Concentrator photovoltaics is a photovoltaic technology that generates electricity from sunlight. Unlike conventional photovoltaic systems, it uses lenses or curved mirrors to focus sunlight onto small efficient, multi-junction solar cells. In addition, CPV systems use solar trackers and sometimes a cooling system to further increase their efficiency. Ongoing research and development is improving their competitiveness in the utility-scale segment and in areas of high insolation. Systems using high-concentration photovoltaics have the potential to become competitive in the near future, they possess the highest efficiency of all existing PV technologies, a smaller photovoltaic array reduces the balance of system costs. CPV is not used in the PV rooftop segment and is far less common than conventional PV systems. For regions with a high annual direct normal irradiance of 2000 kilowatt-hour per square meter or more, the levelized cost of electricity is in the range of $0.08–$0.15 per kWh and installation cost for a 10-megawatt CPV power plant was identified to lie between €1.40–€2.20 per watt-peak.
In 2016, cumulative CPV installations reached 350 megawatts, less than 0.2% of the global installed capacity of 230,000 MW. Commercial HCPV systems reached instantaneous efficiencies of up to 42% under standard test conditions and the International Energy Agency sees potential to increase the efficiency of this technology to 50% by the mid-2020s; as of December 2014, the best lab cell efficiency for concentrator MJ-cells reached 46%. Under outdoor, operating conditions, CPV module efficiencies have exceeded 33%. System-level AC efficiencies are in the range of 25-28%. CPV installations are located in China, the United States, South Africa and Spain. HCPV directly competes with concentrated solar power as both technologies are suited best for areas with high direct normal irradiance, which are known as the Sun Belt region in the United States and the Golden Banana in Southern Europe. CPV and CSP are confused with one another, despite being intrinsically different technologies from the start: CPV uses the photovoltaic effect to directly generate electricity from sunlight, while CSP – called concentrated solar thermal – uses the heat from the sun's radiation in order to make steam to drive a turbine, that produces electricity using a generator.
As of 2012, CSP is still more common than CPV. Research into concentrator photovoltaics has taken place since the mid 1970s spurred on by the energy shock from a mideast oil embargo. Sandia National Laboratories in Albuquerque, New Mexico was the site for most of the early work, with the first modern-like photovoltaic concentrating system produced there late in the decade, their first system was a linear-trough concentrator system that used a point focus acrylic Fresnel lens focusing on water-cooled silicon cells and two axis tracking. Cell cooling with a passive heat sink and use of silicone-on-glass Fresnel lenses was demonstrated in 1979 by the Ramón Areces Project at the Institute of Solar Energy of the Technical University of Madrid; the 350 kW SOLERAS project in Saudi Arabia—the largest until many years later—was constructed by Sandia/Martin Marietta in 1981. Research and development continued through the 1980s and 1990s without significant industry interest. Improvements in cell efficiency were soon recognized as essential to making the technology economical.
However the improvements to Si-based cell technologies used by both concentrators and flat PV failed to favor the system-level economics of CPV. The introduction of III-V Multi-junction solar cells starting in the early 2000s has since provided a clear differentiator. MJ cell efficiencies have improved from 34% to 46% at research-scale production levels. A substantial number of multi-MW CPV projects have been commissioned worldwide since 2010. Modern CPV systems operate most efficiently in concentrated sunlight, as long as the solar cell is kept cool through the use of heat sinks. Diffuse light, which occurs in cloudy and overcast conditions, cannot be concentrated using conventional optical components only. Filtered light, which occurs in hazy or polluted conditions, has spectral variations which produce mismatches between the electrical currents generated within the series-connected junctions of spectrally "tuned" multi-junction photovoltaic cells; these CPV features lead to rapid decreases in power output when atmospheric conditions are less than ideal.
