محبة الرسول
03-06-2010, 10:52 PM
1- introduction:
a-What is the electrode?
An electrode is a conductor through which electric current is passed. Found in variable forms, electrodes may be wires, plates, or rods. An electrode may be constructed of ****l, such as copper (http://www.wisegeek.com/what-is-copper.htm), silver, lead, or zinc (http://www.wisegeek.com/what-is-zinc.htm). However, an electrode may also be made of a non****l substance, such as carbon (http://www.wisegeek.com/what-is-carbon.htm).
An electrode passes current between a ****llic part and a non****llic part of an electrical circuit (http://www.wisegeek.com/what-is-an-electrical-circuit.htm). Most frequently, conductors that are ****llic carry electrical current. In other circuits, however, current is passed through a non****llic conductor.
In an electrochemical cell, an electrode is called either an anode or a cathode (http://www.wisegeek.com/what-is-a-cathode.htm). An anode is an electrode at which current leaves the cell and oxidation (http://www.wisegeek.com/what-is-oxidation.htm) takes place. For example, an anode is the positive electrode in a storage battery.
A cathode is described as a negative electrode. Current enters the cell at the cathode and reduction takes place. Electrons are repelled from the cathode.
When it comes to certain electric devices, like electric batteries, the anode may actually be negative and the cathode may be positive. In such cases, a conversion of non-electric energy to electric energy occurs and current flows from the negative electrode to the positive electrode. However, most familiar electronic devices involve current that flows from the positive electrode or anode to the negative electrode or cathode.
Depending on the particular voltage that is applied to a cell, an electrode may become an anode or a cathode. Some electrodes may perform both functions. For instance, a bipolar electrode acts as the anode to one cell, while functioning as the cathode of a different cell.
In a primary cell, the reaction cannot be changed or reversed and the anode and cathode values are fixed. Attempting to change the reaction in a primary cell can be dangerous. For example, attempting to reverse the reaction by recharging could cause a battery to explode. Primary cells are meant to be used just once.
A secondary cell allows for reversing the reaction. During charging the anode takes is place as the positive electrode and the cathode is negative. By contrast, the anode is the negative electrode when the cell is being discharged and the cathode plays the positive role.
b-What is the corrision?
Corrosion is a process that takes place when essential properties within a given material begin to deteriorate, after exposure to elements that recur within the environment. Most often, this deterioration is noticed in ****ls and referred to as rust (http://www.wisegeek.com/what-is-rust.htm). What happens in this case is the chemical (http://www.wisegeek.com/what-is-a-chemical.htm) reactions that are set up by an exposure of the electrons in the ****l to the presence of water and oxygen (http://www.wisegeek.com/what-is-oxygen.htm). As an example, a tin (http://www.wisegeek.com/what-is-tin.htm) roof is exposed to the wind and the rain.
Over time, the basic workings of that exposure will allow the creation of acids that begin to alter the surface of the tin. The top layer becomes encrusted with corrosion in the form of a red-brown substance that lacks the cohesive nature of the tin. Continued development of the corrosion will eventually weaken the entire roof and the tin will eventually become so weak that it will no longer provide adequate protection as a roof material.
One of the ways to fight corrosion is to apply a protective layer to any ****l surface that must come in contact with water and oxygen. For example, some forms of enamel are ideal for protecting ****l surfaces. A polymer (http://www.wisegeek.com/what-are-polymers.htm) coating, such as the paint that is used on automobiles, is another example of adequately protecting ****l objects that cannot be easily replaced. Plating is also used in many instances for a number of ****l objects, such as jewelry, household piping and ****l sports equipment.
Cathodic protection also helps to minimize the potential for corrosion. In the process of electrochemisty, there has to be an agent that acts as a cathode (http://www.wisegeek.com/what-is-a-cathode.htm) of an electrochemical cell. This is often an avenue of protection used when steel is involved in the construction. Essentially, the surface of the steel is polarized until the surface has uniform potential, which helps to prevent the occurrence of the corrosion. The steel will remain strong and usable for a much longer period of time. Polarized steel is used to make ships, fuel pipelines for city systems, platforms for off shoring drilling operations, and pier piles (http://www.wisegeek.com/what-are-piles.htm).
