本文選題：電子束熔煉　切入點(diǎn)：多晶硅　出處：《大連理工大學(xué)》2017年博士論文

【摘要】：多晶硅是太陽(yáng)能電池的主要原材料,急需開發(fā)具有高效率、低成本、工藝穩(wěn)定的多晶硅綠色制造方法。冶金法是針對(duì)制造太陽(yáng)能級(jí)多晶硅而提出的一種專屬提純技術(shù),該方法以冶金級(jí)硅為原料,根據(jù)硅和雜質(zhì)物理化學(xué)性質(zhì)的差異,優(yōu)化集合酸洗、造渣精煉、真空冶煉、電子束熔煉、定向凝固等方法,分步降低硅中雜質(zhì)含量,實(shí)現(xiàn)雜質(zhì)高選擇性、梯度式分離,最終獲得太陽(yáng)能級(jí)硅材料。硅中雜質(zhì)與硅物理化學(xué)性質(zhì)的差異主要表現(xiàn)在蒸發(fā)、分凝和氧化特性上,目前的冶金法各單元技術(shù)都是基于雜質(zhì)的某一種特性,某些雜質(zhì)無(wú)法通過(guò)一次熔煉過(guò)程被去除到目標(biāo)值,需要經(jīng)過(guò)兩次或多次熔煉去除。同時(shí),這些單元技術(shù)之間相互獨(dú)立,缺少有機(jī)耦合,每個(gè)環(huán)節(jié)都在重復(fù)熔化-熔煉-凝固的過(guò)程,存在流程長(zhǎng)、效率低、產(chǎn)品品質(zhì)不穩(wěn)定等問(wèn)題,增加了能耗和成本。電子束熔煉是冶金法制備太陽(yáng)能級(jí)多晶硅的關(guān)鍵技術(shù),其特點(diǎn)是具有高溫、高真空的熔煉環(huán)境,并且熔池內(nèi)的溫度分布可以通過(guò)功率大小、掃描方式、掃描頻率等進(jìn)行精確控制。本研究提出冶金法去除硅中雜質(zhì)的本質(zhì)是固態(tài)硅中的雜質(zhì)通過(guò)冶金過(guò)程完成在硅的三態(tài)(固相、液相、氣相)或其它相(渣相或合金熔體等)中的再分配,并考察了雜質(zhì)在蒸發(fā)、分凝和氧化組成的多個(gè)維度的性質(zhì),發(fā)現(xiàn)雜質(zhì)在多個(gè)維度上具有特征,可通過(guò)多個(gè)特征的結(jié)合深度去除雜質(zhì)。在此基礎(chǔ)上,針對(duì)不同類型的雜質(zhì),利用電子束構(gòu)建特征熔煉環(huán)境,研究雜質(zhì)的傳輸和去除的機(jī)制�；陔娮邮蹮挔顟B(tài)下高溫、高真空的特征環(huán)境,建立了多晶硅熔煉過(guò)程中蒸發(fā)性雜質(zhì)在硅液相-氣液界面-氣相傳輸?shù)娜蚩刂颇Ｐ?解析硅中雜質(zhì)去除的動(dòng)力學(xué)機(jī)制。研究表明,雜質(zhì)從硅熔體中去除的總傳質(zhì)系數(shù)主要受到溫度和和真空室壓力的影響,溫度越高、真空室壓力越小,傳質(zhì)系數(shù)值越大。在相同的熔煉條件下,P、Al、Ca三種雜質(zhì)元素的總傳質(zhì)系數(shù)值大小為kPkAlkCa,表明P更容易通過(guò)蒸發(fā)去除。在電子束熔煉過(guò)程中,P的去除由界面蒸發(fā)和液相邊界層傳質(zhì)共同控制,而Al和Ca的去除由界面蒸發(fā)控制。雜質(zhì)去除的同時(shí)硅也在蒸發(fā)損失,當(dāng)P的去除率接近100%的時(shí)候,硅的損失率可以控制在10%以內(nèi),說(shuō)明電子束熔煉是去除硅中雜質(zhì)P的有效方法。但此時(shí)Al的去除率仍然較低,要想提高其去除率,需要更高的熔煉溫度或更長(zhǎng)的熔煉時(shí)間,這會(huì)導(dǎo)致硅損失率的進(jìn)一步增加。針對(duì)Al、Ca等同時(shí)具有分凝特性和蒸發(fā)特性的雜質(zhì),通過(guò)緩慢降低電子束功率,構(gòu)建熔體內(nèi)部單向溫度場(chǎng)-氣相高真空的特征環(huán)境,實(shí)現(xiàn)了電子束誘導(dǎo)多晶硅定向凝固。研究表明,雜質(zhì)Al和Ca在硅中的分布呈現(xiàn)出了分凝的趨勢(shì),其去除率取決于熔體溫度和凝固速率,合理的控制熔體溫度和凝固速率可以獲得較高的雜質(zhì)去除率和較好的均勻性。在熔煉過(guò)程中,雜質(zhì)的去除受到氣液界面蒸發(fā)的控制,在15kW的功率下,Al和Ca的總傳質(zhì)系數(shù)分別為3.43×10-5m/s和2.04×10-5m/s,熔煉1800s后去除率分別達(dá)到了98%和91%。在電子束誘導(dǎo)定向凝固過(guò)程中,雜質(zhì)的去除受到固液界面分凝和氣液界面蒸發(fā)的耦合控制,其含量進(jìn)一步降低到0.7×10-4wt.%以下。與傳統(tǒng)的電子束熔煉方式相比,在總時(shí)間相同的條件下,電子束誘導(dǎo)定向凝固的方式具有相同甚至更高的雜質(zhì)去除率,且由于凝固時(shí)功率降低減少了能耗。在本實(shí)驗(yàn)中,與使用15kW的功率熔煉3000s相比,當(dāng)熔煉時(shí)間為1800s、降束時(shí)間為1200s時(shí)的能耗降低了 20%。將電子束熔煉去除P雜質(zhì)和電子束誘導(dǎo)定向凝固去除金屬雜質(zhì)的方法應(yīng)用于產(chǎn)業(yè)化電子束熔煉設(shè)備上,使硅中的P含量降低到0.3×10-4wt.%以下,Fe、Al、Ca的含量降低到10-6～10-5wt.%的數(shù)量級(jí),且這些金屬雜質(zhì)的分凝效果優(yōu)于相同凝固速率下傳統(tǒng)定向凝固的分凝效果。在此基礎(chǔ)上,結(jié)合數(shù)值模擬和實(shí)驗(yàn)手段,從能量利用角度對(duì)現(xiàn)有熔煉工藝進(jìn)行優(yōu)化,通過(guò)在坩堝內(nèi)部增加石墨襯底提高能量利用率。發(fā)現(xiàn)在硅和水冷銅坩堝之間增加一層石墨襯底,相當(dāng)于增加了熱阻,減少了熱量的損失。使用石墨襯底的熔煉方式,在熔煉功率相同時(shí),熔池溫度提高,加速了 P的去除。實(shí)際熔煉過(guò)程中,在達(dá)到相同的除P效果時(shí),可以縮短熔煉時(shí)間,使單位質(zhì)量能耗從29.3kW·h/kg降低到19.5kW·h/kg。
