Chromium dioxide or chromium(IV) oxide is an
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...
with the formula CrO
2. It is a black
synthetic Synthetic things are composed of multiple parts, often with the implication that they are artificial. In particular, 'synthetic' may refer to:
Science
* Synthetic chemical or compound, produced by the process of chemical synthesis
* Synthetic o ...
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
solid.
It once was widely used in
magnetic tape
Magnetic tape is a medium for magnetic storage made of a thin, magnetizable coating on a long, narrow strip of plastic film. It was developed in Germany in 1928, based on the earlier magnetic wire recording from Denmark. Devices that use mag ...
emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althou ...
.
With the increasing popularity of
CDs and
DVDs, the use of chromium(IV) oxide has declined. However, it is still used in
data tape applications for enterprise-class storage systems. It is still considered by many oxide and tape manufacturers to have been one of the best magnetic recording particulates ever invented.
Preparation and basic properties
CrO
2 was first prepared by
Friedrich Wöhler
Friedrich Wöhler () FRS(For) HonFRSE (31 July 180023 September 1882) was a German chemist known for his work in inorganic chemistry, being the first to isolate the chemical elements beryllium and yttrium in pure metallic form. He was the first ...
by decomposition of
chromyl chloride.
Acicular chromium dioxide was first synthesized in 1956 by Norman L. Cox, a chemist at
E.I. DuPont, by decomposing
chromium trioxide
Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.
This compound is a dark-purple ...
in the presence of water at a temperature of and a pressure of 200
MPa. The balanced equation for the
hydrothermal synthesis
Hydrothermal synthesis includes the various techniques of crystallizing substances from high-temperature aqueous solutions at high vapor pressures; also termed "hydrothermal method". The term "hydrothermal" is of geologic origin. Geochemists an ...
is:
:3
CrO3 +
Cr2O3 → 5 CrO
2 +
O2
The magnetic crystal that forms is a long, slender glass-like rod — perfect as a magnetic pigment for recording tape. When commercialized in the late 1960s as a recording medium, DuPont assigned it the tradename of ''Magtrieve''.
CrO
2 adopts the
rutile structure (as do many metal dioxides). As such, each Cr(IV) center has
octahedral coordination geometry and each oxide is
trigonal planar.
[
]
Uses
The crystal's magnetic properties, derived from its ideal shape such as anisotropy
Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's phys ...
which imparted high coercivity
Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
and remanent magnetization
Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...
intensities, resulted in exceptional stability and efficiency for short wavelengths, and it almost immediately appeared in high performance audio tape used in audio cassettes for which treble response and hiss
Hiss or Hissing may refer to:
* Hiss (electromagnetic), a wave generated in the plasma of the Earth's ionosphere or magnetosphere
* Hiss (surname)
* ''Hissing'' (manhwa), a Korean manhwa series by Kang EunYoung
* Noise (electronics) or electroni ...
were always problems. Unlike the imperfectly formed ferric oxide
Iron(III) oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron(II) oxide (FeO), which is rare; and iron(II,III) oxide (Fe3O4), which also occurs naturall ...
coating commonly used, the chromium dioxide crystals were perfectly formed and could be evenly and densely dispersed in a magnetic coating leading to higher signal-to-noise ratios in audio recordings. Chrome tapes did, however, require audio cassette recorders to be equipped with a higher-bias
Bias is a disproportionate weight ''in favor of'' or ''against'' an idea or thing, usually in a way that is closed-minded, prejudicial, or unfair. Biases can be innate or learned. People may develop biases for or against an individual, a group ...
current capability (roughly 50% greater) than that used by ferric oxide to properly magnetize the tape particles. Also introduced was a new equalization (70 μs
A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available.
A microsecond is equal to 1000 ...
) that traded some of the extended high-frequency response for lower noise, resulting in a 5–6 dB improvement in signal-to-noise ratio over ferric oxide audio tapes. These bias and EQ settings were later carried over to "chrome-equivalent" cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, ...
-modified tapes introduced in the mid-1970s by TDK, Maxell, and others. Later research significantly increased the coercivity of the particle by doping
Doping may refer to:
* Doping, adding a dopant to something
* Doping (semiconductor), intentionally introducing impurities into an extremely pure semiconductor to change its electrical properties
* Aircraft dope, a lacquer that is applied to fabr ...
or adsorbing rare elements such as iridium
Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of ...
onto the crystal matrix or by improving the axial length-to-deprecated ratios. The resulting product was potentially a competitor to metallic iron pigments but apparently achieved little market penetration.
