This form of aluminum oxide, commercially known as activated alumina, is used for drying gases and certain liquids. It also serves as a carrier for catalysts of various chemical reactions. Anodic aluminum oxide AAO , typically produced via the electrochemical oxidation of aluminum, is a nanostructured aluminum-based material with a very unique structure.
AAO contains cylindrical pores that provide for a variety of uses. It is a thermally and mechanically stable compound while also being optically transparent and an electrical insulator.
The pore size and thickness of AAO can easily be tailored to fit certain applications, including acting as a template for synthesizing materials into nanotubes and nanorods. Another major compound is aluminum sulfate , a colourless salt obtained by the action of sulfuric acid on hydrated aluminum oxide. The commercial form is a hydrated crystalline solid with the chemical formula Al 2 SO 4 3.
It is used extensively in paper manufacture as a binder for dyes and as a surface filler. Aluminum sulfate combines with the sulfates of univalent metals to form hydrated double sulfates called alums.
The most important of such salts is aluminum potassium sulfate, also known as potassium alum or potash alum. These alums have many applications, especially in the production of medicines, textiles, and paints. The reaction of gaseous chlorine with molten aluminum metal produces aluminum chloride ; the latter is the most commonly used catalyst in Friedel-Crafts reactions —i.
It is one of several aluminum salts employed by the cosmetics industry. With hydrogen, aluminum forms aluminum hydride , AlH 3 , a polymeric solid from which are derived the tetrohydroaluminates important reducing agents. Lithium aluminum hydride LiAlH 4 , formed by the reaction of aluminum chloride with lithium hydride, is widely used in organic chemistry—e.
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The Aluminum Advantage. Aluminum Advantage. Product Markets. The track on the CD begins at the inside and spirals outward so a disc played from beginning to end slows its rotation rate during playback.
The program area is With a scanning speed of 1. A disc with data packed slightly more densely is tolerated by most players though some old ones fail. Using a linear velocity of 1. The change in height between pits and lands results in a difference in the way the light is reflected. Because the pits are indented into the top layer of the disc and are read through the transparent polycarbonate base, the pits form bumps when read.
This causes partial cancellation of the laser's reflection from the surface. By measuring the reflected intensity change with a photodiode , a modulated signal is read back from the disc. To accommodate the spiral pattern of data, the laser is placed on a mobile mechanism within the disc tray of any CD player.
This mechanism typically takes the form of a sled that moves along a rail. The sled can be driven by a worm gear or linear motor. Where a worm gear is used, a second shorter-throw linear motor, in the form of a coil and magnet, makes fine position adjustments to track eccentricities in the disk at high speed.
Some CD drives particularly those manufactured by Philips during the s and early s use a swing arm similar to that seen on a gramophone. This mechanism allows the laser to read information from the center to the edge of a disc without having to interrupt the spinning of the disc itself.
The pits and lands do not directly represent the 0's and 1's of binary data. Instead, non-return-to-zero, inverted encoding is used: a change from either pit to land or land to pit indicates a 1, while no change indicates a series of 0's.
There must be at least 2, and no more than 10 0's between each 1, which is defined by the length of the pit. This, in turn, is decoded by reversing the eight-to-fourteen modulation used in mastering the disc, and then reversing the cross-interleaved Reed—Solomon coding , finally revealing the raw data stored on the disc.
CDs are susceptible to damage during handling and from environmental exposure. Pits are much closer to the label side of a disc, enabling defects and contaminants on the clear side to be out of focus during playback. Consequently, CDs are more likely to suffer damage on the label side of the disc.
Scratches on the clear side can be repaired by refilling them with similar refractive plastic or by careful polishing. The data integrity of compact discs can be measured using surface error scanning , which is able to measure the rates of different types of data errors, known as C1 , C2 , CU and extended finer-grain error measurements known as E11 , E12 , E21 , E22 , E31 and E32 , of which higher rates indicate a possibly damaged or unclean data surface, low media quality, deteriorating media and recordable media written to by a malfunctioning CD writer.