[Abstract]:Polysilicon is the main raw material for solar battery, it is urgent to develop high efficiency, low cost, green manufacturing process stable polysilicon. Metallurgical method for manufacturing solar grade polysilicon and proposed an exclusive purification technology, the metallurgical grade silicon as raw materials, according to the differences of physical and chemical properties of silicon and impurity, optimized pickling, slag refining, vacuum metallurgy, electron beam melting, directional solidification method, step by step to reduce the impurity content in silicon impurities, achieve high selectivity, gradient separation, finally obtain solar grade silicon material. Different impurities in silicon and silicon physical and chemical properties mainly in evaporation, segregation and oxidation characteristics, metallurgical method at present each unit is a kind of technology based on the characteristics of impurities, some impurities can not be removed by a melting process to the target value, need to go through two or more times with smelting removal. When these technologies are independent of each other, the lack of organic coupling, each link in the process of melting and solidification of the repeated melting process, there are long, low efficiency, unstable product quality problems, increase the energy consumption and cost. Electron beam melting is the key technology of metallurgical preparation of solar grade polysilicon, which is characterized by high temperature, high vacuum melting atmosphere, and the temperature distribution in the molten pool through the power of the size, scanning, scanning frequency and precise control. The essence of this research put forward the method of metallurgical removal of impurities in silicon is solid impurities in silicon in three states of silicon by metallurgical process (solid phase, liquid phase and gas phase) or other phase (slag or alloy melt etc.) in redistribution, and the effects of impurities in the evaporation properties of multi dimensions of segregation and oxidation of impurities, with features found in many dimensions, through multiple features With the depth of removal of impurities. On this basis, according to the different types of impurities, construction characteristics of smelting environment by electron beam transmission mechanism research and impurity removal. Electron beam melting state under high temperature environment based on the characteristics of high vacuum, a global control model of the evaporation of impurities in silicon liquid - gas phase gas-liquid interface the transmission of polysilicon in the process of smelting, the kinetic mechanism of removal of impurities in silicon analysis. The research results show that the effect of the total mass transfer coefficient to remove impurities from the silicon melt is mainly affected by the temperature and pressure of the vacuum chamber, high temperature, vacuum chamber pressure is small, the mass transfer coefficient greater melting. In the same conditions, P. Al, the total mass transfer coefficient of size Ca three impurity elements kPkAlkCa, P showed more easily through evaporation removal. In electron beam melting process, the removal of P by interfacial evaporation and liquid phase boundary layer mass transfer co control, Al Ca was controlled by the interfacial evaporation. At the same time also in the removal of impurities in silicon evaporation loss, when the removal rate of P is close to 100% of the time, silicon loss rate can be controlled within 10%, indicating electron beam melting is an effective method to remove the impurities