2-Electrode Potential (http://www.engineeringtoolbox.com/electrode-potential-d_482.html):
The potential difference between an anode and a cathode can be measured by a voltage measuring device. The absolute potential of the anode and cathode cannot be measured directly. Defining a standard electrode, all other potential measurements can be made against this standard electrode. If the standard electrode potential is set to zero, the potential difference measured can be considered as the absolute potential.
Standard Hydrogen Electrode
· The half-cell in which the hydrogen reaction takes place is called the Standard Hydrogen Electrode - SHE
Standard Electrode Potential
· The potential difference measured between ****l M, and the Standard Hydrogen Electrode - SHE
The electrochemical series consists of a list of ****ls which have been arranged in order of their standard electrode potentials.
IMPORTANT! ****ls which are higher in the electrochemical series displace ****ls which are lower in the sequence, which means when connecting two ****ls, the ****l with lowest potential will corrode.
3-The Copper-copper(II) sulfate electrode is a type of reference electrode (http://en.wikipedia.org/wiki/Reference_electrode), based on the redox (http://en.wikipedia.org/wiki/Redox) reaction with participation of the ****l (copper (http://en.wikipedia.org/wiki/Copper)) and its salt - copper(II) sulfate (http://en.wikipedia.org/wiki/Copper(II)_sulfate). It is used for measuring electrochemical potential (http://en.wikipedia.org/wiki/Electrochemical_potential) and is the most commonly used reference electrode for testing cathodic protection (http://en.wikipedia.org/wiki/Cathodic_protection) corrosion (http://en.wikipedia.org/wiki/Corrosion) control systems.
The corresponding equation can be presented as follow:
Cu2+ + 2e- → Cu0(****l)
This reaction characterized by fast electrode kinetics, meaning that a sufficiently high current can be passed through the electrode with the 100% efficiency of the redox reaction (dissolution (http://en.wikipedia.org/wiki/Solvation) of the ****l or cathodic deposition (http://en.wikipedia.org/wiki/Deposition_(chemistry)) of the copper-ions).
The Nernst equation (http://en.wikipedia.org/wiki/Nernst_equation) below shows the dependence of the potential of the copper-copper(II) sulfate electrode on the activity (http://en.wikipedia.org/wiki/Activity_(chemistry)) or concentration (http://en.wikipedia.org/wiki/Concentration) copper-ions:
Commercial reference electrodes consist of a plastic tube holding the copper rod and saturated solution of copper sulfate. A porous plug on one end allows contact with the copper sulfate electrolyte (http://en.wikipedia.org/wiki/Electrolyte). The copper rod protrudes out of the tube. A voltmeter (http://en.wikipedia.org/wiki/Voltmeter) negative lead is connected to the copper rod.
The potential of a copper copper sulfate electrode is +0.314 volt (http://en.wikipedia.org/wiki/Volt) with respect to the standard hydrogen electrode (http://en.wikipedia.org/wiki/Standard_hydrogen_electrode)
4-Galvanic (and thermogalvanic) corrosion
Two ****ls having different potentials (http://electrochem.cwru.edu/ed/dict.htm#e66) in a conducting (http://electrochem.cwru.edu/ed/dict.htm#c50)electrolyte (http://electrochem.cwru.edu/ed/dict.htm#e12) result in the more anodic (http://electrochem.cwru.edu/ed/dict.htm#a02) ****l usually being attacked by galvanic corrosion. Because of electrical contact between two different ****ls, galvanic corrosion differs from the other forms of corrosion described previously in that the anodic and cathodic (http://electrochem.cwru.edu/ed/dict.htm#c03) sites of the corrosion cell reside separately on the two coupled different ****ls comprising the corrosion cell, while for the other forms of corrosion, the cathodes and anodes exist on the surface of the same ****l.
