Capacitor discharge sintering (CDS) [1] is an electric current assisted sintering (ECAS) technique. [2] The technique is based on storage of electromagnetic energy in a high voltage capacitor bank, and discharge into the sintering apparatus at low voltage (<30 V) and high current through step-down transformers on a pre-compacted powder compact which is kept under pressure. The sintering mould and Electrodes are similar to those employed in field assisted sintering techniques (FAST) such as spark plasma sintering and single electromagnetic pulse sintering technologies.
The method, analogous to resistive sintering, is a direct evolution of a welding technology named Capacitor Discharge Welding (Kondensator-Impulsschweißen). CDS seems like an improvement of the less powerful capacitor discharge compaction patented by W.Knoess and M.Schlemmer (EP 0671232, US Patent 5529746).
Advantages of the technique are:
The technique has been studied by Element Six [3] under the name of electro-discharge sintering. This name has been adopted by many authors in the past [4] to describe a range of different technologies which typically adopt very high voltages and completely different machines. [5] [6] [7] For this reason the technique which employs low voltages and high currents adapted from capacitor discharge welding has been named capacitor discharge sintering. Other authors also refer to this technology as spark plasma compaction (in reference to the well known Spark Plasma Sintering with whom it has in common only the use of electric currents).
Capacitor discharge sintering is at an experimental/research stage of development in Germany at the Ruhr-Universität Bochum where a prototype machine is installed. [8]
Electrical discharge machining (EDM), also known as spark machining, spark eroding, die sinking, wire burning or wire erosion, is a metal fabrication process whereby a desired shape is obtained by using electrical discharges (sparks). Material is removed from the work piece by a series of rapidly recurring current discharges between two electrodes, separated by a dielectric liquid and subject to an electric voltage. One of the electrodes is called the tool-electrode, or simply the tool or electrode, while the other is called the workpiece-electrode, or work piece. The process depends upon the tool and work piece not making physical contact.
Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it to the point of liquefaction.
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current". This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the filament of ionized gas. Usually, the action of ionizing the gas is violent and disruptive, often leading to sound, light and heat.
Powder metallurgy (PM) is a term covering a wide range of ways in which materials or components are made from metal powders. PM processes can avoid, or greatly reduce, the need to use metal removal processes, thereby drastically reducing yield losses in manufacture and often resulting in lower costs.
Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group differs. The most common definition includes five elements: two of the fifth period and three of the sixth period. They all share some properties, including a melting point above 2000 °C and high hardness at room temperature. They are chemically inert and have a relatively high density. Their high melting points make powder metallurgy the method of choice for fabricating components from these metals. Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, and chemical reaction vessels in corrosive environments. Partly due to the high melting point, refractory metals are stable against creep deformation to very high temperatures.
Electrical breakdown or dielectric breakdown is a process that occurs when an electrical insulating material, subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. All insulating materials undergo breakdown when the electric field caused by an applied voltage exceeds the material's dielectric strength. The voltage at which a given insulating object becomes conductive is called its breakdown voltage and depends on its size and shape. Under sufficient electrical potential, electrical breakdown can occur within solids, liquids, gases or vacuum. However, the specific breakdown mechanisms are different for each kind of dielectric medium.
Titanium diboride (TiB2) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability and wear resistance. TiB2 is also a reasonable electrical conductor, so it can be used as a cathode material in aluminium smelting and can be shaped by electrical discharge machining.
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An arc discharge is characterized by a lower voltage than a glow discharge and relies on thermionic emission of electrons from the electrodes supporting the arc. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".
Spark plasma sintering (SPS), also known as field assisted sintering technique (FAST) or pulsed electric current sintering (PECS), or Plasma Pressure Compaction (P2C) is a sintering technique.
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael Faraday described this phenomenon as "the beautiful flash of light attending the discharge of common electricity".
Thermal spraying techniques are coating processes in which melted materials are sprayed onto a surface. The "feedstock" is heated by electrical or chemical means.
Spark ionization is a method used to produce gas phase ions from a solid sample. The prepared solid sample is vaporized and partially ionized by an intermittent discharge or spark. This technique is primarily used in the field of mass spectrometry. When incorporated with a mass spectrometer the complete instrument is referred to as a spark ionization mass spectrometer or as a spark source mass spectrometer (SSMS).
Lithium titanate is a compound with the chemical formula Li2TiO3. It is a white powder with a melting point of 1,533 °C (2,791 °F).
Atmospheric-pressure plasma is a plasma in which the pressure approximately matches that of the surrounding atmosphere – the so-called normal pressure.
Hot pressing is a high-pressure, low-strain-rate powder metallurgy process for forming of a powder or powder compact at a temperature high enough to induce sintering and creep processes. This is achieved by the simultaneous application of heat and pressure.
Plasma is one of the four fundamental states of matter, first systematically studied by Irving Langmuir in the 1920s. It consists of a gas of ions – atoms or molecules which have one or more orbital electrons stripped, and free electrons.
Cladding is the bonding together of dissimilar metals. It is different from fusion welding or gluing as a method to fasten the metals together. Cladding is often achieved by extruding two metals through a die as well as pressing or rolling sheets together under high pressure.
Carbon nanotube metal matrix composites (CNT-MMC) are an emerging class of new materials that mix carbon nanotubes into metals and metal alloys to take advantage of the high tensile strength and electrical conductivity of carbon nanotube materials.
A supercapacitor (SC), also called an ultracapacitor, is a high-capacity capacitor with a capacitance value much higher than other capacitors, but with lower voltage limits, that bridges the gap between electrolytic capacitors and rechargeable batteries. It typically stores 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerates many more charge and discharge cycles than rechargeable batteries.
Ultra-high-temperature ceramics (UHTCs) are a class of refractory ceramics that offer excellent stability at temperatures exceeding 2000 °C being investigated as possible thermal protection system (TPS) materials, coatings for materials subjected to high temperatures, and bulk materials for heating elements. Broadly speaking, UHTCs are borides, carbides, nitrides, and oxides of early transition metals. Current efforts have focused on heavy, early transition metal borides such as hafnium diboride (HfB2) and zirconium diboride (ZrB2); additional UHTCs under investigation for TPS applications include hafnium nitride (HfN), zirconium nitride (ZrN), titanium carbide (TiC), titanium nitride (TiN), thorium dioxide (ThO2), tantalum carbide (TaC) and their associated composites.
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