RUNNING HOT. Sub-topics. Fuel cells Casting Solidification

Transcription

1 RUNNING HOT Sub-topics 1 Fuel cells Casting Solidification

2 CONCEPT OF FUEL CELLS International concerns regarding the emission of greenhouse gases and the trend toward distributed power generation are of current interest to the technical community. A fuel cell is an electrochemical cell that produces electricity from a replenishable fuel tank. Fuel cells can operate virtually continuously as long as the necessary flows (reactions) are maintained (they consume reactant from an external source, which must be replenished). 2

3 FUEL CELL The electricityis generated through the reaction, triggered in the presence of an electrolyte, between the fuel (on the anode side) and an oxidant (on the cathode side). The reactants flow into the cell, and the reaction products flow out of it, while the electrolyte remains within it. Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols. Other oxidants include chlorine and chlorine dioxide 3

4 MEMBRANE FUEL CELL 4

5 FUEL CELLS DESIGN Electrolyte Air Electrode Air Flow Interconnection Fuel Electrode The electrolyte must conduct ions, but not electrons, while the electrodes must conduct the electrons generated by the electrode reactions. In addition, the tubes in structural components must be gastight and mechanically stable at high temperatures. Schematic diagram of a SOFC bundle configuration Developing the technology for producing components that meet these property requirements requires processing schemes that produce specific types of micro- and macrostructures. This requires minimizing thermal expansiondifferences among the components, and developing gastight seals for the high 5 temperature use.

6 FUEL CELLS (CONT) In addition, the composite components must be chemically compatible with each other and with the fuel. Recent advances in materials selection and microstructure, combined with fabrication of electrode-supported thin-electrolyte planar geometries, has resulted in tremendous performance gains. In addition to oxide ceramics, silicon-based ceramics such as SiC, Si3N4, and sialons along with other borides, carbides, nitrides, silicides, and diamond and diamond-like materials are now common high T materials of scientific and technological interest in both bulk and coating configurations Current advanced planar SOFCs have demonstrated ~2 W/cm2 at the cell level, at 700 C. These power densities are greater than previous generation cells at 1000 C, thus, providing the opportunity to utilize less expensive metal interconnects. However, the use of metal interconnects brings with it new challenges in 6 high temperature corrosion prevention.

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8 HEAT What is a heat? Heat is atoms in motion. In solids, atoms vibrate about their mean position with a frequency v (about /second) with an average energy (kinetic + potential), of RT. Heat from the sun is the driving force of life on Earth. In physics and thermodynamics, heat is the process of energy transfer from one body or system due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body. Temperature is used as a measure of the internal energy or enthalpy, that is the 8 level of elementary motion giving rise to heat transfer.

9 CASTING 9

10 CASTING PROCESS Casting techniques are employed when Casting is a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape (1)the finished shape is so large or complicated that any other method would be impractical, (2)a particular alloy is so low in ductility that forming by either hot or cold working would difficulties, and (3) in comparison to other fabrication processes, casting is the most economical. 10

11 SAND MOLD CASTING A two-piece mold is formed by packing sand around a pattern that has the shape of the intended casting The sand casting process involves the use of a furnace, metal, pattern, and sand mold. The metal is melted in the furnace and then ladled and poured into the cavity of the sand mold, which is formed by the pattern. The sand mold separates along a parting line and the solidified casting can be removed. 11

12 INJECTION MOLDING Injection molding is the most commonly used manufacturing process for the fabrication of plastic parts. The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part. The common thin-walled products include different types of open containers, such as buckets. Injection molding is also used to produce several everyday items such as toothbrushes or small plastic toys. Many medical devices, including valves and syringes, are manufactured using injection molding as well. 12

13 DIE CASTING Die casting is a process that can produce geometrically complex metal parts through the use of reusable molds, called dies. The die casting process involves the use of a furnace, metal, die casting machine, and die. The metal, typically a non-ferrous alloy such as aluminumor zinc, is melted in the furnace and then injected into the dies. After the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting. Metal housings for a variety of appliances and equipment are often die cast. Several automobile components are also manufactured using die casting, including pistons, cylinder heads, and engine blocks. Other common die cast parts include propellers, gears, bushings, pumps, and valves. 13

14 CENTRIFUGAL CASTING Centrifugal casting, sometimes called rotocasting, is a metal casting process that uses centrifugal force to form cylindrical parts. This differs from most metal casting processes, which use gravity or pressure to fill the mold. In centrifugal casting, a permanent moldmade from steel, cast iron, or graphite is typically used. The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available). 14