To produce equal or greater energy per rated watt than conventional PV systems, CPV systems must be located in areas that receive plentiful direct sunlight. This is specified as average DNI greater than 5.5-6 kWh/m2/day or 2000kWh/m2/yr. Otherwise, evaluations of annualized DNI vs. GNI/GHI irradiance data have concluded that conventional PV should still perform better over time than presently available CPV technology in most regions of the world. CPV research and development has been pursued in over 20 countries for more than a decade; the annual CPV-x conference series has served as a primary networking and exchange forum between university, government lab, industry participants. Government agencies have continued to encourage a number of specific technology thrusts. ARPA-E announced a first round of R&D funding in late 2015 for the MOSAIC Program to further combat the location and expense challe
Soldering is a process in which two or more items are joined together by melting and putting a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal. Unlike welding, soldering does not involve melting the work pieces. In brazing, the filler metal melts at a higher temperature. In the past, nearly all solders contained lead, but environmental and health concerns have dictated use of lead-free alloys for electronics and plumbing purposes. There is evidence. Soldering and brazing are thought to have originated early in the history of metal-working before 4000 BC. Sumerian swords from c. 3000 BC were assembled using hard soldering. Soldering was used to make jewelry items, cooking ware and tools, as well as other uses such as in assembling stained glass. Soldering is used in plumbing and metalwork from flashing to jewelry and musical instruments. Soldering provides reasonably permanent but reversible connections between copper pipes in plumbing systems as well as joints in sheet metal objects such as food cans, roof flashing, rain gutters and automobile radiators.
Jewelry components, machine tools and some refrigeration and plumbing components are assembled and repaired by the higher temperature silver soldering process. Small mechanical parts are soldered or brazed as well. Soldering is used to join lead came and copper foil in stained glass work. Electronic soldering connects electrical wiring and electronic components to printed circuit boards by utilizing a metallic alloy substance called solder; this special alloy is melted by using a soldering iron, a wave bath, or a specialized oven, as it joins conductors to PCBs, wires. Musical instruments brass and woodwind instruments, use a combination of soldering and brazing in their assembly. Brass bodies are a soldered together, while keywork and braces are most brazed. Soldering filler materials are available in many different alloys for differing applications. In electronics assembly, the eutectic alloy of 63% tin and 37% lead has been the alloy of choice. Other alloys are used for plumbing, mechanical assembly, other applications.
Some examples of soft-solder are tin-lead for general purposes, tin-zinc for joining aluminium, lead-silver for strength at higher than room temperature, cadmium-silver for strength at high temperatures, zinc-aluminium for aluminium and corrosion resistance, tin-silver and tin-bismuth for electronics. A eutectic formulation has advantages when applied to soldering: the liquidus and solidus temperatures are the same, so there is no plastic phase, it has the lowest possible melting point. Having the lowest possible melting point minimizes heat stress on electronic components during soldering. And, having no plastic phase allows for quicker wetting as the solder heats up, quicker setup as the solder cools. A non-eutectic formulation must remain still as the temperature drops through the liquidus and solidus temperatures. Any movement during the plastic phase may result in cracks. Common solder formulations based on tin and lead are listed below; the fraction represent percentage of tin first lead, totaling 100%: 63/37: melts at 183 °C 60/40: melts between 183–190 °C 50/50: melts between 183–215 °C For environmental reasons, lead-free solders are becoming more used.
They are suggested anywhere young children may come into contact with, or for outdoor use where rain and other precipitation may wash the lead into the groundwater. Most lead-free solders are not eutectic formulations, melting at around 250 °C, making it more difficult to create reliable joints with them. Other common solders include low-temperature formulations, which are used to join previously-soldered assemblies without unsoldering earlier connections, high-temperature formulations which are used for high-temperature operation or for first assembly of items which must not become unsoldered during subsequent operations. Alloying silver with other metals changes the melting point and wetting characteristics, tensile strength. Of all the brazing alloys, silver solders have the broadest applications. Specialty alloys are available with properties such as higher strength, the ability to solder aluminum, better electrical conductivity, higher corrosion resistance; the purpose of flux is to facilitate the soldering process.
