Magnesium oxide, also known as bitter soil, is a kind of magnesium oxide, the chemical formula is MgO, the molecular structure is a typical NaCl structure, and magnesium atoms and oxygen atoms are arranged in sequence.
Magnesium oxide is a high-performance fine inorganic material that can be widely used in the fields of catalysis, electronics and ceramics. Magnesium oxide, with its large specific surface area, is a promising catalyst support that can be applied to a variety of reactions, as well as a promising chemical adsorbent for destructive adsorption of various pollutants. In addition to the above-mentioned applications, due to its unique characteristics, magnesium oxide can also be used to make effect pigments and sensor materials under certain conditions.
01
Preparation of Magnesium Oxide with Special Morphology
The microscopic morphology and properties of materials are often strongly correlated, so the morphology control of inorganic materials has become one of the hotspots in the field of materials research in recent years. In recent years, domestic and foreign reports on magnesium oxide with special morphology and microstructure have found that some magnesium oxide with special morphology has very effective applications in many aspects through experimental research. The following is a brief introduction to some magnesium oxide with special morphology. The preparation is outlined.
1.1
Spherical magnesium oxide
Magnesium oxide with a spherical structure is calcined from the precursor spherical basic magnesium carbonate or spherical magnesium hydroxide or spherical basic magnesium oxalate. Taking spherical basic magnesium carbonate as an example, the spherical structure of basic magnesium carbonate is formed by the accumulation of sheet-like basic magnesium carbonate. The basic magnesium carbonate is calcined, and the spherical basic magnesium carbonate is thermally decomposed to obtain spherical magnesium oxide [2].
There are two main technical routes for preparing spherical magnesium oxide: (1) first obtain the precursor for preparing spherical magnesium oxide with magnesium salt as raw material, and heat treat the precursor to obtain spherical magnesium oxide; (2) mix magnesium oxide powder with solvent and adhesive After mixing the mixture (in some cases, the binder may not be used), the spherical magnesium oxide is obtained by mechanical molding, and then the spherical magnesium oxide product is obtained through heat treatment.
The spherical magnesium oxide obtained by Zhang[3] by precipitation method has a diameter of 15-17um, and continued to study the influence of time on its reaction process, and found that the magnesium oxide was first flake-like during the synthesis process, and then gradually became filamentous. Finally, it becomes spherical [3].
1.2
Tubular magnesium oxide
The diameter of the tubular magnesium oxide reported in the current literature is generally nano-scale, and the length is generally micron-scale. It usually has the characteristics of directional growth, good crystallinity and definite crystal plane orientation. Magnesium oxide nanotubes can be prepared by carbon-thermal evaporation. During the preparation, an appropriate amount of Ga2O3 is added to the mixture of magnesium oxide and carbon. Ga2O3 plays a vital role in the formation of magnesium oxide nanotubes. Carbon reduces Ga2O3 to obtain gallium vapor, and the condensed gallium droplets catalyze the anisotropic growth of MgO nanotubes in situ. The obtained magnesium oxide nanotubes are single crystals with an average outer diameter of 200nm, a wall thickness of 20nm, and a length of up to 50um [4].
1.3
Flake Magnesium Oxide
The key to obtaining flaky magnesium oxide by ultrasonic treatment is that the impact of the blasting bubbles on the layered magnesium oxide in the ultrasonic cavitation phenomenon makes the layered magnesium oxide peel off. Ultrasonic waves will cause liquid molecules to be continuously compressed and stretched, resulting in alternating positive and negative pressure zones, causing water molecules to undergo alternating changes in density and density, resulting in microbubbles in sparse places and growing up in negative pressure zones. When the microbubbles burst, the surrounding water phase will quickly rush into the center of the bubbles, releasing a strong pressure pulse. The pressure of this pulse is often higher than 104KPa. This phenomenon is also called ultrasonic cavitation effect.