in silicon P. But the Al removal rate is still low, to to improve the removal rate, melting time requires higher melting temperature or longer, it will lead to further increase of the silicon loss rate for Al, Ca also has the impurity segregation characteristics and evaporation characteristics, reduce the beam power through the slow construction characteristics of environment inside the melt temperature and gas phase unidirectional high vacuum the realization of the electron beam induced polysilicon directional solidification. The results show that the distribution of Al and Ca impurities in silicon showing a segregation trend, its removal rate depends on the melt temperature and solidification rate, the reasonable control of melt temperature and solidification rate can Get a higher impurity removal rate and good uniformity. In the process of melting, removal of impurities is controlled by evaporation in the gas-liquid interface, the power of 15kW, the total mass transfer coefficient Al and Ca were 3.43 * 10-5m/s and 2.04 * 10-5m/s, melting after 1800s removal rate reached 98% and 91%. induced in the electronic beam directional solidification process, impurity removal by coupling the solid-liquid interface segregation and gas-liquid interfacial evaporation, the content is further reduced to 0.7 * 10-4wt.%. Compared with the traditional way of electron beam melting, in the condition of the same total time, electron beam induced directional solidification method has the same even higher impurity removal the solidification rate, and reduce the power consumption. In this experiment, compared with the power of melting of 3000s using 15kW, when the melting time of 1800s, falling time is 1200s when the beam energy is reduced by 20%. electron beam The removal of P impurity and electron beam melting method by directional solidification removal of metal impurities used in industrial electron beam melting equipment, the content of P in silicon is reduced to less than 0.3 * 10-4wt.%, Fe, Al, Ca content reduced to 10-6 ~ 10-5wt.% magnitude, and the effect of these metal impurity segregation is better than points coagulation effect is the same as the solidification rate of traditional directional solidification. On this basis, combined with numerical simulation and experimental methods, from the viewpoint of energy utilization of existing smelting process is optimized through the interior of the crucible increases the graphite substrate to improve the energy utilization rate. It is found that adding a layer of graphite substrate between the silicon and water cooled copper crucible, equivalent to an increase of thermal resistance and the heat loss is reduced. The use of graphite substrate melting, melting in the same power, the temperature of the molten pool increase, accelerate the removal of P. The actual smelting process, in addition to P to achieve the same effect, The melting time can be shortened and the unit mass energy consumption is reduced from 29.3kW to h/kg to 19.5kW. H/kg.

【學(xué)位授予單位】：大連理工大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2017
【分類號(hào)】：TQ127.2

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