The influence of grinding particle size on powder particles is mainly reflected in three aspects. They’re physical properties, process performance, and final applications.
1. Changes in Physical and Chemical Properties
Specific Surface Area and Surface Energy:
As the particle size is reduced to the micrometer or nanometer level, the specific surface area increases exponentially. For example, the specific surface area of 1μm particles is 100 times greater than that of 100μm particles. The surface activity is significantly enhanced. For example, the dissolution rate of drugs increases by 3-5 times after nano-sizing. And the catalytic reaction efficiency can increase by more than 30%.
Crystal Structure Distortion:
Mechanical grinding leads to an increase in the lattice defect rate. The dislocation density of typical metal powders after grinding can reach 10^12/cm². It directly affects the reduction of sintering activation energy by about 15-20%.
Deterioration of Fluidity:
When D50 < 20μm, the angle of repose of the powder generally exceeds 45°, requiring the addition of 0.5-1.2% nano-silica and other flow aids to improve flowability.
2. Influence on Process Adaptability
Forming Process Window:
The forming of ceramic green bodies requires the particle size distribution span to be controlled between 1.2 and 1.5. A span that is too narrow leads to insufficient packing density (< 55%). However, a span that is too wide causes delamination defects.
Sintering Dynamics:
When 3Y-TZP ceramic powder is refined from 1μm to 0.3μm, the sintering temperature can be reduced by 150°C. The density increases from 92% to 99.5%.
Dispersion Stability:
Slurries with a particle size of D90 < 5μm require the addition of 0.1-0.5 wt% polycarboxylate dispersants. The absolute value of Zeta potential needs to be > 30 mV. So it can maintain a sedimentation rate of < 5% over 30 days.
3. Terminal Performance Correlation
Mechanical Properties:
For every 0.1μm reduction in tungsten carbide particle size in WC-Co cemented carbide, the hardness increases by 50 HV, while the fracture toughness decreases by 0.5 MPa·m^1/2.
Optical Properties:
When the D50 of titanium dioxide increases from 0.3μm to 0.5μm, the hiding power decreases by 15%, but the weather resistance improves by 20%.
Bioavailability:
When ibuprofen is micronized to D90 < 10μm, the in vitro dissolution T80 time is shortened from 45 minutes to 12 minutes.
Control of Key Parameters in Grinding Technology
Energy Consumption Optimization:
When using a planetary Molino de bolas to grind alumina, the optimal speed ratio is 1:2 (tank: chassis). A 30% filling rate saves 40% energy compared to a 60% filling rate.
Reverse Grinding Threshold:
After 8 hours of grinding, α-Al2O3 particles tend to reagglomerate. To extend the critical grinding time to 12 hours, 0.2% triethanolamine needs to be added.
Temperature Control:
When grinding nano-silicon powder, the slurry temperature should be maintained below 40°C. Above this temperature, the grain growth rate increases by 3 times.
Current Technical Bottleneck: The online detection error for submicron powder remains ±8%, and the tap density of ultrafine powder (D50 < 1μm) is 20-30% lower than the theoretical value. It is recommended to use a multi-stage grinding process (coarse grinding + fine grinding + surface modification) with an online particle size monitoring system to achieve stable control of the particle size distribution, with a CV value <5%.
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