Electroforming Definitions

Electroforming Definitions

Electroforming Key Definitions

Ampere

The measurement unit of current. Symbol: “A” or “amp”.

Acid

A compound that dissociates to produce hydrogen (H+) cations when dissolved in water. Contrast with base.

Active Metal

A metal that is easily oxidized (corroded) in air. For example, sodium will violently react with air, aluminum will always have an air-formed oxide film on its surface, and iron is easily rusted. These metals have high negative standard electrode potentials and are high the on the electromotive series. Contrast with noble metal.

Anodizing

A process to produce an oxide film or coating on metals and alloys by electrolysis. The metal to be treated is made the anode in an electrolytic cell and its surface is electrochemically oxidized. Anodization can improve certain surface properties, such as corrosion resistance, abrasion resistance, hardness, appearance, etc. One metal very often anodized is aluminum, all the above properties improve, furthermore, since the surface film is porous, the aluminum metal can even be colored by the application of pigments or dies in the pores.

Anode

The electrode where oxidation occurs in an electrochemical cell. It is the positive electrode in an electrolytic cell, while it is the negative electrode in a galvanic cell. The current on the anode is considered a positive current according to the international convention; however, in electroanalytical chemistry, the anodic current is often considered negative. Contrast with the cathode.

Anion

A negatively charged ion.

AC

Stands for alternating current. However this term is also used in connection with AC voltage, that is, an “alternating” voltage that will cause an “AC current” to flow in a conductor, and also in connection with AC power.

Atom

An atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutron).

Base

A compound that dissociates to produce hydroxyl (OH-) anions when dissolved in water (also called “caustic” or “alkali”). Contrast with acid.

Cathode

The electrode where reduction occurs in an electrochemical cell. It is the negative electrode in an electrolytic cell, while it is the positive electrode in a galvanic cell. The current on the cathode is considered a negative current according to the international convention; however, in electroanalytical chemistry, the cathodic current is often considered positive.

Cation

A positively charged ion.

Caustic

See base. Sometimes it specifically refers to sodium hydroxide.

Current

The movement of electrical charges in a conductor; carried by electrons in an electronic conductor (electronic current) or by ions in an ionic conductor (ionic current). “By definition” the electrical current always flows from the positive potential end of the conductor toward the negative potential end, independent of the actual direction of motion of the differently charged current carrier (or “charge carrier”) particles. Two kinds of currents must be distinguished: “direct current (DC)” and “alternating current (AC)”. Direct current is the continuous unidirectional flow of current while alternating current is the oscillating (back and forth) flow of current. In electrochemistry, we almost always use direct current. Consequently, the term “current” always designates “DC” in this dictionary unless specifically stated to be “AC”. The normal household current is an alternating current. The measurement unit of current is the ampere.

As mentioned above, the “defined” current flows from the positive terminal of the current source, trough the load, to the negative terminal of the source. Consequently, inside the “source” (whether it is electromechanical or electrochemical), the current must flow from the negative terminal to the positive terminal since there must be a complete circuit. This concept is especially important in electrochemistry because an electrochemical cell can be either a current “source” (galvanic cell) or a “load” (electrolytic cell). Furthermore, a rechargeable battery operates as a “source” during discharge and as a “load” during charge.

Current Density

Electroplating current divided by the total surface area of the part. Influences deposition rate, and plating quality. The higher the current density, the faster the plating rate will be.

Current Efficiency

The fraction, usually expressed as a percentage, of the current passing through an electrolytic cell (or an electrode) that accomplishes the desired chemical reaction. Inefficiencies may arise from reactions other than the intended reaction taking place at the electrodes, or side reactions consuming the product. The expected production can be theoretically calculated and compared with the actual production.

Counterion

The mobile ion in ion exchange. The ion with opposite charge to that of the fixed site on the ion-exchange resin. Contrast with fixed ion.

Desalination

A process to produce clean (potable) water from brackish or seawater. Electrodialysis is an electrochemical technique often used for this purpose.

