1. Structure of SiC Ceramic Membranes

The molecular formula of silicon carbide is SiC, with a density of 3.16-3.2 g/cm³. Silicon carbide is a covalent compound, and its crystal structure belongs to a tetrahedral coordination type, with structural units generally being CSi₄ or SiC₄. Both carbon and silicon are non-metal elements, and the strong covalent bonds formed during their combination create the compound SiC. Approximately 78% of the bond energy in Si-C is ionic, granting silicon carbide advantages such as high hardness, high strength, and excellent resistance to acid and alkali corrosion.

2. Classification of SiC Ceramic Membranes

SiC ceramic membranes can be classified into symmetric and asymmetric membranes. Symmetric membranes have a simple structure, poor filtration performance, high susceptibility to contamination, and a short lifespan, making them rarely used in practice. In contrast, asymmetric membranes typically consist of three layers: a support layer, a transition layer, and a separation layer.

3. Characteristics of SiC Ceramic Membranes

1. High Porosity

SiC ceramic membranes are characterized by numerous pores of varying sizes. Closed pores primarily act as thermal insulators, barriers to sound transmission, and particle transmission, while open pores mainly serve for filtration.

2. High Flexural Strength

SiC ceramic products, prepared using silicon carbide, silica, alumina, and various processing techniques, exhibit high flexural strength. After drying and pre-sintering, the particles within the product undergo melting and bonding at high temperatures during the sintering process, thereby enhancing the product’s flexural strength.

3. Stable Physical and Chemical Properties

SiC ceramics are resistant to high temperatures, acid, and alkali corrosion. They feature a large specific surface area, withstand high pressures, do not pollute the environment, conserve resources, and have low costs.

4. High Hydrophilicity and Oleophobicity

SiC ceramic membranes can separate oil from emulsions—a feat difficult to achieve with traditional technologies. This property makes them widely applicable in the treatment of oily wastewater and industrial effluents.

5. Non-Polluting

SiC ceramic membranes are eco-friendly and classified as green materials, ensuring they do not cause secondary pollution.

6. Consistent Water Quality Output

SiC ceramic membranes maintain stable water production quality over time. Compared to other technologies or membrane materials, they require less weight and space.

There are many methods for preparing SiC ceramic membranes. Based on different pore structures and formation mechanisms, they can generally be divided into four categories: particle stacking, template replication, sacrificial template, and direct foaming. Typically, the preparation of SiC ceramic membranes involves processes such as mixing, molding, and sintering. Besides the preparation methods, the sintering process greatly influences the material’s performance. Common sintering methods for SiC ceramic membranes include reaction sintering, pressureless sintering (solid-phase sintering and liquid-phase sintering), and recrystallization sintering.

The particle stacking method is one of the simplest and most direct approaches to constructing pore structures during the preparation of SiC ceramic membranes. During the green body formation process, SiC particles and additives are stacked together, leaving gaps between particles due to spatial hindrance. After sintering, smaller gaps are eliminated through volume shrinkage and grain diffusion, while larger residual gaps form the pore structure of the SiC ceramic membrane. Green bodies can be formed using dry pressing, gel casting, tape casting, and other methods.

In the template replication method, organic materials are used as templates. SiC ceramic slurry or precursor solutions are impregnated or coated onto the template using specific techniques. After sintering, porous SiC ceramic membranes with structures similar to the template are obtained. Many synthetic and naturally porous materials can be used as templates for fabricating porous SiC ceramics. Because this method directly replicates the pore structure of other materials, it allows precise control over the pore structure, pore size, and distribution of the SiC ceramic membrane.

The sacrificial template method is commonly used to prepare SiC ceramic membranes. It typically involves a continuous matrix phase composed of ceramic particles or precursors and a sacrificial template phase uniformly dispersed within the continuous phase. The template is eventually removed, forming a porous structure. The resulting pore structure directly corresponds to the morphology of the removed sacrificial template. The preparation of a two-phase mixed green body can be achieved by:

1. Pressing mixed powders of two different phases into a green body.

2. Preparing a mixed slurry of two phases and forming the green body through various techniques (e.g., casting, tape casting).

3. Impregnating the sacrificial template into ceramic slurry or precursor solutions to form the green body.

Depending on the pore-forming material, the sacrificial template method can be further classified into techniques such as freeze-drying, emulsion pore-forming, and direct pore-forming agent addition.

