Dr Shi 珺 on polycrystalline silicon ingot heating and melting process

Polaris solar PV net news: polycrystalline silicon ingot is usually divided into heat, melt, high temperature annealing, cooling, crystallization, in six stages. This paper describes the process heat and melt the two.

(A) heating ready

Mounted after good material, and then to once again check the furnace, furnace. Generally, vacuuming the oven first. Step by step vacuum pump is turned on, start heating. Vacuum process is very simple, usually is the first open primary pumps (machines, Rotary or rotary piston pump), drawn from the atmosphere after 2000Pa, and then open the roots pump, vacuum by 10Pa; clean material after cleaning, vacuum as long as you follow these steps in the order of it. About vacuum is less than 1000Pa, you can turn on the heating power.

In Silicon material heating Shi, can according to experience used constant power heating, considered to Silicon material of hot conduction sex poor, and RDS3.0 furnace used of is four week heating way, melting Shi, is from around began melting, then, due to temperature points in Crucible Central, so, around may has melting and to has is high of temperature, but middle of temperature does not high, so, if used temperature set of control way, may power will plus have is big. Power, easily lead to the rising temperature of the walls of the Crucible and Crucible because it is using Quartz material, once the temperature exceeds 1600 deg will easily react with silicon, resulting in erosion of crucible; if the temperature is rising to 1700 ℃, the Crucible and the Silicon reacts violently, leading Silicon liquid splashes, Crucible to melt through. Seriously, the silicone fluid might spill on top, cause damage to the insulation layer of graphite and. Therefore, the usual way of melting stage power control should be used, based on theoretical and empirical values, brings the heating power according to certain values, so you can ensure that the Crucible temperature could not be overestimated.

Furnace for RDS4.0, as a result of bottom and top heating method, the situation will be better, but if the power is too high, also occurred in the bottom of the Crucible temperature is too high, however, since both the top and bottom have infrared temperature, so temperature is too high. But if the IR-jam or out of control, then there is also danger of overheating.

No matter what kind of furnace, heater outside, so the Silicon internal and external temperature is always there, which is why warming up for some time, to carry out thermal insulation in order to let external heat transmitted to the inside, to avoid excessive temperature difference between inside and outside, leading to melting of the prone to accidents. Which is why in molten Silicon during heating curve have the insulation on process.

(B) the Silicon melt

Silicon melt, because of the need to absorb a lot of heat, so close to the melting point, the temperature rise will be slow. Note that in General, the melting point of Silicon is 1414, but when melted, SI-1420 c higher temperatures usually, will it be possible to make the Silicon melts smoothly. Otherwise, Silicon material may occurred melting Hou, due to around of Silicon also is solid, and began solidified of phenomenon, this is why sometimes temperature will appeared upper and lower repeatedly of situation; because, Silicon material in melting Shi, sometimes local in melting Hou, temperature will beyond melting point many, but around may also did melting of silicon block; Dang melting of Silicon liquid flow to silicon block following Hou, silicon block and show to, led to infrared temperature instrument displayed of temperature and below melting point.

Such phenomena tend to last for a few hours, duration of the heater power-related. Sometimes, when the majority of Silicon melt, there are large blocks of Silicon does not melt, once those chunks of Silicon drift into the field of thermometers, infrared thermometer shows the temperature occurs all of a sudden lowering of, serious and even infra-red temperature below Silicon liquid temperature of 50 ° c above. At this point, don’t mistake it for a Silicon fluid solidification, it may only be a temporary phenomenon.

Resolve incomplete melting method is all the more power to Silicon liquid until completely melted. Typically, silicone fluid heating up to 1490, and then stay there for a while, from the IR thermometer to see Silicon liquid surfaces become clear later, began to fade. Silicon liquid all melted, into the vacuum melting process.

(C) the powder melts

During the process of ingot, powder is sometimes encountered. These powders are usually produced by FDR in circulating fluidized bed technology. Because the powder at a lower price, therefore, many cast ingot the ingot manufacturers hope to powder. For powder handling requires great attention. Because, powder material in atmospheric State Xia is easy was vacuum pump by produced of airflow with up in furnace within flying, Silicon Powder flying not only will caused Silicon material of waste, in flying process in the Silicon Powder also will stained in heating pieces, and insulation body Shang, such, once these Silicon Powder melting, is easy infiltrated graphite material and insulation material, led to heating pieces and insulation body damaged; also, Silicon powder of flying will with gas to vacuum pump force, led to vacuum pump of damaged. Therefore, in the melting process of powder, to prevent dust of powder.

In order to solve this problem, we must first understand powder in any case easy to fly. Flying under the three stages of powder easily.

The first stage is to begin vacuum of the moment. Powder in the Crucible, gap is the atmosphere of the powder particles. When you start to vacuum, furnace air pressure under the action of mechanical pump suddenly decreases, gases due to a pressure difference between particles upward movement of air, the airflow can be very powerful, and will boost Silicon Powder dust.