DC

Stands for direct current. However this term is also used in connection with dc voltage, that is, a steady voltage that will cause a “DC current” to flow in a conductor, and also in connection with DC power.

Depolarization

The partial or complete elimination or counteraction of polarization

Dielectric

An insulating material with special characteristics. When a constant electrical field is applied to a dielectric, an electrical current will flow temporarily. However this is not a true current, it only generates localized charges, the field polarizes the molecules of the dielectric, producing some charge on its surfaces that create an electrical field equal to but opposed in direction to the original field causing the current to stop. This phenomenon is utilized in capacitors to store charge.

Diffusion

The movement of chemical species (ions or molecules ) under the influence of concentration difference. The species will move from the high concentration area to the low concentration area till the concentration is uniform in the whole phase. Diffusion in solutions is the most important phenomenon in electrochemistry, but diffusion will also occur in gasses and solids.

Electrical field

A region of space, associated with a distribution of electric charge, in which forces due to that charge act upon other electric charges.

Electrolysis

A process that decomposes a chemical compound into its elements or produces a new compound by the action of an electrical current. The electrical current is passed trough an electrolytic cell and oxidation/reduction reactions occur at the electrodes. E.g., water can be decomposed into hydrogen and oxygen, or a metal can be electroplated by electrolysis.

Electroforming

A process to produce metallic objects with a technique that is essentially precision electroplating. The metal is deposited onto a “mandrel” or “former” of suitable shape to the desired thickness, followed by the removal of the mandrel to produce a free standing metal object. One of the advantages of this production technique is that very complicated shapes can be produced with a single operation.

It is often used to produce very precise optical elements, and solid-state electronic devices (integrated circuit boards, computer chips). Other applications are the production of flat or perforated metal sheets, seamless perforated metal tubes, and metal bellows. Two very prominent past applications of this technique were the production of “stampers” for the old-fashioned musical (phonograph) records and “electrotypes” for the printing industry.

Practically any metal or alloy that can be electroplated can also be used for electroforming. The preparation of the removable mandrel is an important step in this process. One example is the use of machined copper or brass that is surface treated to permit electroplating that will closely follow the mandrel surface but will not permit strong adhesion of the electroformed piece.

Electrode

The two electronically conducting parts of an electrochemical cell. See also anode and cathode. These can be simple metallic structures (rods, sheets, etc.) or much more complicated, composite structures. E.g., the electrodes in a rechargeable battery will also “contain” the chemicals being converted during its operation. The term “electrode” is also used to denote complex assemblies that include an electrode in a small vessel, which contains an electrolyte and is equipped with an ion-permeable separator. Reference electrodes are such assemblies.

Electrodeposition

A process for depositing solid materials on an electrode surface using electrolysis. It is a somewhat loosely used term that is applied to many technologies. There are a number of metal deposition technologies. However, not only metals but also different compounds can be electrodeposited. This is used most often for the formation of oxides (such as manganese dioxide and lead dioxide) by anodic oxidation of dissolved salts. Deposition can also be achieved electrophoretically.

Electrolyte

A chemical compound (salt, acid, or base) that dissociates into electrically charged ions when dissolved in a solvent. The resulting electrolyte (or electrolytic) solution is an ionic conductor of electricity. Very often, the so formed solution itself is simply called an “electrolyte.” Also, molten salts and molten salt solutions are often called “electrolyte” when used in electrochemical cells, see ionic liquid.

Electrolytic Degreasing

Process for removal of grease, oil, etc from metal surfaces in preparation for electroplating. The metal is made the cathode in an electrolytic cell containing strongly basic (sometimes hot) solution that dissolves these coatings. The strong hydrogen evolution occurring on the cathode may reduce some of the coatings, and the strong bubble evolution removes the coatings mechanically, while the agitation of the solution helps the chemical dissolution of the coatings by the base. Also called: “electrolytic cleaning”, “electrochemical cleaning”, “electrocleaning”, and “electrochemical degreasing”.