One of the obstacles to a successful solder joint is an impurity at the site of the joint, for example, oil or oxidation. The impurities can be removed by mechanical cleaning or by chemical means, but the elevated temperatures required to melt the filler metal encourages the work piece to re-oxidize; this effect is accelerated as the soldering temperatures increase and can prevent the solder from joining to the workpiece. One of the earliest forms of flux was charcoal, which acts as a reducing agent and helps prevent oxidation during the soldering process; some fluxes go beyond the simple prevention of oxidation and provide some form of chemical cleaning. Many fluxes act as a wetting agent in the soldering process, reducing the surface tension of
A heat pipe is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to transfer heat between two solid interfaces. At the hot interface of a heat pipe a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface; the vapor travels along the heat pipe to the cold interface and condenses back into a liquid – releasing the latent heat. The liquid returns to the hot interface through either capillary action, centrifugal force, or gravity, the cycle repeats. Due to the high heat transfer coefficients for boiling and condensation, heat pipes are effective thermal conductors; the effective thermal conductivity varies with heat pipe length, can approach 100 kW/ for long heat pipes, in comparison with 0.4 kW/ for copper. A typical heat pipe consists of a sealed pipe or tube made of a material, compatible with the working fluid such as copper for water heat pipes, or aluminum for ammonia heat pipes.
A vacuum pump is used to remove the air from the empty heat pipe. The heat pipe is filled with a working fluid and sealed; the working fluid mass is chosen so that the heat pipe contains both vapor and liquid over the operating temperature range. Below the operating temperature, the liquid can not vaporize into a gas. Above the operating temperature, all the liquid has turned to gas, the environmental temperature is too high for any of the gas to condense. Whether too high or too low, thermal conduction is still possible through the walls of the heat pipe, but at a reduced rate of thermal transfer. Working fluids are chosen according to the temperatures at which the heat pipe must operate, with examples ranging from liquid helium for low temperature applications to mercury and indium for high temperatures; the vast majority of heat pipes for room temperature applications use ammonia, alcohol or water as the working fluid. Copper/water heat pipes have a copper envelope, use water as the working fluid and operate in the temperature range of 20 to 150 °C.
Water heat pipes are sometimes filled by filling with water, heating until the water boils and displaces the air, sealed while hot. For the heat pipe to transfer heat, it must contain its vapor; the saturated liquid vaporizes and travels to the condenser, where it is cooled and turned back to a saturated liquid. In a standard heat pipe, the condensed liquid is returned to the evaporator using a wick structure exerting a capillary action on the liquid phase of the working fluid. Wick structures used in heat pipes include sintered metal powder and grooved wicks, which have a series of grooves parallel to the pipe axis; when the condenser is located above the evaporator in a gravitational field, gravity can return the liquid. In this case, the heat pipe is a thermosiphon. Rotating heat pipes use centrifugal forces to return liquid from the condenser to the evaporator. Heat pipes contain no mechanical moving parts and require no maintenance, though non-condensable gases that diffuse through the pipe's walls, resulting from breakdown of the working fluid or as impurities extant in the material, may reduce the pipe's effectiveness at transferring heat.
The advantage of heat pipes over many other heat-dissipation mechanisms is their great efficiency in transferring heat. A pipe one inch in diameter and two feet long can transfer 3.7 kW at 1,800 °F with only 18 °F drop from end to end. Some heat pipes have demonstrated a heat flux of more than 23 kW/cm², about four times the heat flux through the surface of the sun. Heat pipes have an envelope, a wick, a working fluid. Heat pipes are designed for long term operation with no maintenance, so the heat pipe wall and wick must be compatible with the working fluid; some material/working fluids pairs that appear to be compatible are not. For example, water in an aluminum envelope will develop large amounts of non-condensable gas over a few hours or days, preventing normal operation of the heat pipe. Since heat pipes were rediscovered by George Grover in 1963, extensive life tests have been conducted to determine compatible envelope/fluid pairs, some going on for decades. In a heat pipe life test, heat pipes are operated for long periods of time, monitored for problems such as non-condensable gas generation, material transport, corrosion.