The value for the differences in potential (http://electrochem.cwru.edu/ed/dict.htm#e66) between two dissimilar ****ls is usually obtained from a listing of the standard equilibrium potentials (http://electrochem.cwru.edu/ed/dict.htm#s13) for the various ****ls (for reactions of the type given in Appendix (http://electrochem.cwru.edu/encycl/art-c02-corrosion.htm#app1#app1), Equation [1]). However, since the standard potentials assume bare ****l surfaces in a standard solution containing their ions at unit concentration (http://electrochem.cwru.edu/ed/dict.htm#c26) (strictly speaking, activity (http://electrochem.cwru.edu/ed/dict.htm#a10)), while in practice ****ls invariably have films on their surfaces and are exposed to nonstandard environments, these potentials are not always reliable guides to the corrosion tendencies of the anodic member of a corrosion cell produced by two coupled dissimilar ****ls. Another factor besides the coupling of two different ****ls that can lead to galvanic corrosion is a difference in temperature at separated sites on the same ****l surface. Such a situation leads to thermogalvanic corrosion. This kind of corrosion can be encountered in heat exchanger systems where temperature differences are common.
5- A membrane electrode assembly (MEA) is an assembled stack of proton exchange membranes (http://en.wikipedia.org/wiki/Proton_exchange_membrane) (PEMs) or alkali anion exchange membrane (http://en.wikipedia.org/wiki/Alkali_anion_exchange_membrane) (AAEMs), catalyst and flat plate electrode used in a fuel cell (http://en.wikipedia.org/wiki/Fuel_cell)[1] (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_no te-0#cite_note-0).
PEM-MEA
The PEM is sandwiched between two electrodes (http://en.wikipedia.org/wiki/Electrode) which have the catalyst embedded in them. The electrodes are electrically insulated from each other by the PEM. These two electrodes make up the anode (http://en.wikipedia.org/wiki/Anode) and cathode (http://en.wikipedia.org/wiki/Cathode) respectively.
The PEM is a proton permeable but electrical insulator (http://en.wikipedia.org/wiki/Electrical_insulation) barrier. This barrier allows the transport of the protons from the anode to the cathode through the membrane but forces the electrons to travel around a conductive path to the cathode. Companies such as DuPont (http://en.wikipedia.org/wiki/DuPont), Dow and E-TEK produce PEM. DuPont PEM are offered under the trade name (http://en.wikipedia.org/wiki/Trade_name)Nafion (http://en.wikipedia.org/wiki/Nafion). The commonly used Nafion PEM are Nafion 112[2] (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_no te-1#cite_note-1), 115, 117, 105.
The electrodes are heat pressed onto the PEM. Commonly used materials for these electrodes are carbon cloth or Toray carbon fiber paper [3] (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_no te-2#cite_note-2). E-TEK produce a carbon cloth called ELAT which maximizes gas transport to the PEM as well as moves water vapor (http://en.wikipedia.org/wiki/Water_vapor) away from the PEM.
References
1. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-0#cite_ref-0) WIPO patent WO/2008/007108 CURRENT DISTRIBUTION SYSTEM FOR ELECTROCHEMICAL CELLS (http://www.freepatentsonline.com/WO2008007108.html)
2. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-1#cite_ref-1) High-performance PEMFCs at elevated temperatures using Nafion 112 membranes (http://cat.inist.fr/?aModele=afficheN&cpsidt=16613558)
3. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-2#cite_ref-2) Effect of gas diffusion layer compression on PEM fuel cell performance (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH1-4J021R6-7&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=163fbc1006c55540a19afa85ac20e53 b)
4. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-3#cite_ref-3) [1] (http://www.lynntech.com/), allowing different shapes, catalysts or membranes to be evaluated.
5. Corrosion Engineering (2nd edition), M. G. Fontana, McGraw-Hill, New York, 1986.
6. Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering (3rd edition), H. H. Uhlig and R. W. Revie, Wiley, New York
7. www.wisegeek.com (http://www.wisegeek.com/)
8. www.thermo.com (http://www.thermo.com/)
9. en.wikipedia.org
10. www.engineeringtoolbox.com (http://www.engineeringtoolbox.com/)
a-What is the electrode?