Due to its unique structure, SiC ceramic membranes exhibit excellent filtration performance, thanks to advantages such as acid and alkali resistance, high strength, high hardness, and eco-friendliness. These features enable their widespread application across various fields:

Industrial production generates harmful fumes, emissions from straw burning, and wastewater containing toxic metal ions, all of which severely impact the environment and contribute to acid rain, soil contamination, and water pollution. SiC porous ceramic membranes have been adopted and promoted in many countries for filtration and purification. They can filter industrial wastewater and domestic sewage, achieving water purification and environmental protection.

High-temperature gases emitted from factories often carry fine particles that pollute the environment. These particles, found in exhaust gases from industries such as petroleum, chemical, and pharmaceutical, are often corrosive. SiC ceramic membranes’ high-temperature and acid-alkali resistance make them suitable for removing dust from high-temperature gases.

Beyond filtration, SiC ceramic membranes also serve as sound-absorbing materials. Their porous structure disperses the pressure of sound waves in the air, reducing noise. Additional advantages such as long service life, ease of cleaning, recyclability, corrosion resistance, and lightweight properties give them broad potential in scientific research and applications.

Some toxic and polluting substances in wastewater and exhaust gas cannot be removed by filtration alone. In such cases, catalysts can be used to decompose these pollutants. Catalysts require a carrier, and porous SiC ceramic membranes, with properties like acid-alkali resistance, high-temperature resistance, high flexural strength, toughness, and eco-friendliness, are ideal as catalyst carriers.

In industrial production, molten metals often contain small particles that can affect the metal’s formation, hardness, and strength. This is particularly critical in the production of precision metal castings and electronic components, where such impurities can significantly impact product quality. The high-temperature resistance of SiC ceramic membranes makes them suitable for filtering molten metals.

Porous ceramics can be used as sensitive elements to detect various substances in gases or liquids. The principle involves placing the porous SiC ceramic membrane in a medium, where substances present in the medium react with or are adsorbed by the membrane. Changes in electrical current or potential within the membrane are then used to identify the substances in the medium.

Porous ceramics can be used to manufacture kiln furniture. Traditional kiln furniture is often heavy and cumbersome, whereas porous ceramics are lightweight, more convenient to use, and enable faster heat transfer. This helps reduce firing time. When used for firing ferrites or other electronic ceramics, these materials can improve product performance.

Internationally, only a few countries have developed high-quality SiC ceramic membrane products, with notable examples being Denmark’s LQ and France’s SG. Despite decades of development in the membrane separation field in China, SiC ceramic membranes remain underdeveloped, with the first domestic mass-production company emerging only a few years ago.

As the most advanced product in the ceramic membrane sector, all SiC membranes required for water treatment in China are imported. Foreign companies monopolize material prices and supply, resulting in high costs that restrict the adoption and market expansion of SiC ceramic membranes.

Major patent holders include Kurita Water Industries, Kubota, Hitachi, NGK Insulators, NEC, Toray, JSR, Praxair Technology, Nanjing Tech University, the Membrane Technology and Research Institute, and Tsinghua University, all of which hold highly cited patents. Given the differences in focus areas and application domains, there is potential for both competition and strategic collaboration, presenting both opportunities and challenges for industry development.

Membrane separation technology is regarded as an energy-efficient and innovative separation method, offering effective solutions to critical issues such as energy, resource management, and environmental protection. Data indicate that in the early 21st century, global sales of membranes and related equipment exceeded $60 billion annually, with a growth rate of about 30%. Experts predict that membrane technology, along with integrated approaches combining membranes with other technologies, could largely replace traditional separation techniques.

• In 2019, Danish SiC ceramic membrane supplier LQ received a multimillion-dollar order from a shipping company for a water filtration system using SiC ceramic membranes, worth nearly $10 million.

• In 2016, research achievements in SiC ceramic membranes from Wuhan Institute of Technology stood out among Hubei Province’s top 100 scientific and technological breakthroughs. The findings attracted Ezhou Changda Asset Management Co., Ltd. In 2017, with eight patented SiC ceramic membrane technologies, the team contributed RMB 21.28 million in intellectual property for equity, partnering with Changda Asset Management to establish Hubei Dijie Membrane Technology Co., Ltd.

Globally, only a few enterprises are capable of mass-producing SiC ceramic membranes, and the field remains largely underdeveloped domestically. The emergence of one or two pure SiC ceramic membrane manufacturers in recent years highlights the potential of this product, which has vast application prospects and strong market demand. Domestically, the industry offers a clear opportunity: possessing advanced technology ensures access to a promising market.