The most used envelope /fluid pairs include: Copper envelope with water working fluid for electronics cooling. This is by far the most common type of heat pipe. Copper or steel envelope with refrigerant R134a working fluid for energy recovery in HVAC systems. Aluminum envelope with ammonia working fluid for Spacecraft Thermal Control. Superalloy envelope with alkali metal working fluid for high temperature heat pipes, most used for calibrating primary temperature measurement devices. Other pairs include stainless steel envelopes with nitrogen, neon, hydrogen, or helium working fluids at temperatures below 100 K, copper/methanol heat pipes for electronics cooling when the heat pipe must operate below the water range, aluminum/ethane heat pipes for spacecraft thermal control in environments when ammonia can freeze, refractory metal envelope/lithium working fluid for high temperature applications. In addition to standard, Constant Conductance Heat Pipes, there are a number of other types of heat pipes, including: Vapor Chambers (planar heat pipe
Material properties of diamond
Diamond is the allotrope of carbon in which the carbon atoms are arranged in the specific type of cubic lattice called diamond cubic. Diamond is an optically isotropic crystal, transparent to opaque. Diamond is the hardest occurring material known. Yet, due to important structural weaknesses, diamond's toughness is only fair to good; the precise tensile strength of bulk diamond is unknown, however strength up to 60 GPa has been observed, it could be as high as 90–100 GPa in the form of nanometer-sized wires or needles,with a corresponding local maximum tensile elastic strain in excess of 9%. The anisotropy of diamond hardness is considered during diamond cutting. Diamond has a high refractive index and moderate dispersion properties which give cut diamonds their brilliance. Scientists classify diamonds into four main types according to the nature of crystallographic defects present. Trace impurities substitutionally replacing carbon atoms in a diamond's crystal structure, in some cases structural defects, are responsible for the wide range of colors seen in diamond.
Most diamonds are electrical insulators but efficient thermal conductors. Unlike many other minerals, the specific gravity of diamond crystals has rather small variation from diamond to diamond. Known to the ancient Greeks as ἀδάμας – adámas and sometimes called adamant, diamond is the hardest known occurring material, scoring 10 on the Mohs scale of mineral hardness. Diamond is strong owing to the structure of its carbon atoms, where each carbon atom has four neighbors joined to it with covalent bonds; the material boron nitride, when in a form structurally identical to diamond, is nearly as hard as diamond. It has been shown that some diamond aggregates having nanometer grain size are harder and tougher than conventional large diamond crystals, thus they perform better as abrasive material. Owing to the use of those new ultra-hard materials for diamond testing, more accurate values are now known for diamond hardness. A surface perpendicular to the crystallographic direction of a pure diamond has a hardness value of 167 GPa when scratched with a nanodiamond tip, while the nanodiamond sample itself has a value of 310 GPa when tested with another nanodiamond tip.
Because the test only works properly with a tip made of harder material than the sample being tested, the true value for nanodiamond is somewhat lower than 310 GPa. The precise tensile strength of diamond is unknown, however strength up to 60 GPa has been observed, theoretically it could be as high as 90–225 GPa depending on the sample volume/size, the perfection of diamond lattice and on its orientation: Tensile strength is the highest for the crystal direction, smaller for the and the smallest for the axis. Diamond has one of the smallest compressibilities of any material. Cubic diamonds have a perfect and easy octahedral cleavage, which means that they only have four planes—weak directions following the faces of the octahedron where there are fewer bonds—along which diamond can split upon blunt impact to leave a smooth surface. Diamond's hardness is markedly directional: the hardest direction is the diagonal on the cube face, 100 times harder than the softest direction, the dodecahedral plane.