Problems
Until manufacturers developed new ways to mill the oxide, the crystals could easily be broken in the manufacturing process, and this led to excessive print-through
Print-through is a generally undesirable effect that arises in the use of magnetic tape for storing analog information, in particular music, caused by contact transfer of signal patterns from one layer of tape to another as it sits wound concent ...
(echo). Output from a tape could drop about 1 dB or so in a year's time. Although the decrease was uniform across the frequency range and noise also dropped the same amount, preserving the dynamic range, the decrease misaligned Dolby
Dolby Laboratories, Inc. (often shortened to Dolby Labs and known simply as Dolby) is an American company specializing in audio noise reduction, audio encoding/compression, spatial audio, and HDR imaging. Dolby licenses its technologies to ...
noise reduction decoders that were sensitive to level settings. The chrome coating was harder than competitive coatings, and that led to accusations of excessive head wear. Although the tape initially wore hard ferrite heads faster than oxide-based tapes, it actually wore softer permalloy
Permalloy is a nickel–iron magnetic alloy, with about 80% nickel and 20% iron content. Invented in 1914 by physicist Gustav Elmen at Bell Telephone Laboratories, it is notable for its very high magnetic permeability, which makes it useful as ...
heads at a slower rate; and head wear was more a problem for permalloy heads than for ferrite heads. After 500 hours of running across ferrite heads, chrome tape had polished the granular surface enough that there was no more detectable wear, and the gap edges remained sharp and distinct. The head wear scare and licensing issues with DuPont kept blank consumer chrome tapes at a great disadvantage versus the eventually more popular Type II tapes that used cobalt-modified iron oxide, but chrome was the tape of choice for the music industry's cassette releases. Because of its low Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cu ...
of approximately , chrome tape lent itself to high-speed thermomagnetic duplication of audio and video cassettes for pre-recorded product sales to the consumer and industrial markets.
Producers
DuPont licensed the product to Sony
, commonly stylized as SONY, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. As a major technology company, it operates as one of the world's largest manufacturers of consumer and professional ...
in Japan and BASF
BASF SE () is a German multinational chemical company and the largest chemical producer in the world. Its headquarters is located in Ludwigshafen, Germany.
The BASF Group comprises subsidiaries and joint ventures in more than 80 countries ...
in Germany in the early 1970s for regional production and distribution. Japanese competitors developed cobalt-adsorbed (TDK: ''Avilyn'') and cobalt ferrite
Cobalt ferrite is a semi-hard ferrite with the chemical formula of CoFe2O4 (CoO·Fe2O3). The substance can be considered as between soft and hard magnetic material and is usually classified as a semi-hard material.
Applications
It is mainly use ...
(Maxell: '' Epitaxial'') "chrome equivalent" Type II audio cassettes and various videotape formats as substitutes. Added to that was the problem that the production of CrO2 yielded toxic by-products of which Japanese manufacturers had great difficulty properly disposing. BASF eventually became the largest producer of both the chromium dioxide pigment and chrome tapes, basing its VHS & S-VHS video tape, audio cassettes, and 3480 data cartridges on this formulation. DuPont and BASF had also introduced chrome-cobalt "blended" oxide pigments which combined about 70% cobalt-modified iron oxide with 30% chrome oxide into a single coating, presumably to offer improved performance at lower costs than pure chrome. Many high-grade VHS tapes also used much smaller amounts of chrome in their formulations because its magnetic properties combined with its cleaning effects on heads made it a better choice than aluminium oxide or other non-magnetic materials added to VHS tape to keep heads clean. DuPont discontinued its production of chromium dioxide particles in the 1990s. In addition to BASF, which no longer owns a tape manufacturing division, Bayer AG
Bayer AG (, commonly pronounced ; ) is a German multinational pharmaceutical and biotechnology company and one of the largest pharmaceutical companies in the world. Headquartered in Leverkusen, Bayer's areas of business include pharmaceutica ...
of Germany, Toda Kogyo and Sakai Chemical of Japan also do or can produce the magnetic particles for commercial applications.
References
Further reading
*
*
* O'Kelly, Terence (1981). "The Technical Argument for Chromium Dioxide." ''The BASF Inventor's Notebook'' Number 6; http://www.ant-audio.co.uk/Tape_Recording/Library/Chrome.pdf
{{oxygen compounds
Chromium–oxygen compounds
Audio engineering
Transition metal oxides
Ferromagnetic materials
Chromium(IV) compounds