Properties and Characterization of Alumina Ceramics

2025-11-26

1. Experimental Procedure

This experiment used high-purity Alumina Powder (purity 99.9%, average particle size 1.2μm, produced by a new materials company in Shandong) as the raw material to prepare samples via the traditional dry pressing - pressureless sintering process. The specific steps are as follows:

1.1 Raw Material Pretreatment and Forming

First, the alumina powder was mixed with a polyvinyl alcohol (PVA, mass fraction 5%) binder in proportion. The mixture was wet ball-milled in a planetary ball mill at 200 rpm for 6 hours to ensure uniform dispersion of the material. After ball milling, the slurry was placed in a vacuum drying oven at 80°C for 12 hours to remove moisture, then crushed and passed through an 80-mesh sieve to obtain free-flowing Ceramic Powder. Subsequently, a hydraulic forming machine was used to press the powder into disc specimens (Φ30mm×5mm) and bar specimens (4mm×4mm×30mm) under a pressure of 20 MPa, with a holding time of 30 seconds. Five parallel samples were prepared for each temperature group to reduce experimental error.

1.2 Sintering Process

The formed green bodies were placed in a box resistance furnace using a segmented heating schedule: heating from room temperature to 600°C at a rate of 2°C/min, holding for 2 hours to remove the binder (avoiding cracking of the green body due to rapid heating); then heating to the target sintering temperatures (1550°C, 1600°C, 1650°C) at a rate of 5°C/min, holding for 4 hours to allow sufficient grain growth and densification; finally, cooling to room temperature at a rate of 3°C/min to prevent defects in the samples caused by thermal stress. The entire sintering process was carried out in an air atmosphere without the need for special protective gas, reducing industrial application costs.

1.3 Performance Testing and Characterization

Phase Analysis: Phase characterization of the samples was performed using a Japanese Rigaku D/max-2500 X-ray diffractometer (XRD). Test conditions were: Cu target (λ=0.154 nm), tube voltage 40 kV, tube current 40 mA, scanning range 2θ=10°-80°, scanning rate 5°/min. The crystal structure of the samples was analyzed using the XRD patterns, and the relative intensity of characteristic peaks (e.g., the (113) plane of α-Al₂O₃) was calculated to determine the presence of impurity phases (such as SiO₂, MgO, etc.).
Microstructure Observation: The fracture surface microstructure of the samples was observed using a Dutch FEI Quanta 200 scanning electron microscope (SEM). Samples required gold sputtering (coating thickness about 10 nm) to improve conductivity, with an acceleration voltage of 20 kV. Grain size was statistically analyzed from SEM images (using the intercept method, counting over 50 grains per sample), pore distribution and morphology were observed, and sample density was calculated (using the Archimedes water displacement method, formula: ρ = m₁ρ_water / (m₂ - m₃), where m₁ is the dry weight of the sample, m₂ is the suspended weight in water, and m₃ is the wet weight after soaking).
Mechanical Property Testing: Flexural strength of the samples was tested using a Shanghai Sansi CMT5105 universal material testing machine, employing the three-point bending method with a span of 20 mm and a loading rate of 0.5 mm/min. Five samples from each temperature group were tested, and the average value was taken as the final result. Sample hardness was tested using a Vickers hardness tester (HV-1000) with a load of 10 N and a dwell time of 15 seconds. Five different positions were tested on each sample, and the average was taken after removing the maximum and minimum values.
Dielectric Property Testing: The dielectric constant and dielectric loss tangent of the samples were tested using an American Agilent E4980A impedance analyzer. The test frequency range was 1 kHz-1 MHz, the test temperature was room temperature (25°C), and each sample was tested three times to ensure data reproducibility.

1. Experimental Procedure

This experiment used high-purity alumina powder (purity 99.9%, average particle size 1.2μm, produced by a new materials company in Shandong) as the raw material to prepare samples via the traditional dry pressing - pressureless sintering process. The specific steps are as follows:

2. Structural Analysis

2.1 Phase Structure Analysis

XRD test results (Fig. 1) showed that samples sintered at different temperatures only exhibited characteristic diffraction peaks of α-Al₂O₃ (PDF#01-1307), and no impurity phases were detected. This indicates that within the experimental temperature range, the alumina powder completely sintered into pure α-Al₂O₃ phase, and the sintering temperature did not alter the crystal structure of the samples. Among them, the sample sintered at 1600°C showed the highest characteristic peak intensity (intensity of the (113) plane peak was 2800 cps), higher than samples sintered at 1550°C (2200 cps) and 1650°C (2500 cps), indicating the best crystallinity and more regular crystal arrangement at this temperature, which lays a structural foundation for excellent performance.

