引擎排氣電移動度分析儀的奈米微粒與次微米微粒數目濃度分佈量測正確性

Abstract

為評估引擎排氣電移動度分析儀(EEPS, Engine exhaust particle sizer)的奈米微粒與次微米微粒數目濃度分佈與微粒形貌的關係，本研究以蒸發/核凝法產生銀團粒，並使用燒結爐控制銀微粒之形貌(如鍊狀或球形)，比對EEPS與掃描式電移動度分析儀(SMPS, Scanning mobility particle analyzer)(參考儀器)量測不同形貌之銀微粒的微粒數目濃度分佈與粒徑量測值(dm2)，並與團粒單極充電理論(Chang, 1981；Brown and Hemingway, 1995)計算的dm2預估值比較。 多徑銀團粒數目濃度分佈的比對結果顯示，SMPS與EEPS微粒總數目濃度的相對差異約為20 %，且與SMPS相較之下，EEPS的NMD量測值皆較小。而SMPS與EEPS的dm2量測值與銀微粒TEM影像比對結果顯示，量測電移動度30與80 nm銀微粒時，EEPS與SMPS量測值差異較小是由於微粒形貌為球形，而量測大於電移動度80 nm時，儀器量測值的差異持續存在是因為微粒形貌為橢圓形。因此，造成SMPS與EEPS的量測值差異的主因為微粒形貌並非球形，微粒形貌為團聚或橢圓形皆會影響EEPS的量測值。而本研究進一步以團粒充電理論與圓球充電理論(Fuch, 1963)預估EEPS量測之dm2，結果顯示dm2量測值與預測值之差異小於10 %，只有80 nm銀團粒的dm2預測值高估約16 %。因此，根據實驗與理論計算的結果，微粒形貌是導致EEPS的數目濃度分佈偏移的原因。相同電移動度粒徑之團粒與圓球微粒經過EEPS的單極充電器充電後，電移動度(Zp)較高的團粒會坐落在EEPS中較上方的靜電計，造成粒徑的低估，進而影響到數目濃度分佈的偏移。因此使用者在使用EEPS時須注意此方面的問題。

The influence of particle morphology on the nanoparticle size distributions measured by a fast mobility analyzer, the Engine Exhaust Particle Sizer (EEPS, TSI 3090), was studied using the Scanning Mobility Particle Sizer (SMPS, TSI 3936) as a reference. The EEPS shows consistently smaller number median diameters (NMDs) (18.4 ± 0.5, 33.43 ± 0.1 and 47.8 ± 1.04 nm) in comparison to the SMPS (21.8 ± 0.9, 41.3 ± 0.2 and 66.8 ± 0.4 nm) for polydisperse silver nanoparticles generated from a tubular furnace at the temperature of 1000, 1100 and 1200 oC, respectively. To provide quantitative explanation of the difference, generated polydisperse nanoparticles were further classified as monodisperse particles with the initial equivalent mobility diameter dm1 and sintered in the second furnace at different temperatures, from room temperature (no sintering) to 600 oC, to change their morphologies for the comparison tests. Without sintering, results show that the measured mobility diameter, dm2, of the EEPS is smaller than that of the SMPS when dm1 is larger than 30 nm and the difference increases as dm1 is increased from 30 to 300 nm. But the difference decreases as the morphology of particles changes from agglomerates to spheres for dm1 less than 80 nm and the sintering temperature higher than 200 oC. In comparison, the difference between the dm2 of the EEPS and SMPS persists for dm1 larger than 80 nm even at the sintering temperature of 600 oC when the particles are still not sintered into spherical shape but appear to be oblate spheroids instead. The projected properties like the aspect ratio and primary particle size of silver nanoparticles were used for the theoretical calculations of the mean charge per agglomerated particle (Np, agg). Results show that the Np, agg is more than the mean charge per particle of sphere (Np, sphere) with the same dm1 resulting in overestimation of the electrical mobility and underestimation of the dm2 by the EEPS. The present theory predicts the measured dm2 values by the EEPS quantitatively which agree well with the experimental data.