15 WHY IS PROCESS OF SOLIDIFICATION IMPORTANT? Solidification is an important industrial process since most metals are melted and then cast into a semi-finished or finished shape. SZe4k4&feature=related 15

16 WHY TO STUDY SOLIDIFICATION? 80% of ALL industry involves a casting and solidification process of materials in various ways The initial microstructure of the material forms during solidification process where the melted alloy becomes a (crystalline) solid During the last century, by examining metal alloys with an optical microscope after polishing and etching the surface, it was discovered that the microstructures influenced the material's properties. Clearly, it is important to understand this subject 16

17 NUCLEATION AND GROWTH OF GRAINS When a liquid solidifies, solid first has to appear from somewhere, after which the interface between solid and liquid can migrate to enable atoms to switch from one phase to the other at the boundary the two stages are nucleation and growth

18 SOLIDIFICATION OF METALS The steps of solidification: Thermal gradients define the shape of each grain. 18

19 SOLIDIFICATION: METAL CASTING In casting, a liquid above its melting point is poured into a mold where it cools by thermal conduction it is relatively cheap and well suited for complex 3-d shapes New solid forms by nucleation new crystals form in the melt, on the walls of the mold, or on foreign particles Crystals grow in opposing directions and impinge on one another to form grain boundaries

20 FUNDAMENTALS Solidification is a change from liquid to solid state Recall the atomic arrangements in a liquid and solid 2 step process of NUCLEATION and GROWTH Solidification - the liquid cools to just below its freezing (or melting) temperature, because the energy associated with the crystalline structure of the solid is less than the energy of the liquid. 20

21 FORMATION OF STABLE NUCLEI Two main mechanisms: Homogenous and heterogeneous. Homogenous Nucleation : Metal itself will provide atoms to form nuclei. Metal, when significantly cooled (below freezing T), has several slow moving atoms which bond each other to form nuclei. Cluster of atoms below critical size is called embryo (continuously being formed and re-dissolved in a molten metal). If clusters of atoms reach critical size, they grow into crystals. Else get dissolved. Cluster of atoms that are grater than critical size are called nucleus. 21 The critical radius is the minimum size of a crystal that must be formed by atoms clustering together in the liquid before the solid particle is stable and begins to grow.

22 THE RELATIONSHIP BETWEEN FREE ENERGY AND TEMPERATURE At the melting point, both phases have the same free energy and can co-exist Abovethe melting point, liquid is in the state of lower free energy; If a liquid is cooled beyond its melting point, its free energy is greater than that of a solid; The system can release energy if it solidifies this is the driving force for phase transformation Energy difference between the liquid and 22 the solid is the driving force for solidification.

23 ENERGIES INVOLVED IN HOMOGENOUS NUCLEATION Two kinds of energy should be considered 23

24 FREE ENERGY CHANGE Retarding energy Energy opposing to the formation of embryos, the energy to form the surface of these particles ~ specific surface free energy Driving energy Energy is released by the liquid to solid transformation 24

25 TOTAL FREE ENERGY Total free energy associated with the formation of embryo 25

26 CONDITIONS FOR NUCLEATION Stable cluster nucleation solidification Assume spherical cluster of radius R Total energy = Volume energy (negative) + Surface energy (positive) Total energy E T = 4/3πR 3 G v + 4πR 2 γ de T /dr = 0 for energy to be minimum de T /dr = 4 πr 2 G v + 8πR γ=0 r* is the critical radius for a stable nucleus 26

27 CRITICAL RADIUS AND TEMPERATURE The greater the degree of undercooling, the greater the change in volume free energy. Surface energy does not change much with T. Cluster stabilitydepends on energy: Energy change is positive: instable cluster Energy change is negative: stable cluster 27

28 CRITICAL RADIUS VERSUS UNDERCOOLING Homogeneous nucleation occurswhen the undercooling becomes large enough to cause the formation of a stable nucleus. The latent heat of fusion (entalphy) represents the heat given o during the liquid-to-solid transformation. 28

29 UNDERCOOLING The undercooling (T) is the differencebetween the equilibrium freezing temperature and the actual temperature of the liquid. As the extent of undercooling increases, the thermodynamic driving force for the formation of a solid phase from the liquid overtakes the resistance to create a solid-liquid interface. 29

30 PROBLEM Problem: Calculate the critical radius of a homogeneous nucleus that forms when pure liquid copper solidifies. Assume T of undercooling = 0.2 Tmelt Calculate the number of atoms in the critical-sized nucleus. 30

31 HETEROGENEOUS NUCLEATION Contact angle between solid and liquid The solid nucleating agent 31 must be wetted by the liquid metal.