The octahedral plane is intermediate between the two extremes. The diamond cutting process relies on this directional hardness, as without it a diamond would be nearly impossible to fashion. Cleavage plays a helpful role in large stones where the cutter wishes to remove flawed material or to produce more than one stone from the same piece of rough. Diamonds crystallize in the diamond cubic crystal system and consist of tetrahedrally, covalently bonded carbon atoms. A second form called lonsdaleite, with hexagonal symmetry, has been found, but it is rare and forms only in meteorites or in laboratory synthesis; the local environment of each atom is identical in the two structures. From theoretical considerations, lonsdaleite is expected to be harder than diamond, but the size and quality of the available stones are insufficient to test this hypothesis. In terms of crystal habit, diamonds occur most as euhedral or rounded octahedra and twinned, flattened octahedra with a triangular outline. Other forms cubes.
There is evidence that nitrogen impurities play an important role in the formation of well-shaped euhedral crystals. The largest diamonds found, such as the Cullinan Diamond, were shapeless; these diamonds therefore contain little if any nitrogen. The faces of diamond octahedrons are lustrous owing to their hardness. A diamond's fracture may be conchoidal or irregular. Diamonds which are nearly round, due to the formation of multiple steps on octahedral faces, are coated in a gum-like skin; the combination of stepped faces, growth defects, nyf produces a "scaly" or corrugated appearance. Many diamonds are so distorted; some diamonds found in Brazil and the Democratic Republic of the Congo are polycrystalline and occur as opaque, darkly colored, radial masses of tiny crystals.
Cookware and bakeware
Cookware and bakeware are types of food preparation containers found in a kitchen. Cookware comprises cooking vessels, such as saucepans and frying pans, intended for use on a stove or range cooktop. Bakeware comprises cooking vessels intended for use inside an oven; some utensils are considered both bakeware. The choice of material for cookware and bakeware items has a significant effect on the item's performance in terms of thermal conductivity and how much food sticks to the item when in use; some choices of material require special pre-preparation of the surface—known as seasoning—before they are used for food preparation. Both the cooking pot and lid handles can be made of the same material but will mean that, when picking up or touching either of these parts, oven gloves will need to be worn. In order to avoid this, handles can be made of non-heat-conducting materials, for example bakelite, plastic or wood, it is best to avoid hollow handles because they are difficult to dry. A good cooking pot design has an "overcook edge", what the lid lies on.
The lid has a dripping edge that avoids condensation fluid from dripping off when handling the lid or putting it down. The history of cooking vessels before the development of pottery is minimal due to the limited archaeological evidence; the earliest pottery vessels, dating from 19,600±400 BP, were discovered in Xianrendong Cave, China. The pottery may have been used as cookware, manufactured by hunter-gatherers. Harvard University archaeologist Ofer Bar-Yosef reported that "When you look at the pots, you can see that they were in a fire." It is possible to extrapolate developments based on methods used by latter peoples. Among the first of the techniques believed to be used by stone age civilizations were improvements to basic roasting. In addition to exposing food to direct heat from either an open fire or hot embers it is possible to cover the food with clay or large leaves before roasting to preserve moisture in the cooked result. Examples of similar techniques are still in use in many modern cuisines.
Of greater difficulty was finding a method to boil water. For people without access to natural heated water sources, such as hot springs, heated stones could be placed in a water-filled vessel to raise its temperature. In many locations the shells of turtles or large mollusks provided a source for waterproof cooking vessels. Bamboo tubes sealed at the end with clay provided a usable container in Asia, while the inhabitants of the Tehuacan Valley began carving large stone bowls that were permanently set into a hearth as early as 7,000 BC. According to Frank Hamilton Cushing, Native American cooking baskets used by the Zuni developed from mesh casings woven to stabilize gourd water vessels, he reported witnessing cooking basket use by Havasupai in 1881. Roasting baskets covered with clay would be filled with the product to be roasted; when the thus-fired clay separated from the basket, it would become a usable clay roasting pan in itself. This indicates a steady progression from use of woven gourd casings to waterproof cooking baskets to pottery.
