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Ceramic foams are of significant technological interest as they offer favorable properties including low density, low thermal conductivity, variable mechanical strength, high thermal shock resistance and high surface area. Although focused research on cellular ceramics has been going for at least three decades, to date there has been little investigation on electroceramic foams. BaTiO3 foams were synthesized via direct foaming method. In order to control the microstructure and by extension the properties, synthesis parameters such as foaming agent, composition, sintering time, and sintering temperature were varied to determine their influence. The effect of each parameter on the microstructure was measured and characterized using scanning electron microscopy, image analysis, density measurements, and mercury porosimetry. Microstructure was classified in terms of strut stability, average grain size, average pore size, porosity, and grain boundary integrity. For example, foams with 30% ceramic volume were synthesized using a laboratory developed polyurethane system and a commercial silicon-free polyurethane system. When sintered at 1400 °C for 8 hours they produced stable struts and uniform pore-size distributions. The laboratory foam had a porosity of 67 %, an average grain size of 44.4 µm and an average pore size of 66.7 µm. The commercial silicon-free foam had a porosity of 73 %, an average grain size of 20 µm and an average pore size of 99 µm. The synthesized foams have potential mechanical and electrical applications.


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Published online 25 March 2009​

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Fig. 1. The crystal structure of CaCu3Ti4O12 , an ordered perovskite.​

Perovskites are ceramics with a structure that gives them remarkable abilities to store electric charge. The crystalline structure consists of a network of titanium and oxygen octahedra, in between which large metal ions such as calcium (Ca) and copper (Cu) are located. However, the metals are too large to fit comfortably, causing the octahedra to shift and twist. The result is an unstable internal structure that can be oriented with an electric field, making perovskites excellent materials for energy-storing capacitors.
A new type of perovskite, CaCu3Ti4O12 (Fig. 1) is of interest because of its intriguing electrical properties. Current passing through this material behaves nonlinearly—characteristics which could allow the ceramic to be developed as a long-life sensor.
Dopants such as the rare earth atom lanthanum (La) are normally added to CaCu3Ti4O12 to improve device performance. However, because the perovskite crystal is unstable, the dopant substitution position can dramatically alter the electronic behaviour. Now, Sung-Yoon Chung and colleagues1 have used electron microscopy and spectroscopy to precisely pinpoint the location of dopant atoms in the CaCu3Ti4O12 lattice.
The researchers prepared pure CaCu3Ti4O12 crystals into thin sections so that the beam of a scanning transmission electron microscope (STEM) could pass through them. The resulting images showed an array of round dots arranged into a checkerboard pattern, with each dot corresponding to a column of atoms. Because each atom has a different size, the dot’s brightness reveals its identity.
Half of the rows in the CaCu3Ti4O12 crystal were alternately light and dark. These were identified as columns of pure Cu and mixed Ca-Cu atoms. And, since Ca is a lighter atom with a lower atomic number, the mixed columns appear darker in the images.
Doping the CaCu3Ti4O12 with 5% La caused a dramatic change in the atomic images. Suddenly, metal rows that were formerly light and dark now appeared universally bright. Image simulations and spectroscopic measurements proved that La exclusively replaced Ca in the perovskite.
“Our study shows that La is selectively substituted for Ca rather than Cu,” says Chung. “Progress in STEM allows not only better image resolution, but also sensitive detection of the atomic columns due to the enhanced beam current​

منبع : www.natureasia.com​

Annealing metallurgy​

From Wikipedia, the free encyclopedia​

Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the re-crystallization temperature and maintaining a suitable temperature, and then cooling. Annealing is used to induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties
In the cases of copper, steel, silver, and brass this process is performed by substantially heating the material (generally until glowing) for a while and allowing it to cool slowly. In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming
Thermodynamics of annealing
Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent) in ceramics. This alteration in dislocations allows metals to deform more easily, so increases their ductility.
The amount of process-initiating Gibbs free energy in a deformed metal is also reduced by the annealing process. In practice and industry, this reduction of Gibbs free energy is termed "stress relief".
The relief of internal stresses is a thermodynamically spontaneous process; however, at room temperatures, it is a very slow process. The high temperatures at which the annealing process occurs serve to accelerate this process.
The reaction facilitating the return of the cold-worked metal to its stress-free state has many reaction pathways, mostly involving the elimination of lattice vacancy gradients within the body of the metal. The creation of lattice vacancies is governed by the Arrhenius equation, and the migration/diffusion of lattice vacancies are governed by Fick’s laws of diffusion.[1]
Mechanical properties, such as hardness and ductility, change as dislocations are eliminated and the metal's crystal lattice is altered. On heating at specific temperature and cooling it is possible to bring the atom at the right lattice site and new grain growth can improve the mechanical properties.
Stages of annealing​

