alumina ceramic that have been powdered, molded, and sintered contain pores to varying degrees, which are the gaseous phase of the alumina ceramic. The proportion of pores in different alumina ceramic can range from nearly zero to more than 90%, and it is of practical interest to test and analyze the content, distribution and shape of pores in materials.The analysis of the content, distribution and shape of the pores in the material is of practical interest. Since the gas phase is fundamentally different from the solid phase in many respects, materials with a low porosity can be characterized as a continuous solid phase, whereas materials with a particularly high porosity content are very close to a continuous gas phase. With the exception of porous ceramics for special applications, in most cases it is desirable that the porosity of the alumina ceramic be as high as possible.In most cases, it is desirable that the porosity of alumina ceramic be as low as possible, but it is difficult to reach zero.
The pores in alumina ceramic can be divided into two categories according to their permeability: open pores and closed pores. Open pores are pores that are connected to the surface, also known as “visual pores”, and include both capillary and non-capillary pores. The permeability is not only related to the size and shape of the pores, but also related to the nature of the fluid.The permeability is not only related to the size and shape of the pores, but also related to the nature of the fluid, generally the open pores through which the fluid can flow are called permeable pores. Before sintering, the pores in the powder are mostly open pores trapped in the grain boundaries. After sintering, some of the pores may merge and remain in the body to become closed pores and the alumina becomes dense. Since the open pores are located on the surface, they have an effect on the permeability and vacuum tightness of the material, as well as on the effective surface for catalytic reactions and chemical corrosion.
The morphology and distribution of pores in alumina can be directly observed by imaging with an electron microscope. However, due to focusing and other problems, the accuracy of the direct imaging observation of stomatal dimensions using electron microscopy is low, and should be combined with other test methods, such as X-ray small-angle and large-angle diffraction, apparent density measurement, water displacement method, compressive density measurement, etc.The measurement of density in alumina ceramic bodies is called apparent density measurement, water displacement method, mercury compression method, and so on. The measured density of a alumina ceramic body is called the apparent density, and the total porosity of alumina can be estimated by comparing the difference between the apparent density of the material and the theoretical density of the constituent materials. For alumina that are soluble in water, a very thin layer of wax is applied to the surface of the sample to isolate it from water prior to testing.The same method of measuring apparent density can be used to calculate porosity.