To avoid this problem, for powder material, in began pumping vacuum Shi, cannot like General of block material as directly with mechanical pump vigorously pumping, and should will valve first closed, then slowly open, best is closed competent road, used a bypass of fine pipeline, with fine pipeline Shang of Regulation valve, from closed to open, gradually increases, such, pumping vacuum Shi, furnace within of pressure will compared slow to declined, does not produced suddenly of pressure drop, thus solution Silicon Powder flying of problem.

When the vacuum pressure fell to below 100Pa, due to low pressure and buoyancy of gases to Silicon Powder is very small, you can gradually open the main valve, using normal valve pumping air, at this time, because the air force is very small, not enough to take gas out and, therefore, can be pumped to vacuum.

Second stage is the stage of silicon materials to heat up to about 1000 degrees Celsius. First, 900 ℃ above, Silicon powder on will produced sintering phenomenon, and due to Silicon powder of thermal conductivity sex is poor, can said almost is adiabatic material, so, dang infrared temperature instrument measurement to Silicon Powder temperature to 1000 ℃ around of when, Crucible side of temperature may has reached close melting of problem, even has began melted, dissolved of Silicon encountered around of Silicon powder and will solidified and knot shell; and then, Crucible most central part of Silicon Powder due to adiabatic role, temperature may only 100 ℃ not to, such, Silicon Powder adsorption of moisture, as well as gas, has not been fully released. Dang external Silicon Powder knot shell formed closed body of when, with internal Silicon Powder temperature of gradually rose, gas expansion, pressure increased, in Silicon Powder internal gradually formed closed of high-pressure ball mission, dang pressure arrived must degree, on will occurred burst; or although internal itself of pressure no reached burst pressure, but knot shell Hou, due to heating body of role makes shell body again melting variable thin Hou, internal of gas will burst rushed out, produced airflow, will Silicon Powder splashed.

This process of Silicon Powder dust, difficult to avoid, the better way is in the upper part of the Silica fume to compare brittle material (sizes 10-30mm), covered with a layer thickness of about 50-100mm, so that even if the powder have splashes, block block, so that it does not spill too far, and reached the body of heating or insulation.

In addition, there is a way to make Silicon powder packed in, using thin steel, internal spacing bar a few small holes in Silicon Powder, from the top to the bottom, so that the gap around the hole larger, Silica fume is not easy to form a closed ball, this test is more efficient way.

Third easy flying phase of Silicon Powder, melting Silicon powder. Melting Silicon Powder, on the one hand and the sintering stage of the same phenomenon, on the other hand, in the stage of melting Silicon Powder, powder substance on the surface oxide layer of powder in particular, can react with silicone fluid, Silicon monoxide gas produced, as well as other gases, the gas will be in the form of splashes, leading Silicon or silicon liquid splashed out on the surface of it. Since this time, entire crucible of Silicon material will focus on similar effects, therefore, although local effects do not necessarily have the first two stages of serious, but the overall effect may be the splash is the most powerful.

Avoid at this stage and the second stage, it’s hard to avoid. Similarly, laid above the Silicon material of Silicon Powder, and pierced holes also have certain effects. A process approach, you can for your reference. Is, in heating Shi control power don’t high, makes temperature rose time also compared slow (prevent Silicon Powder internal language around of temperature had big), in melting point zhixia of a temperature, such as 1380 ℃ (also can 1400 ℃, but because temperature points and around has temperature, so, too close melting point Shi, may around of Silicon Powder has melting), insulation a time, makes whole Crucible within of Silicon Powder are reached 1380 ℃, then again warming melting. In this way, because the synchronization of high temperature, melting for a short period and, therefore, produce sintered and form a shell’s chances are greatly reduced. The technology tested and effective.

When most of Silicon Powder is melted in a crucible, Silica fume can form dough floats on the surface of silicone fluid, just like flour in the water similar to. Relationships within the powder because powder adiabatic, and therefore more difficult to melt, and the surface of the dough much lower temperatures than silicone fluid, when the dough quantity is large, may result in temperature are not allowed, or large temperature fluctuations of temperature measurement.

Therefore, only through the method of high temperature will gradually all the powder melts and continue to process until the surface is clean. Which is why the melting temperature of powder to be taller than the block reason.

Some people said that melting Silicon powder and her small grains of this somewhat supernatural means. But quite possibly melting, melting Silicon Powder is not complete, many fine particles cause the silicone fluid in the still, therefore, nucleating more reasons. Solution methods, should still be high temperatures keep confirming that Silicon melts. But melt later, might as well in high temperature area to wait for a period of time, because our observation window can be observed only the Crucible is a small piece of the surface, there may be many not melting Silicon powder around. When the particles of powder when it is fully dissolved, metallurgy of powder should be exactly like the stuff.

Original title: Shi 珺 PhD on polycrystalline silicon ingot heating and melting process

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