Natural graphite, how to choose the sintering temperature of ceramic composites
Release time:
2025-07-07
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Abstract
1. Core influencing factors
(1) Material component properties
Natural graphite:
Temperature limit: natural graphite starts to oxidise in oxidising atmosphere above 450°C (need inert atmosphere protection, e.g. Ar, N₂), in vacuum or inert environment it can withstand more than 3000°C, but the actual sintering temperature is usually much lower than this (to avoid structural damage).
Graphitization: ≥2500°C if graphite crystallinity needs to be increased (e.g. to enhance electrical conductivity), but at the expense of ceramic phase stability.
Ceramic phase:
Melting point/decomposition temperature: e.g. Al₂O₃ (2050°C), SiC (2700°C), Si₃N₄ (1900°C decomposition), need to be lower than 90%~95% of their melting point.
Sintering activity: nanoceramic powders may require lower temperatures (e.g. Al₂O₃ nanopowder can densify at 1300°C).
(2) Target properties of composites
High electrical/thermal conductivity: graphite network needs to be preserved, sintering temperature ≤ ceramic phase sintering temperature (e.g. graphite-SiC selected 1800~2000°C).
High strength/wear resistance: preferred ceramic densification, higher temperatures may be required (e.g. graphite-Al₂O₃ selected 1600~1700°C), but graphite oxidation needs to be balanced.
(3) Interface reaction control
Avoid harmful phases: e.g. graphite-Al₂O₃ at ≥1700°C may generate Al₄C₃ (brittle phase), which needs to be suppressed by addition of sintering additives (e.g. Y₂O₃) or by lowering temperature.
2. Sintering Temperature Selection Steps
Step 1: Determine the Ceramic Phase Sintering Window
Refer to the sintering temperature of the ceramic single phase (e.g., T₁ = 0.7 x melting point, T₂ = 0.9 x melting point), e.g.:
SiC: 1900~2400°C (melting point 2700°C)
Al₂O₃: 1400~1600°C (melting point 2050°C)
Step 2: Evaluate graphite stability
Test the graphite loss of weight in the target atmosphere (e.g. TGA analysis) and determine its upper safe temperature limit (e.g. ≤ 2200°C in N₂).
Step 3: Optimise the temperature interval
Initial range: take the intersection of the lower limit of the ceramic sintering temperature and the lower limit of the graphite stability temperature (e.g. graphite-Al₂O₃: 1400~1600°C).
Experimental validation: the reaction peaks were detected by differential thermal analysis (DSC), and the temperature was adjusted to avoid the violent reaction interval.
3. Process optimisation strategies
Sintering aids: e.g. B₄C reduces the SiC sintering temperature to below 1800°C and reduces damage to graphite.
Rapid sintering technology: SPS or Flash Sintering (FS) can be done in minutes to inhibit graphite oxidation (e.g. graphite-Al₂O₃ at 1450°C/5min).
Atmosphere control:
Vacuum: suitable for high temperatures (>2000°C), but graphite sublimation needs to be prevented.
N₂/Ar: common protective atmosphere, note O₂ residue <10ppm.
4. Summary Recommendations
Preliminary experiments: test the thermal behaviour of the composites in a Differential Scanning Calorimeter (DSC) to determine the reaction onset temperature.
Gradient sintering: Trial sintering at 50°C intervals in the target range, favouring the interval with the best mechanical/electrical properties.
Industrial scale-up: Considering equipment limitations (e.g. size of the homogeneous zone of a vertical graphitising furnace), temperatures may need to be adjusted by ±20°C.
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Natural graphite, how to choose the sintering temperature of ceramic composites
2025-07-07
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