Magnesium oxide (MgO), or magnesia, is a white
hygroscopic solid
mineral that occurs naturally as
periclase and is a source of
magnesium (see also
oxide). It has an
empirical formula of MgO and consists of a
lattice of Mg2+ ions and O2− ions held together by
ionic bonding.
Magnesium hydroxide forms in the presence of water (MgO + H2O → Mg(OH)2), but it can be reversed by heating it to remove moisture.
Magnesium oxide was historically known as magnesia alba (literally, the white mineral from
Magnesia), to differentiate it from magnesia nigra, a black mineral containing what is now known as
manganese.
Related oxides
While "magnesium oxide" normally refers to MgO, the compound
magnesium peroxide MgO2 is also known. According to evolutionary crystal structure prediction,[11] MgO2 is thermodynamically stable at pressures above 116 GPa (gigapascals), and a semiconducting
suboxide Mg3O2 is thermodynamically stable above 500 GPa. Because of its stability, MgO is used as a model system for investigating vibrational properties of crystals.[12]
Calcining at different temperatures produces magnesium oxide of different reactivity. High temperatures 1500 – 2000 °C diminish the available surface area and produces dead-burned (often called dead burnt) magnesia, an unreactive form used as a
refractory. Calcining temperatures 1000 – 1500 °C produce hard-burned magnesia, which has limited reactivity and calcining at lower temperature, (700–1000 °C) produces light-burned magnesia, a reactive form, also known as caustic calcined magnesia. Although some decomposition of the carbonate to oxide occurs at temperatures below 700 °C, the resulting materials appear to reabsorb carbon dioxide from the air.[citation needed]
Applications
Refractory insulator
MgO is prized as a
refractory material, i.e. a solid that is physically and chemically stable at high temperatures. It has the useful attributes of high thermal conductivity and low electrical conductivity. According to a 2006 reference book:[14]
By far the largest consumer of magnesia worldwide is the refractory industry, which consumed about 56% of the magnesia in the United States in 2004, the remaining 44% being used in agricultural, chemical, construction, environmental, and other industrial applications.
It is used extensively as an electrical insulator in tubular construction
heating elements as in electric
stove and
cooktop heating elements. There are several
mesh sizes available and most commonly used ones are 40 and 80 mesh per the
American Foundry Society. The extensive use is due to its high dielectric strength and average thermal conductivity. MgO is usually crushed and compacted with minimal airgaps or voids.
Sorel cement uses MgO as the main component in combination with MgCl2 and water.
Fertilizer
MgO has an important place as a commercial plant fertilizer[15] and as animal feed.[16]
Fireproofing
It is a principal fireproofing ingredient in construction materials. As a construction material,
magnesium oxide wallboards have several attractive characteristics: fire resistance, termite resistance, moisture resistance, mold and mildew resistance, and strength, but also a severe downside as it attracts moisture and can cause moisture damage to surrounding materials [17][14][1]
Medical
Magnesium oxide is used for relief of heartburn and indigestion, as an
antacid, magnesium supplement, and as a short-term
laxative. It is also used to improve symptoms of
indigestion. Side effects of magnesium oxide may include nausea and cramping.[18] In quantities sufficient to obtain a laxative effect, side effects of long-term use may rarely cause
enteroliths to form, resulting in
bowel obstruction.[19]
Waste treatment
Magnesium oxide is used extensively in the soil and
groundwater remediation, wastewater treatment, drinking water treatment, air emissions treatment, and waste treatment industries for its acid buffering capacity and related effectiveness in stabilizing dissolved heavy metal species.[according to whom?]
Many heavy metals species, such as
lead and
cadmium, are least soluble in water at mildly basic conditions (pH in the range 8–11). Solubility of metals increases their undesired bioavailability and mobility in soil and groundwater. Granular MgO is often blended into metals-contaminating soil or waste material, which is also commonly of a low pH (acidic), in order to drive the
pH into the 8–10 range. Metal-hydroxide complexes tend to
precipitate out of aqueous solution in the pH range of 8–10.
MgO is packed in bags around
transuranic waste in the disposal cells (panels) at the
Waste Isolation Pilot Plant, as a CO2 getter to minimize the complexation of
uranium and other
actinides by
carbonate ions and so to limit the
solubility of
radionuclides. The use of MgO is preferred over
CaO since the resulting
hydration product (Mg(OH) 2) is less soluble and releases less
hydration heat. Another advantage is to impose a lower
pH value (about 10.5) in case of accidental water ingress into the dry salt layers, in contast to the more soluble Ca(OH) 2 which would create a higher pH of 12.5 (strongly
alkaline conditions). The Mg2+ cation being the second most abundant cation in
seawater and in
rocksalt, the potential release of magnesium ions dissolving in
brines intruding the
deep geological repository is also expected to minimize the
geochemical disruption.[20]
Pressed MgO is used as an optical material. It is transparent from 0.3 to 7 μm. The
refractive index is 1.72 at 1 μm and the
Abbe number is 53.58. It is sometimes known by the
Eastman Kodak trademarked name Irtran-5, although this designation is obsolete. Crystalline pure MgO is available commercially and has a small use in infrared optics.[25]
An aerosolized solution of MgO is used in library science and collections management for the
deacidification of at-risk paper items. In this process, the alkalinity of MgO (and similar compounds) neutralizes the relatively high acidity characteristic of low-quality paper, thus slowing the rate of deterioration.[26]
Magnesium oxide is used as an oxide barrier in
spin-tunneling devices. Owing to the crystalline structure of its thin films, which can be deposited by
magnetron sputtering, for example, it shows characteristics superior to those of the commonly used amorphous Al2O3. In particular,
spin polarization of about 85% has been achieved with MgO[27] versus 40–60 % with aluminium oxide.[28] The value of
tunnel magnetoresistance is also significantly higher for MgO (600% at room temperature and 1,100 % at 4.2 K[29]) than Al2O3 (ca. 70% at room temperature[30]).
^Tatekawa Y, Nakatani K, Ishii H, et al. (1996). "Small bowel obstruction caused by a medication bezoar: report of a case". Surgery Today. 26 (1): 68–70.
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^Parkin, S. S. P.; Kaiser, C.; Panchula, A.; Rice, P. M.; Hughes, B.; Samant, M.; Yang, S. H. (2004). "Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers". Nature Materials. 3 (12): 862–867.
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^Ikeda, S.; Hayakawa, J.; Ashizawa, Y.; Lee, Y. M.; Miura, K.; Hasegawa, H.; Tsunoda, M.; Matsukura, F.; Ohno, H. (2008). "Tunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB/MgO/CoFeB pseudo-spin-valves annealed at high temperature". Applied Physics Letters. 93 (8): 082508.
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^Wang, D.; Nordman, C.; Daughton, J. M.; Qian, Z.; Fink, J.; Wang, D.; Nordman, C.; Daughton, J. M.; Qian, Z.; Fink, J. (2004). "70% TMR at Room Temperature for SDT Sandwich Junctions with CoFeB as Free and Reference Layers". IEEE Transactions on Magnetics. 40 (4): 2269.
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