An electrode is a conductor through which electric current is passed. Found in variable forms, electrodes may be wires, plates, or rods. An electrode may be constructed of ****l, such as copper (http://www.wisegeek.com/what-is-copper.htm), silver, lead, or zinc (http://www.wisegeek.com/what-is-zinc.htm). However, an electrode may also be made of a non****l substance, such as carbon (http://www.wisegeek.com/what-is-carbon.htm).
An electrode passes current between a ****llic part and a non****llic part of an electrical circuit (http://www.wisegeek.com/what-is-an-electrical-circuit.htm). Most frequently, conductors that are ****llic carry electrical current. In other circuits, however, current is passed through a non****llic conductor.
In an electrochemical cell, an electrode is called either an anode or a cathode (http://www.wisegeek.com/what-is-a-cathode.htm). An anode is an electrode at which current leaves the cell and oxidation (http://www.wisegeek.com/what-is-oxidation.htm) takes place. For example, an anode is the positive electrode in a storage battery.
A cathode is described as a negative electrode. Current enters the cell at the cathode and reduction takes place. Electrons are repelled from the cathode.
When it comes to certain electric devices, like electric batteries, the anode may actually be negative and the cathode may be positive. In such cases, a conversion of non-electric energy to electric energy occurs and current flows from the negative electrode to the positive electrode. However, most familiar electronic devices involve current that flows from the positive electrode or anode to the negative electrode or cathode.
Depending on the particular voltage that is applied to a cell, an electrode may become an anode or a cathode. Some electrodes may perform both functions. For instance, a bipolar electrode acts as the anode to one cell, while functioning as the cathode of a different cell.
In a primary cell, the reaction cannot be changed or reversed and the anode and cathode values are fixed. Attempting to change the reaction in a primary cell can be dangerous. For example, attempting to reverse the reaction by recharging could cause a battery to explode. Primary cells are meant to be used just once.
A secondary cell allows for reversing the reaction. During charging the anode takes is place as the positive electrode and the cathode is negative. By contrast, the anode is the negative electrode when the cell is being discharged and the cathode plays the positive role.
b-What is the corrision?
Corrosion is a process that takes place when essential properties within a given material begin to deteriorate, after exposure to elements that recur within the environment. Most often, this deterioration is noticed in ****ls and referred to as rust (http://www.wisegeek.com/what-is-rust.htm). What happens in this case is the chemical (http://www.wisegeek.com/what-is-a-chemical.htm) reactions that are set up by an exposure of the electrons in the ****l to the presence of water and oxygen (http://www.wisegeek.com/what-is-oxygen.htm). As an example, a tin (http://www.wisegeek.com/what-is-tin.htm) roof is exposed to the wind and the rain.
Over time, the basic workings of that exposure will allow the creation of acids that begin to alter the surface of the tin. The top layer becomes encrusted with corrosion in the form of a red-brown substance that lacks the cohesive nature of the tin. Continued development of the corrosion will eventually weaken the entire roof and the tin will eventually become so weak that it will no longer provide adequate protection as a roof material.
One of the ways to fight corrosion is to apply a protective layer to any ****l surface that must come in contact with water and oxygen. For example, some forms of enamel are ideal for protecting ****l surfaces. A polymer (http://www.wisegeek.com/what-are-polymers.htm) coating, such as the paint that is used on automobiles, is another example of adequately protecting ****l objects that cannot be easily replaced. Plating is also used in many instances for a number of ****l objects, such as jewelry, household piping and ****l sports equipment.
Cathodic protection also helps to minimize the potential for corrosion. In the process of electrochemisty, there has to be an agent that acts as a cathode (http://www.wisegeek.com/what-is-a-cathode.htm) of an electrochemical cell. This is often an avenue of protection used when steel is involved in the construction. Essentially, the surface of the steel is polarized until the surface has uniform potential, which helps to prevent the occurrence of the corrosion. The steel will remain strong and usable for a much longer period of time. Polarized steel is used to make ships, fuel pipelines for city systems, platforms for off shoring drilling operations, and pier piles (http://www.wisegeek.com/what-are-piles.htm).