This method is to use the mechanism of ultrasonic cavitation, use ultrasonic waves to cause a large number of microbubbles with the continuous relaxation and compression of the liquid phase, and make the microbubbles continuously generate shock waves in the positive pressure area to hit the surface of the material, making a layer of layered magnesium oxide One layer was peeled away, creating magnesium oxide nanosheets.
1.4
MgO whiskers
Magnesium oxide whiskers are mainly prepared by the precursor calcination method, that is, the precursor whiskers such as basic magnesium sulfate, basic magnesium chloride, basic magnesium carbonate, and magnesium carbonate are first prepared, and then the magnesium oxide whiskers are obtained by heat treatment.
Using active magnesium oxide and magnesium chloride as raw materials, basic magnesium chloride whiskers were first synthesized under hydrothermal conditions. The morphology of basic magnesium chloride whiskers can be maintained after pyrolysis, thereby obtaining magnesium oxide whiskers. The length of the whiskers is about 200um, and the diameter is about 0.5um.
Magnesium sulfate and sodium hydroxide were used as raw materials, and the precursor basic magnesium sulfate with good crystallization and fibrous shape was prepared through reaction hydrothermal crystallization at room temperature. By controlling the decomposition rate of the precursor to slowly decompose at low temperature to maintain the whisker-like shape, and then sintering at high temperature, magnesium oxide whiskers with good sintering, uniform dispersion and large aspect ratio can be obtained[5].
1.5
Mesoporous MgO
Mesoporous magnesium oxide is mainly prepared by using mesoporous carbon, cotton fiber, etc. as hard templates. A solution with a volume of 200mL containing 1mol L-1 Mg(NO3)2 was controlled in a water bath at 30°C, and the temperature was room temperature, the volume was 200mL, and the concentration was 0.8mol L-1 K2CO3 solution (stoichiometric ratio). At this time, the solution changed from clarification to a liquid-solid mixed state, and kept stirring for 30 minutes. The suspension was separated from solid to liquid, and soluble ions were removed with distilled water. The samples prepared in this experiment had good filterability, and then the obtained samples were heated to 110°C for drying. After the dried samples were preliminarily crushed, they were placed in The alumina crucible was heated to 600°C with the furnace, and the phase inversion time was 2 hours. The obtained sample was rod-shaped mesoporous magnesium oxide [3].
1.6
Preparation and Research Status of Other Special Morphology Magnesium Oxide
Yu et al. used chemical precipitation method, using magnesium nitrate and potassium carbonate as raw materials, mixed potassium carbonate and magnesium nitrate in an oil bath at 120°C, continued to stir for 1min, aged at 120°C for 2h, filtered and washed, and roasted at 700°C 4h, nest-shaped and flower-shaped magnesium oxide crystals were obtained by changing the ratio of magnesium nitrate and potassium carbonate.
Sutradhar et al. used hydrothermal method, using magnesium nitrate and ammonium carbonate as raw materials, to synthesize flake, rod, flower and spherical magnesium oxide by hydrothermal method, and studied its application as catalyst support.
Wang et al. mixed the MgCl2 solution with the benzoic acid additive, adjusted the pH value of the solution with sodium hydroxide to allow magnesium to settle, and synthesized a sheet-shaped magnesium hydroxide precursor by a hydrothermal method, and replaced the additive with citric acid or ethylenediaminetetraacetic acid. Disodium salt was obtained in the form of fibrous and discoid magnesium oxide [4].
02
Application of Special Morphology Magnesium Oxide
Nanostructured magnesium oxide is a material that has received widespread attention in recent years. Compared with traditional magnesium oxide, nanostructured magnesium oxide often has a smaller particle size and a larger specific surface area, and by deliberately controlling its growth conditions More specific crystal faces can be exposed, providing a higher density of active adsorption sites. Therefore, compared with traditional magnesium oxide materials, it has better optical, electrical, magnetic, thermal, and chemical properties, and is one of the hot spots for scientific research and industrial applications.