32 DOES WATER REALLY FREEZE AT 0 C? This process is dependent on the contact angle for the nucleating phase and the surface on which nucleation occurs. a radius of curvature greater than the critical radius is achieved with very little total surface between the solid and liquid. Relatively few atoms must cluster together to produce a solid particle that has the required radius of curvature. The rate of nucleation (the number of nuclei formed per unit time) is a function of temperature. Prior to solidification, there is no nucleation. As T drops, the driving force for nucleation increases; however, as T decreases, atomic diffusion becomes slower, hence slowing the nucleation process. Much less undercooling is required to achieve the critical size, so nucleation occurs more readily. a typical rate of nucleation 32 reaches a maximum at some T below the transformation T

33 GROWTH OF CRYSTALS AND FORMATION OF GRAIN STRUCTURE Nucleus grow into crystals in different orientations. Crystal boundaries are formed when crystals join together at complete solidification. Crystals in solidified metals are called grains. Grains are separated by grain boundaries. More the number of nucleation sites available, more the number of grains formed. When will we obtain fine-grained structures? 33

34 SOLIDIFICATION(COOLING) CURVES Pure metal Alloy L L Soldification begins T m L S L + S S T L T S Solidification complete S Alloys are used in most engineering applications. Example: Cartridge brass is binary alloy of 70% Cu and 30% Zinc. Iconel is a nickel based superalloy with about 10 elements. 34

35 RATE OF TRANSFORMATION The rate of nucleation (the number of nuclei formed per unit time) is a function of temperature. Prior to solidification there is no nucleation. At T above the freezing point, the rate is zero. As the temperature drops, the driving force for nucleation increases. However, as the temperature becomes lower, atomic diffusion becomes slower, hence slowing the nucleation process. Thus, a typical rate of nucleation reaches a maximum at some temperature below the transformation temperature 35

36 COARSE-GRAINED OR FINE-GRAINED? The size of the particles depends on transformation temperature. For transformations that occur at T near to melting point corresponding to low nucleation and high growth rates, few nuclei form that grow rapidly. Thus, the resulting microstructure will consist of few and relatively large phase particles (e.g., coarse grains). For transformations at lower T, nucleation rates are high and growth rates low, which results in many small particles (e.g., fine grains). When a material is cooled very rapidly to a relatively low T where the rate is extremely low, it is possible to produce nonequilibrium phase structures 36

37 CAN MATERIALS BE STRENGTHENING DURING SOLIDIFICATION? Grain structure of Aluminum cast with and without grain refiners. When a metal casting freezes, impurities in the melt and walls of the mold in which solidification occurs serve as heterogeneous nucleation sites. To produce cast ingots with fine grain size, grain refiners are added. Example: For aluminumalloy, small amount of Titanium, Boron or Zirconium is added. The greater grain boundary area provides grain size strengthening in metallic materials. 37

38 WHAT IS DENDRITE? If the liquid is undercooled, a protuberance on the solid-liquid interface can grow rapidly as a dendrite. The latent heat of fusion (enthalpy) is removed by raising the temperature of the liquid back to the freezing temperature. 38

39 SOLIDIFICATION IN CASTING Dendritic growth continues until the undercooled liquid warms to the freezing temperature. Any remaining liquid then solidifies by planar growth. 39

40 TYPES OF GRAINS Equiaxed Grains: M Crystals, smaller in size, grow equally in all directions. M Formed at the sites of high concentration of the nuclie. Columnar Grains: M Long thin and coarse. M Grow predominantly in one direction. M Formed at the sites of slow cooling and steep temperature gradient. M Example: Grains that are away from the mold wall. Columnar Grains Equiaxed Grains 40

41 VIEW OF THE SOLIDIFIED INGOTS The colomnar grains have grown perpendicular to the mold faces since large thermal gradients are presented in those directions 41

42 SOLIDIFICATION OF SINGLE CRYSTALS The most widely used technique for making single-crystal silicon is the Czochralski process, in which a seed of single-crystal silicon contacts the top of molten silicon. As the seed is slowly raised, atoms of the molten silicon solidify in the pattern of the seed and extend the single-crystal structure. 42