Aquadag
   HOME

TheInfoList



OR:

Aquadag is a trade name for a water-based
colloidal A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exten ...
graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
coating commonly used in
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms on an oscilloscope, a ...
s (CRTs). It is manufactured by Acheson Industries, a subsidiary of ICI. The name is a shortened form of "Aqueous Deflocculated Acheson Graphite", but has become a generic term for conductive graphite coatings used in vacuum tubes. Other related products include Oildag, Electrodag and Molydag. Deflocculation refers to the distribution of powdered high purity graphite in an aqueous solution containing approximately 2% to 10% by weight of various Tannic/Gallotannic acid variants and separating the colloidal graphite suspension from the remaining unsuspended graphite particulates. The product names are often printed with DAG in upper case (e.g. AquaDAG). It is used as an electrically conductive coating on insulating surfaces, and as a lubricant.


Properties

Aquadag consists of a dispersion of colloidal graphite in distilled water. It is provided in concentrated paste form and is usually diluted with distilled water to a desired consistency before application. It can be applied by brushing, swabbing, spraying, or dipping, after which the surface is dried, leaving a layer of pure graphite. After drying the coating is electrically conductive. Its resistance and other electrical properties vary with degree of dilution and application method. When diluted 1:1 and applied by brush its resistance is: :Air-dried ~800 ohms per square :Heated to 200 °C ~500 ohms per square :Heated to 300 °C ~20–30 ohms per square


Use in cathode ray tubes

A conductive aquadag coating applied to the inside of the glass envelope of
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube containing one or more electron guns, which emit electron beams that are manipulated to display images on a phosphorescent screen. The images may represent electrical waveforms on an oscilloscope, a ...
s, serves as a high-voltage electrode. The coating covers the inside walls of the "bell" of the CRT tube, from just inside the neck, and stops just short of the screen. Due to the graphite, it is electrically conductive and forms part of the high-voltage positive
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can consist of a varie ...
, the second
anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...
, which accelerates the electron beam. The second anode is a metal cylinder inside the neck of the tube, connected to a high positive voltage of 18 to 25 kilovolts. It has spring clips, which press against the walls of the tube, making contact with the aquadag coating so it also carries this high positive voltage. The
electron beam Since the mid-20th century, electron-beam technology has provided the basis for a variety of novel and specialized applications in semiconductor manufacturing, microelectromechanical systems, nanoelectromechanical systems, and microscopy. Mechani ...
from the
electron gun file:Egun.jpg, Electron gun from a cathode-ray tube file:Vidicon Electron Gun.jpg, The electron gun from an RCA Vidicon video camera tube An electron gun (also called electron emitter) is an electrical component in some vacuum tubes that produc ...
in the neck of the tube is accelerated by the high voltage on the anode and passes through it to strike the screen. The aquadag coating has two functions: it maintains a uniform electric field inside the tube near the screen, so the electron beam remains collimated and is not distorted by external fields, and it collects the electrons after they have hit the screen, serving as the return path for the cathode current. When the electron beam hits the screen, in addition to causing the
fluorescent Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...
phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or ...
coating to give off light, it also knocks other electrons out of the surface. These ''
secondary electrons Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation In physics, radiation is the emission or transmission of energy in the form of waves or particle ...
'' are attracted to the high positive voltage of the coating and return through it to the anode power supply. Without the coating a negative space charge would develop near the screen, deflecting the electron beam. A typical value of beam current collected by the anode coating is 0.6 mA. In some CRTs the aquadag coating performs a third function, as a
filter capacitor Capacitors have many uses in electronic and electrical systems. They are so ubiquitous that it is rare that an electrical product does not include at least one for some purpose. Capacitors allow only AC signals to pass when they are charged blocki ...
for the high-voltage anode supply. A second conductive coating is applied to part of the outside of the tube facing the inside coating. This outside coating is connected to the ground side of the anode supply, thus the full anode voltage is applied between the coatings. The sandwich of the two coatings separated by the
dielectric In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...
glass wall of the tube form a final
capacitor In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
to filter out ripple from the anode supply. Although the capacitance is small, around 500 pF, due to the low anode current it is sufficient to act as a filter capacitor. In the television tube manufacturing industry, the manufacturing step that applies the aquadag is called "dagging".


Other uses

Aside from its use in the production of CRTs, Aquadag is used in many types of high-voltage lab apparatus where a conductive coating is needed on an insulating surface. The surfaces of some metals (most notably aluminum) can develop nonconductive oxide layers, which tend to disrupt the electrostatic field produced around the surface of the metal when used as an electrode. Aquadag is not subject to such effects and provides a completely uniform equipotential surface for electrostatics. Producers of continuous filament fiberglass will coat their product with Aquadag when a conductive property is required. Aquadag was also used in the production of some copper oxide rectifiers, to help make the ohmic connections to their counterelectrodes.


Other dags

There are also deflocculated graphite products dispersed in liquids other than water. Acheson has extended the use of the dag
brandname A brand is a name, term, design, symbol or any other feature that distinguishes one seller's goods or service from those of other sellers. Brands are used in business, marketing, and advertising for recognition and, importantly, to create and ...
to non-graphite products e.g. the copper-based Electrodag 437 conductive paint.


References

{{reflist


External links


AquaDAG Product data sheet
fro

Vacuum tubes