Custom atmospheres’ influence over sintered ceramic materials.
Ceramic materials are non-metallic solid compounds characterized by a crystalline or semi-crystalline structure. Hard but brittle, strong in compression but weak in shearing and tension, characterized by good chemical inertness and high-temperature resistance, ceramic materials can be both found in nature (mostly oxides) or artificially synthesized to create products with custom properties. Due to their specific mechanical, electrical and optical properties, ceramic materials represent a family of compounds used in an extremely vast field of applications.
Cemented carbides, also known as hard metals, are a group of ceramics materials characterized by a mixture of metal carbides embedded in a matrix of cobalt or nickel. Hard metals’ physical properties make them especially suitable for the production of cutting tools and anti-wear components.
Both large-scale industrial production and R&D sectors exploit the wide range of characteristics related to ceramic materials. Compounds with different microstructure and/or composition are constantly developed in order to obtain electronic devices with enhanced performances. Semiconductors, dielectrics and ferroelectric compounds are fundamental components for every modern electronic application but ceramic materials are also promising substrates for the creation of semiconductors fuel cells (electricity production), solid state devices (pure oxygen production), superconductors (improved NMR devices) and many others.
The fabrication of ceramic materials is achieved with many advanced techniques, one of the most common is sintering.
Sintering is the techniques this application note is focused on due to the controlled atmosphere, required during the synthesis process to obtain satisfactory products. MCQ offers its Gas Blenders Series as the ideal instruments to create highly accurate custom atmospheres, easily adaptable to R&D projects aiming at the improvement of sintering results.
The basic principle of this technique is the possibility to create objects from powders, exploiting the diffusion phenomenon. The process consists in holding the powders inside a mold (properly shaped to obtain the desired object) and then heating the system to enhance the diffusion between the powder particles in order to achieve grains’ coalescence. The chemical bonds that are established between the grains effectively create a new solid object from the starting powders. The heating process continues until the connections are considered satisfactory. This mechanism shares some similarities with the common “melting and solidifying” process, but the sintering technique offers some noticeable advantages:
• Working temperature. Reaching the melting point (m.p.), especially with tungsten or molybdenum based materials (m.p.>2600°C) is generally troublesome and often industrially unattractive. Sintering process occurs at significantly lower temperatures (around half of the m.p.), making the production process easier and more affordable.
• Densification control. The mechanism of coalescence between the powder particles leads to the formation of porous products. Densification, i.e. the gradual reduction of porosity that naturally occurs during the sintering process, can be controlled by managing the working parameters.
• Grain size control. The growth of grains, strictly connected with the densification, is another phenomenon commonly observed during sintering. Along with the porosity, the grain size strongly affects the chemical and physical properties of sintered ceramic materials.
The versatility of this technique allows to create materials with specific features by managing the process conditions. Quality and properties of final products are not only affected by working temperature but also by additives (compounds mixed with the powders in small quantities), working pressure and system atmosphere’s composition. The working atmosphere is an especially critical parameter. Many sintering processes prove their effectiveness with standard air as working atmospheres but pure gases or custom mixtures are often required to achieve the desired results.
Pure gases represent the standard working condition for sintering. Nitrogen or argon usually replace air when the presence of oxygen can negatively affect the process outcome, while pure oxygen can be used to create extremely oxidant conditions. When severe reducing conditions are needed, sintering can be conducted under pure hydrogen. Process conditions’ strong influence on sintered materials leads many experiments to be carried out in different atmospheres, switching between various pure gases to verify any possible improvement related to the changed working parameters. However, the choice of pure gases represent a limiting condition, particularly for R&D purposes.
Many literature works, during the past decade, proved custom gas mixtures’ fundamental role to achieved specific advanced results. The use of custom atmospheres greatly enhances basic experimentation, increasing the range of working conditions and consequently widening the properties’ spectrum of sintered materials.
Oxygen is a key element for sintering experimentation, its presence drastically affecting the final product. The effectiveness and quality of mixed-conducting membranes (applied in gas/vapor separations and catalytic chemical reactions) and piezoresistive inks (commonly used for pressure sensor development) strongly depend on electrical conductivity, which can be easily modulated with the regulation of oxygen % in the sintering atmosphere. The relative amount of oxygen also affects the final density of sintered materials, influences the process of densification, alters the sintering temperature, and overall contributes to the parameters’ optimization process.
• CO, CO2, H2
Other gas compounds, whose chemical properties are exploited for sintering, are carbon oxides and hydrogen. Mixtures containing a combination of carbon dioxide (CO2) and monoxide (CO) has been used for the synthesis of spinels (base material for electric/acoustic devices) or metallurgic powder preparations, while hydrogen has been used in combination with nitrogen or argon for various reducing mixtures experimentation.
MCQ has recently developed the Gas Blenders Series, the ideal products for up to 6 components custom gas mixtures management. Designed following the “Lab in a box” principle, which replaces the standard bulky configuration of the Mass Flow Controllers connected with an external control unit, the Gas Blenders Series are professional instruments that allow intuitive creation and instantly control of desired atmospheres with high precision (1% accuracy for each channel), high repeatability (0,16% of reading value) and the fastest response time for setpoint value changes available on the market. Gas Blenders settings and configurations can be easily managed with the MCQ Gas Mixer Manager software, bundled with the instruments and compatible with desktop and laptop working with any Windows operating system (starting from Windows XP).
An example of MCQ Gas Blenders Series hardware configuration is represented in the image. The instruments work with dry, non-aggressive gases. The gas sources can be both pure or mixtures (in our example pure gases have been chosen for simplicity). The gas cylinders are connected to the instruments through 6 mm diameter tubes and a check valve is installed along each line as backflow prevention devices. Each gas media is connected and controlled by a dedicated channel of the Gas Blenders. Another 6 mm tube finally connects the instruments to the sintering furnaces in which the experiment takes place. A PC is connected to the Gas Blenders through a simple USB connection. All the instruments features and the gas mixture properties can then be manage with the Gas Mixer Manager software.