Magnesium oxide is a common basic oxide and the main raw material for producing magnesium hydroxide and metallic magnesium. In ceramic applications, thanks to its high melting point of 2800℃ and several exceptional properties, MgO is highly valued in advanced ceramics. It can be directly sintered into magnesia ceramics or used as an additive.
As a common additive in the production of advanced ceramics, what roles does magnesium oxide play? What changes does it bring to ceramics during sintering? The following cases illustrate these effects.
Effects of MgO on Alumina Ceramics
Influence of MgO on Sintering Temperature and Densification of Alumina Ceramics
First, as a typical sintering aid, MgO effectively lowers the sintering temperature of alumina ceramics. Using high-purity α-Al₂O₃ powder as raw material and MgO as sintering aid, alumina ceramics were prepared via spark plasma sintering (SPS). Proper addition of MgO reduces the sintering temperature, inhibits grain growth, and improves density. The optimal mass fraction of MgO is 0.25%.
Influence of MgO on Mechanical Properties and Grain Growth of Alumina Ceramics
Alumina ceramics were fabricated from alumina powder with MgO as an additive using a two-step sintering process. As the MgO content increases, the relative density, flexural strength, and hardness of the sintered samples first rise and then slightly decrease.
When the MgO content is below its solid solubility limit, Mg accelerates grain-boundary diffusion and refines grains, leading to better density and mechanical properties. When MgO exceeds the solid solubility limit, although grain refinement is enhanced, magnesium aluminate spinel formed at grain boundaries hinders pore removal.
Regarding grain growth, grain size and uniformity first improve and then slightly decline with increasing MgO. At 0.25% MgO, the average grain size is minimized with concentrated grain distribution, yielding optimal performance. At 0.5% MgO, grains are finer and more uniform, with a maximum grain size of only 2.2 μm, but density is the poorest.
Influence of MgO on Optical Properties of Transparent Alumina Ceramics
The effects of MgO on the mechanical properties and densification of transparent alumina ceramics follow similar trends to those of conventional alumina ceramics: appropriate MgO addition positively impacts mechanical performance, densification, and grain growth inhibition.
For optical properties, low MgO doping results in high light transmittance, as moderate MgO restrains rapid grain-boundary migration, enabling complete pore elimination and higher densification. However, as doping increases beyond the solid solubility in Al₂O₃, local second phases form and act as light-scattering centers, reducing transmittance.
Effects of MgO Doping on ZnO Linear Ceramics
ZnO linear ceramic resistors in industrial manufacturing feature a wide resistivity range, high current density, low nonlinear coefficient, and small temperature coefficient of resistance (TCR), making them widely used in power electronics, transportation, communications, and household appliances.
Nonetheless, conventional ZnO composite ceramics suffer from issues such as poor structural uniformity, low industrial production repeatability, poor stability, and insufficient theoretical research. MgO addition helps improve the TCR of ZnO ceramic resistors. Proper MgO promotes sintering and densification, while excessive addition reduces density.
Effects of MgO on Ferroelectric Ceramics
Influence of MgO on Structure and Properties of Barium Strontium Titanate (BST) Ceramics
BST ferroelectric ceramics show great promise for phasers in phased arrays and tunable microwave devices due to high tunability and low dielectric loss. As existing ferroelectric materials have limitations, improving overall performance is critical for large-scale BST applications.
Besides rare-earth ion doping at A-sites, adding compounds such as MgO, MgTiO₃, and Mg₂SiO₄ to BST ceramics and thin films reduces dielectric constant and dielectric loss.
Influence of MgO on Properties of BaTiO₃-Based Ceramics
Uniform precipitation coating of MgO onto BaTiO₃-based ceramic powders effectively inhibits grain growth, producing uniform-grained ceramics. This grain-refining effect arises from MgO segregation at grain boundaries. MgO promotes the formation of core–shell structured grains, flattens and broadens the dielectric constant peak of BaTiO₃-based ceramics, and increases resistivity and breakdown strength.
Effects of MgO on Sintering Performance of YAG Transparent Ceramics
YAG transparent ceramics exhibit a high melting point (1950℃), high strength, high thermal conductivity, and excellent physicochemical stability, making them promising for both functional and structural applications. MgO is a commonly used sintering aid in YAG fabrication.
High-quality YAG transparent ceramics can be prepared via vacuum solid-state reaction sintering with trace MgO as a sintering aid. MgO helps control grain-boundary diffusion, grain growth, and pore elimination. However, excessive MgO leads to second-phase formation or rapid sintering shrinkage that traps pores within grains, drastically reducing YAG transmittance.
Effects of MgO on Mechanical Properties of Sialon Ceramics
Sialon is a solid solution based on Si₃N₄. During Sialon synthesis, the low diffusion coefficient of Si₃N₄ causes decomposition at sintering temperatures above 1800℃, making low-temperature sintering a development trend for Sialon ceramics.
Fan Guofeng et al. found that MgO increases the density of β-Sialon ceramics and lowers the sintering temperature. Flexural strength and fracture toughness first increase and then decrease with rising sintering temperature. At 1600℃, the β-Sialon ceramic is densely structured, with maximum flexural strength and fracture toughness.
Summary
The above describes the multifaceted effects of MgO on the mechanical properties and microstructure of several ceramic materials. It is evident that MgO plays a vital role as an additive in the production of advanced ceramics.