There are three stages in the annealing process, with the first being the recovery phase, which results in softening of the metal through removal of crystal defects (the primary type of which is the linear defect called a dislocation) and the internal stresses which they cause. Recovery phase covers all annealing phenomena that occur before the appearance of new strain-free grains.[2] The second phase is recrystallization, where new strain-free grains nucleate and grow to replace those deformed by internal stresses.[2] If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties.
Annealing in a controlled atmosphere​

The low temperature of annealing (about 50 °F above C3 line) may result in oxidation of the metal’s surface, resulting in scale. If scale is to be avoided, annealing is carried out in an oxygen-, carbon-, and nitrogen-free atmosphere (to avoid oxidation, carburization, and nitriding respectively) such as endothermic gas (a mixture of carbon monoxide, hydrogen gas, and nitrogen[clarification needed]).
The magnetic properties of mu-metal (Espey cores) are introduced by annealing the alloy in a hydrogen atmosphere.
Setup and Equipment​

Typically, large ovens are used for the annealing process. The inside of the oven is large enough to place the workpiece in a position to receive maximum exposure to the circulating heated air. For high volume process annealing, gas fired conveyor furnaces are often used. For large workpieces or high quantity parts Car-bottom furnaces will be used in order to move the parts in and out with ease. Once the annealing process has been successfully completed, the workpieces are sometimes left in the oven in order for the parts to have a controlled cooling process. While some workpieces are left in the oven to cool in a controlled fashion, other materials and alloys are removed from the oven. After being removed from the oven, the workpieces are often quickly cooled off in a process known as quench hardening. Some typical methods of quench hardening materials involve the use of media such as air, water, oil, or salt.
Diffusion annealing of semiconductors​

In the semiconductor industry, silicon wafers are annealed, so that dopant atoms, usually boron, phosphorus or arsenic, can diffuse into substitutional positions in the crystal lattice, resulting in drastic changes in the electrical properties of the semiconducting material.
Specialized annealing cycles​

Normalization​

Normalization is an annealing process in which a metal is cooled in air after heating in order to relieve stresses.
This process is typically confined to hardenable steel. It is used to refine grains which have been deformed through cold work, and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper critical point. It is soaked for a short period then allowed to cool in air. Small grains are formed which give a much harder and tougher metal with normal tensile strength and not the maximum ductility achieved by annealing. It eliminates columnar grains and dendritic segregation that sometimes occurs during casting. Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes.
Process annealing​

Process annealing, also called "intermediate annealing", "subcritical annealing", or "in-process annealing", is a heat treatment cycle that restores some of the ductility to a work piece allowing it be worked further without breaking. Ductility is important in shaping and creating a more refined piece of work through processes such as rolling, drawing, forging, spinning, extruding and heading. The piece is heated to a temperature typically below the austenizing temperature, and held there long enough to relieve stresses in the metal. The piece is finally cooled slowly to room temperature. It is then ready again for additional cold working. This can also be used to ensure there is reduced risk of distortion of the work piece during machining, welding, or further heat treatment cycles.
The temperature range for process annealing ranges from 500 °F to 1400 °F, depending on the alloy in question.
Full anneal​

Full annealing temperature ranges​

A full anneal typically results in the second most ductile state a metal can assume for metal alloy. It creates an entirely new homogeneous and uniform structure with good dynamic properties. To perform a full anneal, a metal is heated to its annealing point (about 50°C above the austenic temperature as graph shows) and held for sufficient time to allow the material to fully austenitize, to form austenite or austenite-cementite grain structure. The material is then allowed to cool slowly so that the equilibrium microstructure is obtained. In some cases this means the material is allowed to air cool. In other cases the material is allowed to furnace cool. The details of the process depend on the type of metal and the precise alloy involved. In any case the result is a more ductile material that has greater stretch ratio and reduction of area properties but a lower yield strength and a lower tensile strength. This process is also called LP annealing for lamellar pearlite in the steel industry as opposed to a process anneal which does not specify a microstructure and only has the goal of softening the material. Often material that is to be machined, will be annealed, then be followed by further heat treatment to obtain the final desired properties.
Short cycle anneal​

Short cycle annealing is used for turning normal ferrite into malleable ferrite. It consists of heating, cooling, and then heating again from 4 to 8 hours.
: See also​

: References

1. ^ Van Vlack, L.H. Elements of Materials Science and Engineering, Addison-Wesley, 1985, p 134
2. ^ a b Verhoeven, J.D. Fundamentals of Physical Metallurgy, Wiley, New York, 1975, p. 326

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