Properties and Applications of Alumina Ceramics
The advancement of electronic ceramics is essential to the miniaturization and high integration of electronic components. In addition to having excellent electrical properties like high insulation, low dielectric loss, and stable electrical properties, alumina ceramics also have high strength, heat resistance, thermal shock resistance, corrosion resistance, and other properties of ceramic materials. Alumina raw material resources are also plentiful. As a result of its abundant supply, reasonable cost, and well-developed manufacturing process, it finds widespread applications in multilayer wiring ceramic substrates, electronic packaging, and high-density packaging substrates.
There are numerous homogeneous crystals of alumina, with α-Al2O3, β-Al2O3, and γ-Al2O3 are the most prevalent. Ceramic materials containing α-Al2O3 as the primary crystal phase have high mechanical strength, good thermal conductivity, high electric strength and insulation resistance, low dielectric loss, stable electrical properties with temperature and frequency, and other excellent properties. It is simple to produce, has a uniform and flat surface, and is used extensively as an electrical insulating material.
Alumina Ceramics with Various Purity
Among alumina ceramics with various purity, 96% alumina is the standard for thick film substrates and is frequently used in the production of hydrogenated microelectronic circuits. It is a cost-effective option for producing mixing equipment because it is readily available, inexpensive, and of high technical quality. Excellent mechanical strength, good thermal conductivity, chemical resistance, and dimensional stability are all characteristics of the base material made of 96% alumina ceramics.
99.6% alumina is the industry standard for thin-film substrates, which are commonly used for sputtering, evaporation, and chemical vapor deposition of metals to form circuits. The material is smoother with fewer surface defects thanks to the high purity and smaller grain size of 99.6% alumina, and the surface roughness is less than 1um. 99.6% alumina has excellent electrical insulation, low thermal conductivity, high mechanical strength, good dielectric properties, and good resistance to corrosion and wear.
Alumina with a purity of 99.5% is typically used when the need for finer grains is not strict. As a result of the larger grain size, the surface finish of 99.5% alumina will reach 2um. The material has lower flexural strength, thermal conductivity, dielectric strength, and dielectric constant than 99.6% alumina, and one of its current benefits is that it can be used in larger sizes and thicknesses.
Opaque alumina is a dark brown material that is also known as black alumina. This substance is appropriate for the packaging of military integrated circuits with high reliability, strong airtightness, and good light transmission. It can serve as the substrate and packaging shell for products like optical devices, quartz crystal oscillators, etc.
Significance of Black Alumina Ceramics
Semiconductor integrated circuits typically display obvious photosensitivity because semiconductors have five main properties: doping, heat sensitivity, photosensitivity, negative resistivity temperature characteristics, and rectification characteristics. The white alumina substrate is converted into black alumina ceramics to lessen the light transmittance of alumina ceramics in order to reduce the negative impact of light on integrated circuits.
Since "black alumina" is used in electronic technology, its electrical properties, such as resistivity, dielectric constant, etc., should also be taken into consideration in addition to its fundamental physical and mechanical properties and chemical stability (the "black alumina" ceramics here are different from the "black alumina" in the abrasive field, which has an Al2O3 content of 70%–80%, and contains more impurities like silicon oxide and titanium oxide).
Synthesis Technique of Black Alumina Ceramics
One-Time Synthesis
The one-time synthesis method entails preparing black alumina by mixing alumina, colored oxide, and co-solvent in accordance with a specific ratio and procedure. Following are the steps: Al2O3, Fe2O3, CoO, NiO, MnO2, and talc are weighed according to a specific proportion, then wet-ground in a ball mill, sieved, dried, granulated, compressed into tablets, sintered, and finally silver coating.
Secondary Synthesis
Firstly, using some metal oxides to generate black pigments. Secondly, the black alumina is prepared by certain procedures with a specific ratio of black pigments, co-solvents, and alumina. Following are the steps: Weigh Al2O3, CoO, NiO, MnO2 in a specific proportion, calcinate them to synthesize Fo-Co-Ni-Mn black pigment. Then combine alumina, talc, and the above black pigment in accordance with a certain proportion, followed by wet grinding, sieving, drying, granulating, tableting, sintering, and silver coating.
Coloring Mechanism of Black Alumina Ceramics
The selective absorption of visible light by a substance is what gives it its certain color. If a substance is capable of absorbing every wavelength of light that falls within the visible range, then the substance will have a dark appearance.
Transition metal oxides are the primary coloring agents used in Al2O3 ceramics. The d orbital, a non-spherical symmetrical orbit, is the outermost layer of the ions of transition metal elements. Under the influence of the crystal field, the d orbital of the transition metal ion undergoes energy level splitting, resulting in the formation of multiple energy levels. The transition energy ranges between 1-4eV, corresponding to the absorption of a specific wavelength of light, and the wavelength range just falls in the visible light region, so the material is colored, and the color presented by the material is the complementary color of the light wave it absorbs. The Al2O3 ceramic samples prepared under specific conditions can absorb a significant amount of visible light and appear black by varying the proportion of colored oxides in the formula.
An oxide ion's color depends on both its surrounding coordination environment and its own valence state. Black alumina ceramics can contain colored oxides in three different ways. One way is as a solid solution in the α-Al2O3 crystal lattice. The second is to dissolve in the phase of glass at the grain boundary. The third is the interaction of different colored oxides to create a spinel phase. Spinel powder stands out among them for having strong structural stability and good chemical stability. The valence state, coordination environment, and coloring stability of the colored oxide ion are at their highest levels when it is present in the spinel structure.