Other than in many other cultures, Native Americans used and still use the heat source inside the cookware. Cooking baskets are filled with roasting pans with wood coals. Native Americans would form a basket from large leaves to boil water, according to historian and novelist Louis L'Amour; as long as the flames did not reach above the level of water in the basket, the leaves would not burn through. The development of pottery allowed for the creation of fireproof cooking vessels in a variety of shapes and sizes. Coating the earthenware with some type of plant gum, glazes, converted the porous container into a waterproof vessel; the earthenware cookware could be suspended over a fire through use of a tripod or other apparatus, or be placed directly into a low fire or coal bed as in the case of the pipkin. Ceramics conduct heat poorly, however, so ceramic pots must cook over low heats and over long periods of time. However, most ceramic pots will crack if used on the stovetop, are only intended for the oven.
The development of bronze and iron metalworking skills allowed for cookware made from metal to be manufactured, although adoption of the new cookware was slow due to the much higher cost. After the development of metal cookware there was little new development in cookware, with the standard Medieval kitchen utilizing a cauldron and a shallow earthenware pan for most cooking tasks, with a spit employed for roasting. By the 17th century, it was common for a Western kitchen to contain a number of skillets, baking pans, a kettle and several pots, along with a variety of pot hooks and trivets. Brass or copper vessels were common in Asia and Europe, whilst iron pots were common in the American colonies. Improvements in metallurgy during the 19th and 20th centuries allowed for pots and pans from metals such as steel, stainless steel and aluminium to be economically produced. At the 1968 Miss America protest, protestors symbolically threw a number of feminine products into a "Freedom Trash Can", which included pots and pans.
Pottery has been used to make cookware from before dated history. Pots and pans made with this material are inert and non-reactive. Heat is conducted evenly in this material, they can be used for baking in the oven. Metal pots are made from a narrow range of metals because pots and pans need to conduct heat well, but need to be chemically unreactive so that they do not alter the
Convection is the heat transfer due to the bulk movement of molecules within fluids such as gases and liquids, including molten rock. Convection includes sub-mechanisms of advection, diffusion. Convection cannot take place in most solids because neither bulk current flows nor significant diffusion of matter can take place. Diffusion of heat takes place in rigid solids, but, called heat conduction. Convection, additionally may take place in soft solids or mixtures where solid particles can move past each other. Thermal convection can be demonstrated by placing a heat source at the side of a glass filled with a liquid, observing the changes in temperature in the glass caused by the warmer fluid circulating into cooler areas. Convective heat transfer is one of the major types of heat transfer, convection is a major mode of mass transfer in fluids. Convective heat and mass transfer takes place both by diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid.
In the context of heat and mass transfer, the term "convection" is used to refer to the combined effects of advective and diffusive transfer. Sometimes the term "convection" is used to refer to "free heat convection" where bulk-flow in a fluid is due to temperature-induced differences in buoyancy, as opposed to "forced heat convection" where forces other than buoyancy move the fluid. However, in mechanics, the correct use of the word "convection" is the more general sense, different types of convection should be further qualified, for clarity. Convection can be qualified in terms of being natural, gravitational, granular, or thermomagnetic, it may be said to be due to combustion, capillary action, or Marangoni and Weissenberg effects. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere, its oceans, its mantle. Discrete convective cells in the atmosphere can be seen as clouds, with stronger convection resulting in thunderstorms. Natural convection plays a role in stellar physics.
The convection mechanism is used in cooking, when using a convection oven, which uses fans to circulate hot air around food in order to cook the food faster than a conventional oven. The word convection may have different but related usages in different scientific or engineering contexts or applications; the broader sense is in fluid mechanics, where convection refers to the motion of fluid regardless of cause. However, in thermodynamics "convection" refers to heat transfer by convection. Convection occurs on a large scale in atmospheres, planetary mantles, it provides the mechanism of heat transfer for a large fraction of the outermost interiors of our sun and all stars. Fluid movement during convection may be invisibly slow, or it may be obvious and rapid, as in a hurricane. On astronomical scales, convection of gas and dust is thought to occur in the accretion disks of black holes, at speeds which may approach that of light. Convective heat transfer is a mechanism of heat transfer occurring because of bulk motion of fluids.