2.2 Microstructure Analysis

SEM image analysis (Fig. 2) indicated that the sintering temperature significantly affected the microstructure of the alumina ceramics: The sample sintered at 1550°C contained numerous pores (porosity about 8%), had smaller grain size (average grain size 2.5 μm), uneven grain distribution, and contained some insufficiently sintered agglomerates, with a density of only 90%. The fracture surface of the sample sintered at 1600°C was flat, the number of pores was significantly reduced (porosity decreased to 2%), grains grew into uniform equiaxed shapes, the average grain size increased to 4.0 μm, and the density increased to 96.5%, meeting the standard for high-density ceramics (density ≥95%). In contrast, the sample sintered at 1650°C exhibited significant abnormal grain growth (average grain size 6.5 μm), microcracks appeared at grain boundaries, porosity increased to 5%, and density decreased to 93%. This is due to excessive grain growth at excessively high temperatures, weakening the bonding force between grains and generating structural defects.

2.3 Correlation Between Performance and Structure

Combining performance test data and structural analysis, it can be seen that there is a direct correlation between ceramic properties and microstructure: The 1600°C sample, due to its high density (96.5%) and uniform grains (4.0 μm), achieved a flexural strength of 385 MPa, significantly higher than samples sintered at 1550°C (290 MPa) and 1650°C (320 MPa). The Vickers hardness was also highest for the 1600°C sample (1650 HV), while the 1650°C sample, due to abnormal grain growth, had a hardness reduced to 1520 HV. Regarding dielectric properties, the 1600°C sample had a dielectric constant of 9.2 (at 1 kHz) and a dielectric loss tangent of 0.002, exhibiting excellent insulation performance. This is because the low porosity reduces the influence of air (dielectric constant ≈1) on the overall dielectric performance. The 1550°C sample, due to high porosity, had a decreased dielectric constant of 8.5, while the 1650°C sample, due to the presence of microcracks, had an increased dielectric loss tangent of 0.004.

In practical application scenarios, if this alumina ceramic is to be used for electronic packaging substrates, it needs to simultaneously meet high insulation (dielectric loss <0.005) and certain mechanical strength (flexural strength >300 MPa). The 1600°C sintered sample fully meets these requirements. When used for wear-resistant valve cores, which require higher hardness (HV>1600), the 1600°C sample also holds an advantage. This further verifies the rationality of selecting the optimal sintering temperature.

3. Conclusion

This study successfully prepared high-performance 99% alumina ceramics by controlling the sintering temperature. The experimental results show that the sintering temperature significantly regulates the microstructure and properties of the ceramics, and there is a clear correlation among the three: Appropriately increasing the sintering temperature promotes grain growth and densification, improving the mechanical and dielectric properties of the material. However, excessively high temperature leads to abnormal grain growth and structural defects, consequently causing a decline in performance.

1600°C was determined as the optimal sintering temperature for this alumina ceramic. Samples prepared at this temperature exhibited the best overall performance: phase composition was pure α-Al₂O₃, density reached 96.5%, average grain size was 4.0 μm, flexural strength was 385 MPa, Vickers hardness was 1650 HV, dielectric constant was 9.2 (at 1 kHz), and dielectric loss tangent was 0.002. All performance indicators meet the application requirements in fields such as electronic packaging and wear-resistant components.

The traditional dry pressing - pressureless sintering process used in this study offers advantages of simple operation, low cost, and ease of industrial production. The obtained sintering process parameters and performance data can provide direct reference for the large-scale preparation of alumina ceramics. Furthermore, the study clarified the influence mechanism of microstructure (density, grain size, porosity) on ceramic properties, providing theoretical reference for the performance regulation of other ceramic materials (such as Zirconia, silicon nitride ceramics).

Follow-up research could further explore the optimization effects of sintering aids (such as MgO, SiO₂) on the properties of alumina ceramics, or employ advanced processes like hot pressing sintering or microwave sintering to shorten sintering time and reduce sintering temperature, thereby further enhancing material performance and production efficiency, and expanding its application range in extreme environments such as high temperature and high pressure.