Classification of ceramic 3D printing materials
The printing material based on PEP technology is a ceramic-polymer composite material with a particle size of 8-14 mesh near-spherical granular material. The materials commonly used in ceramic 3D printing can be categorized into slurry, powder and filament according to their forms. Paste material is generally made of organic liquid and ceramic powder mixing and stirring, mainly used in SLA technology and IJP technology, powder material is a mixture of ceramic powder organic particles or ceramic powder, mainly used in SLS technology and 3DP technology, filament material is mainly used in FDM technology hot melt filament material.
Commonly used materials for ceramic 3D printing
Magnesium oxide ceramics are ceramics with magnesium oxide as the main crystalline phase and cubic crystal system sodium chloride type structure, white in color, particle size in 8-14 mesh. Magnesium oxide ceramics have good electrical insulation, high melting point, are weakly alkaline materials with good resistance to alkaline slag erosion, and large coefficient of thermal expansion, which is one of the highest among all pure oxide ceramics.
Magnesium oxide ceramics are typical new ceramics and also belong to traditional refractory materials. Under the protection of oxidizing atmosphere or nitrogen, magnesium oxide ceramics can work stably up to 2400℃, so magnesium oxide is the key material in the advanced process of modern metallurgical industry. Many metals such as iron, nickel, uranium, zinc, aluminum, aluminum, magnesium, copper, platinum, etc. do not act with magnesium oxide. Therefore magnesium oxide ceramics can be used as crucibles for melting metals, molds for pouring metals, protective tubes for high temperature thermocouples, and lining materials for high temperature furnaces. Transparent magnesium oxide ceramics, can be used as radar cover, infrared detector cover, chemical window materials.
MgO ceramics theoretical density of 3.85g/cm³, melting point up to 2800 ℃, Mohs hardness of 5-6. The ion stacking density in the lattice of magnesium oxide is close, the symmetry of ion arrangement is high, the lattice defects are few, and it is difficult to sinter. Therefore, in the preparation process of magnesium oxide ceramics, in order to reduce the sintering temperature, it is necessary to add a certain amount of sintering additives, generally mixed rare earth metal oxides (RxOY), TiO2, ZrO2, SiO2 as a sintering additive. The high temperature stability and corrosion resistance of magnesium oxide ceramics are better than that of alumina ceramics, which can be used in the corrosive environment of strong alkali metals.
Preparation of magnesium oxide ceramics
(1) Preparation of magnesium oxide raw materials
Nature is rich in magnesium-containing compounds, it exists in the form of a variety of minerals in the earth’s crust and the ocean, such as magnesite (MgCO3), dolomite (CaMg(CO3)2), magnesite (Mg(OH)2), talc (Mg3[Si4O10](OH)2) and so on. Industrially, MgO is mainly extracted from the above minerals, but recently it is developed to be extracted from seawater.
Extracted from minerals or seawater MgO, most of the first made of magnesium hydroxide or magnesium carbonate, and then calcined decomposition into MgO, this MgO through further chemical treatment or heat treatment can be obtained from high-purity MgO.
(2) Preparation of MgO ceramics
Preparation of MgO raw materials after treatment, according to the composition of the ingredients. In order to promote sintering as well as can make the grain grow a little bit, at the same time in order to reduce the hydration tendency of the preparation, can add some additives, such as TiO2, Al2O3, V2O3, etc.. Table 1 lists the effects of some additives on MgO grain size and sinterability. If MgO ceramics with high purity are required, the method of adding additives to promote sintering and grain growth cannot be used, but the method of activated sintering is used, i.e., Mg(OH)2 is calcined at an appropriate temperature to obtain activated MgO with a lot of lattice defects, which is used to manufacture sintered magnesium oxide ceramics. It is generally appropriate to calcine decomposed Mg(OH)2 at 1200°C.
MgO chemical activity, soluble in acid, hydration capacity, so MgO ceramics manufacturing process must take into account this characteristic of the raw material. In order to reduce the activity of MgO, should be pre-fired to 1100 ~ 1300 ℃. Grinding liner and grinding balls used in grinding raw materials should be made of ceramic materials, can not use steel balls. Dry grinding is used to prevent hydration, and it is better to use vibrating mill for fine grinding, which can get more fine particles. Molding methods can be used semi-dry pressing method, slurry method, hot die casting method and hot pressing method. Semi-dry pressing method using the binder for glycerol, polyvinyl alcohol, beeswax carbon tetrachloride solution, etc., choose fine-grained ingredients, molding under 50 ~ 70MPa pressure. Preparation of slurry molding slurry, need anhydrous ethanol as a medium to avoid MgO hydration and expansion. Preparation of hot die casting with the best raw materials in the fine grinding to add 2% ~ 3% of oleic acid (general dosage of 14% or so), the fine grinding time should not be too long to prevent MgO particles agglomeration. The bulk density of the product obtained by hot pressing method can be close to the theoretical density value. General pressure of 20~30MPa, temperature 1300~1400 ℃, pressurization time 20~40min.
MgO ceramics are mostly produced by slurry injection method. But because MgO is easy to hydrate, so its process is more complex. In the preparation of slurry, in order not to make MgO and water contact, need to use anhydrous ethanol and other organic liquids as a suspension medium. Widely used process is as follows: the MgO raw materials with a sufficient amount of distilled water mixed to form a paste and fully hydrated into Mg(OH)2. stored for a certain period of time, drying below 100 ℃, in 1450 ~ 1600 ℃ in the sealed conditions of calcination, insulation 8h, so that the decomposition of magnesium hydroxide into MgO. then ball milling for 45 ~ 90h, and then add water (the amount of water to add) Then ball mill for 45~90h, then add water (the amount of water added is 50%~60%) and continue to mill for 70~90min. to form a suspension slurry, used for casting molding. In order to improve the performance of the slurry, it can be adjusted by the method of pH value, so that the pH value of 7~8.
After demolding the billet is dried at a temperature of 70°C. In order to remove the moisture from the billet quickly, the humid air should be removed as soon as possible.
Ceramic sintering, is the first in 1250 ℃ temperature for vegetal firing, and then loaded into the corundum porcelain sagger in 1750 ~ 1800 ℃ temperature insulation 2h sintering.