以常壓噴射式電漿製備金屬氧化物奈米粉體之製程研究

Abstract

此實驗設備為在常壓下運用直流脈衝式電源產生的噴射式電漿(atmospheric pressure plasma jet，以下簡稱為APPJ)，此系統可維持長時間的穩定電漿噴流，利用電漿產生具高反應性的自由基，將反應先驅物(precursor)於極短時間內(ms)分解反應並生成產物。 本實驗製程為電漿法與噴霧熱解法(spray pyrolysis, SP)之結合，實驗中將反應先驅物溶液經1.7 MHz超音波霧化器(nebulizer)霧化為液滴狀態，再由載流氣體將液滴載入電漿噴流下游進行反應，最後於反應管之下游置放一盛有去離子水之燒杯以收集產物，並且依照各種粉體應用之需求改善製程並進行探討。 本實驗共分三部分：第一部分以水溶性金屬鹽類當作反應先驅物製備氧化物粉體。此部分我們以含鋅的鹽類製備氧化鋅(zinc oxide, ZnO)粉體，並經由加設前加熱器(pre-heater)與使用鹽類輔助式噴霧熱解法(salt-assisted spray pyrolysis, SASP)改良製程。當使用SASP製程時，所合成之氧化鋅奈米粒子粒徑約為50 nm，且由X射線光電子能譜(XPS)分析中可知其為氮摻雜之氧化鋅(N-doped ZnO)，氮含量約為0.4 %。我們亦證明由系統參數可控制粉體形貌與特性。 第二部分為以有機金屬四異丙烷氧化鈦(titanium(IV) isopropoxide, TTIP)蒸氣當作反應先驅物合成氮摻雜之二氧化鈦(N-doped titanium dioxide, N-doped TiO2)奈米粒子。先驅物之蒸氣經由載流氣體送入電漿中反應。此為氣相轉變為顆粒之機制(gas–to–particles conversion mechanism)，因此所生成之粒徑小且均勻，粒徑約為5 nm，但稍有聚集現象。此粉體經x射線光電子能譜分析後確認其為氮摻雜之二氧化鈦，且由紫外光/可見光光譜中顯示，其具有吸收可見光之能力，可吸收波長480 nm以下的光。 本實驗第三部分為製備多成分金屬氧化物(multiple-component metal oxide)尖晶石鋰鈦氧化合物(spinel lithium titanium oxide, Li4Ti5O12)。與第一部分裝置方法皆相同，經由控制系統參數(電漿工作氣體種類與流量、有無前加熱器、載流氣體流率、先驅物濃度)可得到不同形貌之純相尖晶石鋰鈦氧化合物：無前加熱器時，粒子皆為殼狀粒子，且不受載流氣體之流率所影響；具前加熱器且於低載流氣體流率(0.7 slm)時為實心球，當先驅物LiOH濃度為0.05 ~ 0.4M時可由質量均衡求出其粒徑為118 ~ 2350 nm；具前加熱器於高載流氣體流率(3 slm)時為殼狀粒子，如無前加熱器時所觀察到之形貌；具前加熱器而載流氣體流率適中(1.5 slm)且高濃度(0.4 M)時為中空孔洞球；而具前加熱器而載流氣體流率適中(1.5 slm)且低濃度(0.05, 0.2 M)時為，可同時觀察到中空孔洞球與殼狀之粒子。以無前加熱器情況下所得到之殼狀Li4Ti5O12粉體進行鋰離子電池循環測詴，於50 C下電容量仍可達100 mAhg-1。 最後綜合探討實驗參數對形貌之影響：當僅以電漿系統當作反應器時，液滴中溶劑蒸發速率(solvent evaporation)遠大於溶質擴散速率(solute diffusion)，因此僅表面析出一層固體球殼，而當已析出之鹽類滲透率(permeability)不高時，液滴內部壓力過大而破裂，形成破裂或是有孔洞之殼狀形貌；但於前加熱器中，因液滴中之溶劑蒸發速率約等於溶質擴散速率，因此易形成實心球。更多的機制將於本論文中介紹。

Nanocrystalline N-doped ZnO, N-doped TiO2, and Li4Ti5O12 particles are synthesized by an atmospheric pressure plasma jet (APPJ). This APPJ is sustained using a repetitive pulse source with nitrogen or oxygen as the plasma gas. This system can sustain a stable plasma jet, allowing for a long treatment time. Nebulizer-generated precursor droplets are sprayed into the plasma jet, resulting in the fabrication of metal oxide particles within a very short time (ms). This process is divided into three parts. First, Zn(NO3)2 solutions with NaNO3 are -sized droplets and fed into the downstream region of the APPJ. Salt solution droplets undergo vaporization and react to form solid particles in the downstream region of the jet. This process is a salt-assisted spray pyrolysis(SASP)-like process. During this SASP process, when the droplet temperature exceeds the melting point of the salt, the salts melt and act as high-temperature solvents and the particle size is about 50 nm from SEM images. The particles are collected using de-ionized water. The doping level is changed by controlling plasma flow rate. Second, using the same system, N-TiO2 particles are synthesized by injecting the TTIP vapor into the plasma jet with a bubbler. The particle size, size distribution, and morphology are characterized by TEM, and particle composition is analyzed by XPS spectra. Third, Li/Ti solution droplets undergo O2 and N2 plasma treatment and Li4Ti5O12 particles are synthesized using the same system. The as-prepared powder is crystallized Li4Ti5O12, as observed. The morphologies and particle size are controlled by simple system parameter (with/without pre-heater, carrier gas flow rate, concentration of precursor). We also propose a possible mechanism for the formation of particles with different morphologies. As discussed in this thesis, the evolution of the particle upon entering the pre-heater followed by plasma treatment is sensitive to the carrier gas flow rate (resident time in pre-heater), the presence of the pre-heater, and the precursor concentration. In the plasma, the high temperature resulting solvent evaporation rate much higher than the solute diffusion rate resulting in hollow particles. In pre-heater(Tpre-heater < Tplasma),the solvent evaporate rate is nearly to the solute diffusion rate resulting in dense particles. The experiment of the morphology are summarized in this thesis.