Electrolytic Pickling

Process for removal of oxide scales from metal surfaces in preparation for electroplating. The metal is made the cathode in an electrolytic cell containing strongly acidic (sometimes hot) solution that dissolves the oxide scales. The strong hydrogen evolution occurring on the cathode may reduce some of the oxides, and the strong bubble evolution removes the scales mechanically, while the agitation of the solution helps the chemical dissolution of the scales by the acid.

Electroplating

The process that produces a thin, metallic coating on the surface on another metal (or any other conductor, e.g., graphite). The metal substrate to be coated is made the cathode in an electrolytic cell where the cations of the electrolyte are the positive ions of the metal to be coated on the surface.

When a current is applied, the electrode reaction occurring on the cathode is the reduction of the metal ions to metal. E.g., gold ions can be discharged form a gold solution to form a thin gold coating on a less expensive metal to produce “custome” jewelry. Similarly, chromium coating is often applied to steel surfaces to make them more “rust resistant”.

Electroplating is also used in the production of integrated circuits on computer chips and for other modern electronic instrumentation. The anode material can either be the metal to be deposited (in this case the electrode reaction is electrodissolution that continuously supplies the metal ions) or the anode can be of inert material and the anodic reaction is oxygen evolution (in this case the plating solution is eventually depleted of metal ions). Also called “electrodeposition”.

Farad

The measurement unit of capacitance. Symbol: “F”, which is the same as the symbol of the Faraday number. It is usually obvious from the context which meaning is appropriate. A capacitor has a capacitance of one farad when one coulomb charges it to one volt.

Faraday’s Laws

Some of the most fundamental laws of electrochemistry discovered by Faraday in the 1830’s. They are usually stated as: (1) in any electrolytic process the amount of chemical change produced is proportional to the total amount of electrical charge passed through the cell. (2) The mass of the chemicals changed is proportional to the chemicals’ equivalent weight. The proportionality constant being the Faraday Number.

Fixed Ion

The permanently attached charged fragment in an ion-exchange resin. Contrast with counterion

Gram-mole

An amount of a compound equal in grams to its molecular weight. E.g., the molecular weight of water is 18, so 18 grams of water is called a gram-mole of water. This provides an atomistically fundamental unit because one gram-mole of any material will contain the same number of molecules (this is a very large number, called “Avogadro’s” number). One gram-mole of hydrogen gas contains the exactly same number of molecules as one gram-mole of table salt (sodium chloride), even though the latter is much heavier. The simplified expression of “mole” is often used in place of “gram-mole” and also in place of gram-atom. It is usually obvious from the context which meaning is appropriate. The measurement unit and symbol of the “gram-mole” or “mole” is the “mol”.

Hydrolysis

A chemical reaction in which water reacts with another substance and gives decomposition or other products, often a reaction of water with a salt to create an acid or a base.

Ion

An electrically charged chemical particle. “Anions” are negatively charged, and “cations” are positively charged.

Ion-exchange Membrane

A plastic sheet formed from ion-exchange resin. The utility of such membranes is based on their property that they are permeable preferentially only to either positive ions (cation-exchange membrane) or to negative ions (anion-exchange membrane).

Ion-exchange Membrane

A polymeric resin that contains electrically charged fragments (“fixed ions”) permanently attached to the polymer backbone, electrical neutrality is achieved by attached mobile “counterions” in the solution phase the resin is immersed into. A practical use of such resin is the removal of unwanted ions from a solution by replacing them with other ions. E.g., a cation exchange resin containing fixed negative charges with attached mobile sodium ions can be used to remove “hardness” from water if the calcium and magnesium ions are more strongly attracted to the resin and therefore will replace the sodium ions. Eventually all the sodium ions will go into solution and the ion-exchange process terminates. The resin can be regenerated by soaking in a high concentration sodium salt solution. Such process can also be used to remove unwanted ions from polluted water streams.

Molecule

The smallest physical unit of a substance that retains all the physical and chemical properties of that substance. It may consist of a single atom or a group of atoms bonded together chemically.