Error scanning can reliably predict data losses caused by media deteriorating. Support of error scanning varies among vendors and models of optical disc drives , and extended error scanning known as "advanced error scanning" in Nero DiscSpeed has only been available on Plextor and some BenQ optical drives so far, as of The digital data on a CD begins at the center of the disc and proceeds toward the edge, which allows adaptation to the different size formats available.
Standard CDs are available in two sizes. By far, the most common is millimetres 4. Discs are 1. The official Philips history says this capacity was specified by Sony executive Norio Ohga to be able to contain the entirety of Beethoven's Ninth Symphony on one disc.
Instead, however, the information density was lowered by 30 percent to keep the playing time at 74 minutes. The format is a two-channel bit PCM encoding at a Four-channel sound was to be an allowable option within the Red Book format, but has never been implemented.
Monaural audio has no existing standard on a Red Book CD; thus, the mono source material is usually presented as two identical channels in a standard Red Book stereo track i. CD-Text is an extension of the Red Book specification for an audio CD that allows for the storage of additional text information e.
The information is stored either in the lead-in area of the CD, where there is roughly five kilobytes of space available or in the subcode channels R to W on the disc, which can store about 31 megabytes.
These six bits store the graphics information. This extra data is stored in subcode channels R-W. Introduced in , it was developed by Sony and Philips, the same companies that created the Red Book. CD- MIDI is a format used to store music-performance data, which upon playback is performed by electronic instruments that synthesize the audio. For the first few years of its existence, the CD was a medium used purely for audio.
However, in , the Yellow Book CD-ROM standard was established by Sony and Philips, which defined a non-volatile optical data computer data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive. Overall picture quality is intended to be comparable to VHS video. Poorly compressed VCD video can sometimes be lower quality than VHS video, but VCD exhibits block artifacts rather than analog noise and does not deteriorate further with each use.
This approximates the overall resolution of an analog VHS tape, which, although it has double the number of vertical scan lines, has a much lower horizontal resolution. While no specific limit on SVCD video length is mandated by the specification, one must lower the video bit rate, and therefore quality, to accommodate very long videos.
It is usually difficult to fit much more than minutes of video onto one SVCD without incurring significant quality loss, and many hardware players are unable to play video with an instantaneous bit rate lower than to kilobits per second. Launched in , the discs were designed to hold nearly high-quality images, scanned prints and slides using special proprietary encoding.
They are intended to play on CD-i players, Photo CD players, and any computer with suitable software irrespective of operating system. The images can also be printed out on photographic paper with a special Kodak machine. The Philips Green Book specifies a standard for interactive multimedia compact discs designed for CD-i players This format was supposed to be more compatible with older audio CD players. Enhanced Music CD, also known as CD Extra or CD Plus, is a format which combines audio tracks and data tracks on the same disc by putting audio tracks in a first session and data in a second session.
It was developed by Philips and Sony, and it is defined in the Blue Book. VinylDisc is the hybrid of a standard audio CD and the vinyl record. The vinyl layer on the disc's label side can hold approximately three minutes of music. In , material costs were 30 cents for the jewel case and 10 to 15 cents for the CD. This was done because the apparent value increased. The incremental cost, though, to produce an MP3 is negligible.
A photosensitive dye is then applied, after which the discs are metalized and lacquer-coated. The write laser of the CD recorder changes the color of the dye to allow the read laser of a standard CD player to see the data, just as it would with a standard stamped disc. CD-Rs follow the Orange Book standard. CD-R recordings are designed to be permanent. Over time, the dye's physical characteristics may change causing read errors and data loss until the reading device cannot recover with error correction methods.
Errors can be predicted using surface error scanning. The design life is from 20 to years, depending on the quality of the discs, the quality of the writing drive, and storage conditions. The recordable audio CD is designed to be used in a consumer audio CD recorder.
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