2.1
Application of spherical magnesium oxide
Spherical magnesium oxide is mainly used in the fields of chromatography stationary phase, adsorption of toxic substances, and material additives. High performance liquid chromatography (HPLC) is an effective separation technique, and its separation effect is greatly affected by the column packing. At present, the fillers used are mainly silica-based fillers. When silica-based fillers are used for separation under alkaline conditions, there are problems such as secondary reactions, long retention time, severe tailing, low efficiency, and poor reproducibility. After research and exploration, it is found that the mixture of oxides of magnesium and aluminum and silicon dioxide in a certain proportion as the stationary phase of liquid chromatography can solve this problem well, so the application of spherical magnesium oxide in liquid chromatography is getting more and more The more attention [6].
In addition, the mesoporous nanosheets of spherical magnesium oxide exhibit excellent adsorption properties, which can adsorb common toxic heavy metal ions and organic pollutants, and are expected to be used in wastewater treatment processes. Some people also found that the characteristic diffraction peak at 335nm is due to the induction of defects or defect energy levels to generate new energy levels when conducting XRD characterization of spherical magnesium oxide prepared by the carbonization method. From this feature, it can be predicted that MgO microspheres and nanosheets will be a very promising material in plasma display panels or other optical applications.
2.2
Magnesium Oxide Whiskers for Material Reinforcement
Whisker is a needle-like single crystal material with a diameter of a few tenths to several microns and a length of several microns to hundreds of microns. Due to the complete crystal structure, whiskers have good mechanical strength and are used as plastics, metals and ceramics. Modification additive of substances, showing excellent physicochemical properties and mechanical properties.
The microscopic morphology of magnesium oxide whiskers is fibrous, with high melting point, high strength, high elastic modulus, heat resistance, alkali resistance, insulation, thermal conductivity (the thermal conductivity is three times that of alumina), stability and reinforcement Good toughness, in addition, magnesium oxide whiskers have good high temperature oxidation resistance. Due to the above-mentioned excellent properties, magnesium oxide whiskers are suitable as reinforcing auxiliary materials for composite materials, especially for the preparation of high-temperature composite materials, and are high-tech structural new materials that have developed rapidly in recent years [7].
2.3
Mesoporous magnesium oxide for environmental protection
The negative impact of the greenhouse gas carbon dioxide on the environment has been paid more and more attention, and more and more researches have been done on the capture of carbon dioxide. Adsorption is an effective method for capturing carbon dioxide, and basic oxides are effective carbon dioxide adsorbents. The combination of magnesia to carbon dioxide is a reversible process. The binding force is stronger than that of zeolite and weaker than that of alkali metal oxides. At room temperature, carbon dioxide is adsorbed by magnesia by physical or chemical action, so magnesia is a more suitable carbon dioxide adsorbent. The adsorption capacity of mesoporous magnesium oxide to carbon dioxide is much higher than that of solid magnesium oxide, and the adsorption capacity increases with the increase of temperature. Mesoporous magnesium oxide can also be used to adsorb and remove fluoride ions in water, and the removal efficiency is much higher than that of ordinary commercial magnesium oxide [8].
2.4
Nano Magnesium Oxide for Antibacterial
So far, there are few domestic and foreign research reports on the antibacterial effects of magnesium oxide. The antibacterial performance of magnesium oxide can be traced back to the mid-1990s of the last century. In order to screen out inorganic materials with better antibacterial performance, the famous Japanese researcher Sawai tested a series of ceramic powders by using conductivity research. The experiment found that among the 26 common ceramic powders, 10 metal oxides and carbides such as CaO, ZnO, and MgO have better antibacterial properties. Among them, MgO has been proven to have strong bactericidal and antibacterial capabilities against Gram-positive bacteria, Gram-negative bacteria, and fungi [9].
It is generally believed that the antibacterial mechanism of MgO is caused by active oxygen, and MgO itself is a good desiccant, which can generate a large number of strong oxidants on its surface, thereby inhibiting and killing microorganisms.