2-Electrode Potential (http://www.engineeringtoolbox.com/electrode-potential-d_482.html):
The potential difference between an anode and a cathode can be measured by a voltage measuring device. The absolute potential of the anode and cathode cannot be measured directly. Defining a standard electrode, all other potential measurements can be made against this standard electrode. If the standard electrode potential is set to zero, the potential difference measured can be considered as the absolute potential.
Standard Hydrogen Electrode
· The half-cell in which the hydrogen reaction takes place is called the Standard Hydrogen Electrode - SHE
Standard Electrode Potential
· The potential difference measured between ****l M, and the Standard Hydrogen Electrode - SHE
The electrochemical series consists of a list of ****ls which have been arranged in order of their standard electrode potentials.
IMPORTANT! ****ls which are higher in the electrochemical series displace ****ls which are lower in the sequence, which means when connecting two ****ls, the ****l with lowest potential will corrode.
3-The Copper-copper(II) sulfate electrode is a type of reference electrode (http://en.wikipedia.org/wiki/Reference_electrode), based on the redox (http://en.wikipedia.org/wiki/Redox) reaction with participation of the ****l (copper (http://en.wikipedia.org/wiki/Copper)) and its salt - copper(II) sulfate (http://en.wikipedia.org/wiki/Copper(II)_sulfate). It is used for measuring electrochemical potential (http://en.wikipedia.org/wiki/Electrochemical_potential) and is the most commonly used reference electrode for testing cathodic protection (http://en.wikipedia.org/wiki/Cathodic_protection) corrosion (http://en.wikipedia.org/wiki/Corrosion) control systems.
The corresponding equation can be presented as follow:
Cu2+ + 2e- → Cu0(****l)
This reaction characterized by fast electrode kinetics, meaning that a sufficiently high current can be passed through the electrode with the 100% efficiency of the redox reaction (dissolution (http://en.wikipedia.org/wiki/Solvation) of the ****l or cathodic deposition (http://en.wikipedia.org/wiki/Deposition_(chemistry)) of the copper-ions).
The Nernst equation (http://en.wikipedia.org/wiki/Nernst_equation) below shows the dependence of the potential of the copper-copper(II) sulfate electrode on the activity (http://en.wikipedia.org/wiki/Activity_(chemistry)) or concentration (http://en.wikipedia.org/wiki/Concentration) copper-ions:
Commercial reference electrodes consist of a plastic tube holding the copper rod and saturated solution of copper sulfate. A porous plug on one end allows contact with the copper sulfate electrolyte (http://en.wikipedia.org/wiki/Electrolyte). The copper rod protrudes out of the tube. A voltmeter (http://en.wikipedia.org/wiki/Voltmeter) negative lead is connected to the copper rod.
The potential of a copper copper sulfate electrode is +0.314 volt (http://en.wikipedia.org/wiki/Volt) with respect to the standard hydrogen electrode (http://en.wikipedia.org/wiki/Standard_hydrogen_electrode)
4-Galvanic (and thermogalvanic) corrosion
Two ****ls having different potentials (http://electrochem.cwru.edu/ed/dict.htm#e66) in a conducting (http://electrochem.cwru.edu/ed/dict.htm#c50)electrolyte (http://electrochem.cwru.edu/ed/dict.htm#e12) result in the more anodic (http://electrochem.cwru.edu/ed/dict.htm#a02) ****l usually being attacked by galvanic corrosion. Because of electrical contact between two different ****ls, galvanic corrosion differs from the other forms of corrosion described previously in that the anodic and cathodic (http://electrochem.cwru.edu/ed/dict.htm#c03) sites of the corrosion cell reside separately on the two coupled different ****ls comprising the corrosion cell, while for the other forms of corrosion, the cathodes and anodes exist on the surface of the same ****l.
