not affect the pore structure or membrane filtration performance. Examples of colored compounds that can form on the surface of the silver metal membrane are:

• Ag2S (black)
• Agl (yellow)
• Ag3PO4 (yellow)
• Ag2CrO4 (dark red)
• AgCl (dark brown)
• Ag2O (dark brown)
• AgBr (light yellow)

The most common compounds that form on the silver metal membrane are Ag2S and AgCl. AgCl is a photosensitive salt that can be removed by flushing the membrane with an ammonia solution. Typically, just a brief soak or dip in the ammonia solution will dissolve AgCl. Ag2S is a very stable compound and is very difficult to remove from the membrane without altering the structure. A flush with methyl or ethyl alcohol can be used to remove some of the other compounds.

These compounds should not be confused with the natural grayish white appearance of the silver metal membrane surface. This appearance is due to the microporous structure of the media which reflects light in a manner different than polished silver. The slight difference in color between the two sides of the membrane is due to the manufacturing process and is most noticeable on 3 and 5 micron pores sizes.

Q. What is a Polycarbonate or Polyester Track Etch filter membrane?

A. These types of filter membranes are precise, two-dimensional micro porous screens with straight through, cylindrical pores.

As in the case of other screen-type filters, particle capture takes place only on the surface, therefore there is more accurate separation cut-off. The precision cylindrical pores of Track Etch membranes have the most accurate size cut-off of any membrane. In depth filters, particles get caught throughout the torturous paths within the matrix as well as on the surface of the membrane.

Track Etch filters are also very thin (between 6 - 15 microns thick) but very durable (can withstand over 3,000 psi when properly supported).   They range in color from opaque to almost transparent and black.

Q. Will Sterlitech Track Etch filter membranes keep liquid behind the filter and let gases pass through?

A. PVP-Free Polycarbonate membranes have a water contact angle of approximately 90° and will not spontaneously wet out with liquids that have a surface tension equivalent to or greater than water (1 dyne). Due to the low water contact angle, polycarbonate membranes do not make effective vent filters. Low differential pressures will allow liquid water to break through the pores. We recommend membranes with a higher water entry pressure such as Hydrophobic PTFE, Hydrophobic Polyethylene, and Oleophobic Polyester for venting applications. Effective vent filters will allow permeation of gasses, while blocking liquid from entering the pores. Water vapor and other gases will pass through a hydrophobic vent membrane.

Q. How is the performance of a filter measured?

A. Design and material selection determines the performance of a filter. Three important measures of filter performance are flow rate, throughput and bubblepoint, defined as follows:

Flow Rate: Determines the volume of liquid or air that will flow through the filter at a fixed pressure and temperature. This is usually displayed as ml/minute/cm^2.

Throughput: Describes the dirt handling capacity of a filter. Namely, how long the liquid will continue to flow through the membrane before the membrane clogs. The lower the flow rate and throughput, the longer it takes the researcher to complete the analysis.

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Bubble point: A test to determine the integrity and pore size of a filter. The differential pressure at which a steady stream of gas bubbles is emitted from a wetted filter under specific test conditions. The bubble point test measures the largest pore. Bubble point is generally determined using water or an alcohol (methanol or isopropynol) and is displayed as PSI.

Q. What variables affect the performance of a filter?

A. Viscosity: The viscosity of a liquid determines its resistance to flow; the higher the viscosity, the lower the flow rate and the higher the differential pressure required to achieve a given flow rate.

Porosity: The flow rate of a membrane is directly proportional to the porosity of a membrane, eg. the more pores, the higher the flow rate.

Filter Area: The larger the filter area, the faster the flow rate at a given pressure differential and the larger the expected filter throughput volume prior to "clogging for a given solution."

Q. What are the differences between the crossflow test cells and the Sterlitech HP stirred cell?

A. The Sepa CF, CF042, and CF016 test cells operate in true crossflow mode and have both concentrate and permeate streams. Depending on system design and the fluid being processed, they are operated with user selected pressure and flow parameters and allow for continuous testing and sampling.The HP stirred cell is an enclosed batch system with a maximum feed volume of 300mL that is typically pressured with compressed gas. Stirred cells are operated in normal flow mode and do not have a concentrate stream. Stir bar action is used to simulate cross flow near the membrane surface.

It is not uncommon for the foulant spacer to leave an imprint on the membrane and, in most cases, is not a cause for concern.

However, it is important to verify that the foulant spacer (or the foulant spacer and shim combination) is not thicker than the feed channel. If too thick of a foulant spacer is used, then it may cause damage to the membrane.

The Reynolds number is a dimensionless number that is related to the ratio of inertial forces to viscous forces experienced by a fluid for given flow conditions. The Reynolds number can be used to predict whether flow conditions result in a laminar or turbulent flow.

In theory, the cross section area of the test cell feed channel can be used to calculate the Reynolds number for the feed flow. In practice, it is very difficult to calculate the Reynolds number because of the complex geometry of the foulant spacer occupying the feed channel. There are empirical methods to estimate the Reynolds number by characterizing the relationship between feed flow and differential pressure.

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The user must completely release the pressure before opening a stirred cell. The HP and HPX stirred cells do not have integrated pressure relief valves, but Sterlitech does offer an accessory bleed valve for purchase which can be attached to the regulator's outlet to the high pressure hose upstream of the stirred cell. https://www.sterlitech.com/pressure-relief-bleed-valve--psig-.html . The bleed valve can be slowly opened to release pressure when necessary for sample removal or refilling.

The polymeric stirred cells have integrated pressure relief valves. These valves can simply be opened to release the pressure when necessary.

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