Noble Metal

A metal that resists oxidation (corrosion) in air, and therefore retains its metallic luster. Examples are platinum and gold. These metals have high positive standard electrode potentials and are the lowest ones on the electromotive series. Contrast with active metal.

Ohm’s Law

The relation amongst the current flowing through a resistor and the potential difference between the two ends of the resistor. The potential difference is equal to the product of the current and the resistance (volt = ampere x ohm).

Passivation

The formation of a thin adherent film or layer on the surface of a metal or mineral that acts as a protective coating to protect the underlying surface from further chemical reaction, such as corrosion, electrodissolution, or dissolution. The passive film is very often, though not always, an oxide. A passivated surface is often said to be in a “passive state.” The surface oxidation can result from chemical or electrochemical (anodic) oxidation. During anodic passivation, using linear-sweep voltammetry, the current first increases with potential, then falls to a very small value.

pH

A measure of the acidity/alkalinity (basicity) of a solution. The pH scale extends from 0 to 14 (in aqueous solutions at room temperature). A pH value of 7 indicates a neutral (neither acidic nor basic) solution. A pH value of less than 7 indicates an acidic solution, the acidity increases with decreasing pH value. A pH value of more than 7 indicates a basic solution, the basicity or alkalinity increases with increasing pH value.

The pH of a solution is equal to the negative, ten-based logarithm of the activity of the hydrogen ions in the solution. Neutral water dissociates into equal amounts of hydrogen (H+) cations and hydroxyl (OH-) anions. As the product of the concentrations (activities) of the two ions is always a constant 10-14, water has a pH of 7. In acidic solutions, the hydrogen ions are in excess, while in basic solutions the hydroxyl ions are in excess.

Polarity

Indicates the sign of the potential of an electrode, that is, it can be negative or positive.

Polarization

The change potential of an electrode from its equilibrium potential upon the application of a current. Somewhat confusingly, the term “polarization” is often also used in place of overvoltage. In bioelectrochemistry: the separation and grouping of opposite electrical charges so that two clear groups are perceptible as two distinct poles. Similarly, the term in general means the separation and grouping of opposite electrical charges (across any material) so that two clear groups are perceptible as two distinct poles.

Potentiometer

Can be used in more than one meaning:

1. A continuously variable resistor. More precisely, a resistor with continuously variable tap. This can provide three resistance values, a fixed resistance between the two end connectors, and two variable resistances, one between either end connector and the variable tap connector. The sum of the two variable resistances is the fixed resistance.

2. A somewhat archaic measurement system, based on a resistor with a continuously variable tap, that can be used to measure the electromotive force of electrochemical cells that can be easily polarized by current. It uses a comparison technique to compare a “standard” voltage source to the unknown, under conditions of practically zero current. It is seldom used today because high input resistance voltmeters and electrometers are readily available.

Radical

A molecule or atom possessing an unpaired electron.

Rectifier

Electrical equipment that converts alternating current into direct current.

Resistivity (electrical)

The measure of a material’s inability to carry electrical current. The measurement unit of the resistivity (resistance) is the ohm. See also impedance. The reciprocal of conductivity.

Surface Tension

The work required to increase a surface area, for example, to increase the size of a drop of water. When two phases are involved, it is often called an “interfacial tension,” for example, to increase the size of a mercury droplet under water.

Throwing Power

A qualitative term used in electroplating to describe the ability of the system to produce a uniformly thick deposit on the substrate surface. That is, the “throwing power” is considered “good” when the current distribution is uniform even on an irregularly shaped substrate.

Viscosity

The resistance to flow exhibited by a liquid or gas subjected to deformation.

Volt

The measurement unit of the electrical potential. Symbol: “V”. A term named in honor of Alessandro Volta,.

Watt

The measurement unit of electrical power. Symbol: “W”. Related units are that of power density: watt/kilogram (W/kg) and watt/liter (W/l).

Technical Resources

Electroformed Optics

Optical Coatings

Optical Components