The value for the differences in potential (http://electrochem.cwru.edu/ed/dict.htm#e66) between two dissimilar ****ls is usually obtained from a listing of the standard equilibrium potentials (http://electrochem.cwru.edu/ed/dict.htm#s13) for the various ****ls (for reactions of the type given in Appendix (http://electrochem.cwru.edu/encycl/art-c02-corrosion.htm#app1#app1), Equation [1]). However, since the standard potentials assume bare ****l surfaces in a standard solution containing their ions at unit concentration (http://electrochem.cwru.edu/ed/dict.htm#c26) (strictly speaking, activity (http://electrochem.cwru.edu/ed/dict.htm#a10)), while in practice ****ls invariably have films on their surfaces and are exposed to nonstandard environments, these potentials are not always reliable guides to the corrosion tendencies of the anodic member of a corrosion cell produced by two coupled dissimilar ****ls. Another factor besides the coupling of two different ****ls that can lead to galvanic corrosion is a difference in temperature at separated sites on the same ****l surface. Such a situation leads to thermogalvanic corrosion. This kind of corrosion can be encountered in heat exchanger systems where temperature differences are common.
5- A membrane electrode assembly (MEA) is an assembled stack of proton exchange membranes (http://en.wikipedia.org/wiki/Proton_exchange_membrane) (PEMs) or alkali anion exchange membrane (http://en.wikipedia.org/wiki/Alkali_anion_exchange_membrane) (AAEMs), catalyst and flat plate electrode used in a fuel cell (http://en.wikipedia.org/wiki/Fuel_cell)[1] (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_no te-0#cite_note-0).
PEM-MEA
The PEM is sandwiched between two electrodes (http://en.wikipedia.org/wiki/Electrode) which have the catalyst embedded in them. The electrodes are electrically insulated from each other by the PEM. These two electrodes make up the anode (http://en.wikipedia.org/wiki/Anode) and cathode (http://en.wikipedia.org/wiki/Cathode) respectively.
The PEM is a proton permeable but electrical insulator (http://en.wikipedia.org/wiki/Electrical_insulation) barrier. This barrier allows the transport of the protons from the anode to the cathode through the membrane but forces the electrons to travel around a conductive path to the cathode. Companies such as DuPont (http://en.wikipedia.org/wiki/DuPont), Dow and E-TEK produce PEM. DuPont PEM are offered under the trade name (http://en.wikipedia.org/wiki/Trade_name)Nafion (http://en.wikipedia.org/wiki/Nafion). The commonly used Nafion PEM are Nafion 112[2] (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_no te-1#cite_note-1), 115, 117, 105.
The electrodes are heat pressed onto the PEM. Commonly used materials for these electrodes are carbon cloth or Toray carbon fiber paper [3] (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_no te-2#cite_note-2). E-TEK produce a carbon cloth called ELAT which maximizes gas transport to the PEM as well as moves water vapor (http://en.wikipedia.org/wiki/Water_vapor) away from the PEM.
References
1. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-0#cite_ref-0) WIPO patent WO/2008/007108 CURRENT DISTRIBUTION SYSTEM FOR ELECTROCHEMICAL CELLS (http://www.freepatentsonline.com/WO2008007108.html)
2. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-1#cite_ref-1) High-performance PEMFCs at elevated temperatures using Nafion 112 membranes (http://cat.inist.fr/?aModele=afficheN&cpsidt=16613558)
3. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-2#cite_ref-2) Effect of gas diffusion layer compression on PEM fuel cell performance (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TH1-4J021R6-7&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_version=1&_urlVersion=0&_userid=10&md5=163fbc1006c55540a19afa85ac20e53 b)
4. ^ (http://en.wikipedia.org/wiki/Membrane_electrode_assembly#cite_re f-3#cite_ref-3) [1] (http://www.lynntech.com/), allowing different shapes, catalysts or membranes to be evaluated.
5. Corrosion Engineering (2nd edition), M. G. Fontana, McGraw-Hill, New York, 1986.
6. Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering (3rd edition), H. H. Uhlig and R. W. Revie, Wiley, New York
7. www.wisegeek.com (http://www.wisegeek.com/)
8. www.thermo.com (http://www.thermo.com/)
9. en.wikipedia.org
10. www.engineeringtoolbox.com (http://www.engineeringtoolbox.com/)