Heat is the entity of interest being advected, diffused. This can be contrasted with conductive heat transfer, the transfer of energy by vibrations at a molecular level through a solid or fluid, radiative heat transfer, the transfer of energy through electromagnetic waves. Heat is transferred by convection in numerous examples of occurring fluid flow, such as wind, oceanic currents, movements within the Earth's mantle. Convection is used in engineering practices of homes, industrial processes, cooling of equipment, etc; the rate of convective heat transfer may be improved by the use of a heat sink in conjunction with a fan. For instance, a typical computer CPU will have a purpose-made fan to ensure its operating temperature is kept within tolerable limits. A convection cell known as a Bénard cell is a characteristic fluid flow pattern in many convection systems. A rising body of fluid loses heat because it encounters a cold surface. In liquid, this occurs. In the example of the Earth's atmosphere, this occurs.
Because of this heat loss the fluid becomes denser than the fluid underneath it, still rising. Since it cannot descend through the rising fluid, it moves to one side. At some distance, its downward force overcomes the rising force beneath it, the fluid begins to descend; as it descends, it warms again and the cycle repeats itself. Atmospheric circulation is the large-scale movement of air, is a means by which thermal energy is distributed on the surface of the Earth, together with the much slower ocean circulation system; the large-scale structure of the atmospheric circulation varies from year to year, but the basic climatological structure remains constant. Latitudinal circulation occurs because incident solar radiation per unit area is highest at the heat equator, decreases as the latitude increases, reaching minima at the poles, it consists of two primary convection cells, the Hadley cell and the polar vortex, with the Hadley cell experiencing stronger convection due to the release of latent heat energy by condensation of water vapor at higher altitudes during cloud formation.
Longitudinal circulation, on the other hand, comes about because the ocean has a higher specific heat capacity than land (and thermal conduct
Thermal grease is a thermally conductive compound, used as an interface between heat sinks and heat sources. The main role of thermal grease is to eliminate air gaps or spaces from the interface area in order to maximize heat transfer. Thermal grease is an example of a thermal interface material; as opposed to thermal adhesive, thermal grease does not add mechanical strength to the bond between heat source and heat sink. It will have to be coupled with a mechanical fixation mechanism such as screws, applying pressure between the two, spreading the thermal grease onto the heat source. Thermal grease consists of a polymerizable liquid matrix and large volume fractions of electrically insulating, but thermally conductive filler. Typical matrix materials are epoxies, silicones and acrylates. Aluminum oxide, boron nitride, zinc oxide, aluminum nitride are used as fillers for these types of adhesives; the filler loading can be as high as 70–80% by mass, raises the thermal conductivity of the base matrix from 0.17–0.3 W/ up to about 2 W/.
Silver thermal compounds may have a conductivity of 3 to 8 W/ or more, consist of micronized silver particles suspended in a silicone/ceramic medium. However, metal-based thermal grease can be electrically capacitive; the most effective pastes consist entirely of liquid metal a variation of the alloy galinstan, have thermal conductivities in excess of 13 W/. These are difficult to apply evenly and have the greatest risk of causing malfunction due to spillage; these pastes contain gallium, corrosive to aluminium and cannot be used on aluminium heat sinks. In PCs and laptops, thermal paste is invariably used in between the top of the CPU and a heat sink, but may or may not be used in between the CPU die and its integrated heat spreader. In many CPUs, the die is attached to the heat spreader by solder. However, in others the die is not directly attached to the heat spreader, but maintains contact via a layer of thermal paste. In the latter case, performance enthusiasts sometimes pry the heat spreader from the die, replace the thermal paste, of low quality, with a thermal paste which has a greater thermal conductivity - a process known as "delidding".
Liquid metal thermal pastes are used in such instances. Computer cooling Hot-melt adhesive Phase-change material Thermally conductive pad Thermal adhesive List of thermal conductivities