CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claim priority to U.S. Provisional Patent Application Serial No. 62/237,288, filed 05 October 2015, the disclosure of which is hereby

incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

[0002] The field of the present disclosure relates to isostatic graphite liners for fluidized bed reactors, and, more specifically, to graphite liners having a pre-tensioned wrap for applying a radial compressive force to the liner.

BACKGROUND

[0003] Polycrystalline silicon is a vital raw material used to produce many commercial products

including, for example, integrated circuits and

photovoltaic (i.e., solar) cells. Polycrystalline silicon is often produced by a chemical vapor deposition mechanism in which silicon is deposited from a thermally decomposable silicon precursor compound onto silicon particles in a fluidized bed reactor ("FBR") . Suitable thermally

decomposable silicon compounds include, for example, silane and halosilanes such as trichlorosilane .

[0004] In FBR preparation processes known in the art, decomposable silicon precursor compounds (e.g., silane) and silicon seed particles are continuously fed to the reactor. The bed of silicon particles is maintained at high temperature using appropriate heating devices and is maintained in a fluidized state by the upward flow of gas. Such a bed is referred to as a fluidized bed. Silicon is deposited on the seed particles that continuously grow until they reach a critical size after which they partition to the bottom of the reactor wherein they are removed from the FBR as polycrystalline silicon product (i.e.,

"granular" polycrystalline silicon) .

[0005] Unlike conventional FBRs for preparing common chemical products, limitations are encountered in material selection of the components of the fluidized bed reactor for preparing polycrystalline silicon. The reactor liner is continually in contact with silicon particles fluidizing at high temperature and pressure, and is subject to irregular vibration and severe shear stress caused by the fluidized bed of the particles, as well as differential pressures and differential temperatures across the reactor liner. Material selection for fluidized bed liner

construction is difficult because of the combination of desired high purity of the polycrystalline silicon, desired high material throughput with minimal out-of -service time, exposure to corrosive reaction gasses, and the extreme operation conditions of pressure, temperature, shear stress, vibration and abrasion. For these reasons, FBR reactors and reactor liners have a complex structure impacted by many process variables and combinations thereof .

[0006] FBR liners and liner surfaces in contact with silicon particles are designed to meet mechanical, thermal, electrical and chemical resistance property requirements in order to provide acceptable compatibility for high temperature operation and produce high-purity grade silicon. Problematically however, the surface hardness and roughness of liners known in the art are not optimized to provide for high resistance to erosion that results from contact of polycrystalline silicon particles with the liner surface. Still further, the compressive, flexural and tensile strength of liners known in the art are not sufficient to allow for construction of liners for large FBRs that would enable for improved polycrystalline silicon throughput and production rates as compared to reactors known in the art. Specifically, tensile stresses placed on the exterior of cylindrical graphite liners in a silicon FBR reactor can lead to axial fracture of the graphite cylinder segments that line the reactor and serve as the process inner liner. Axial pressure for maintaining seals and radial pressure for maintaining process purity can create axial cracks or propagate existing graphite cracks axially leading to liner fracture or breakage and run failure.

[0007] Thus a need exists for the prevention or elimination of fractures in FBR liners used in the preparation of polycrystalline silicon with high purity and at high production rates.

[0008] This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure.

Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art . BRIEF DESCRIPTION

[0009] In one aspect, a polysilicon fluidized bed reactor includes an annular outer shell, an annular liner interior of the outer shell such that an inner chamber is defined between the outer shell and the liner. The polysilicon fluidized bed reactor also includes at least one feed gas inlet, at least one vent gas outlet, and a fluidized zone region comprising a volume defined by the diameter of the inner surface of the annular liner and the length of the annular liner from the feed gas inlet location to the vent gas outlet location. The annular liner includes a cylindrical tube extending along an axis. The tube includes an inner surface and an outer surface defining a tube thickness therebetween. The annular liner also includes at least one layer of a wrap wound about the outer surface, wherein the wrap is configured to prevent radial expansion of the tube.

[00010] In another aspect, a polysilicon preparation liner includes a cylindrical tube extending along an axis. The tube includes an inner surface and an outer surface defining a tube thickness therebetween. The polysilicon preparation liner also includes at least one layer of a wrap wound about the outer tube surface . The wrap is configured to prevent radial expansion of the tube.

[00011] Various refinements exist of the features noted in relation to the above-mentioned aspects of the present disclosure. Further features may also be incorporated in the above-mentioned aspects of the present disclosure as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present

disclosure may be incorporated into any of the above- described aspects of the present disclosure, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[00012] FIG. 1 is a schematic of the gas flows entering and exiting a fluidized bed reactor (FBR) system.

[00013] FIG. 2 is a cross -sectional view of an FBR section of the present disclosure with a liner and reaction shell being depicted.

[00014] FIG. 3 is a cross -sectional view of the right side of a first example liner that may be used with the FBR system shown in FIG. 1.

[00015] FIG. 4 is a cross -sectional view of a second example liner that may be used with the FBR system shown in FIG . 1.

[00016] FIG. 5 is a cross -sectional view of a third example liner that may be used with the FBR system shown in FIG . 1.

[00017] FIG. 6 is a side view showing any of the liners shown in FIGS. 3-5.

[00018] FIG. 7 is a cross -sectional view of a fourth example liner that may be used with the FBR system shown in FIG . 1.

[00019] FIG. 8 is a cross -sectional view of a fifth example liner that may be used with the FBR system shown in FIG . 1. [00020] FIG. 9 is a cross -sectional view of a sixth example liner that may be used with the FBR system shown in FIG . 1.

[00021] FIG. 10 is a side view of any of the liners shown in FIGS. 7-9.

[00022] Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION

[00023] Referring now to FIG. 1, an example of one fluidized bed reactor (FBR) within the scope of the present disclosure is designated as 1. The FBR 1 includes a reaction chamber 10 and a gas distribution unit 2. A source of a first gas 5 and a source of second gas 7 are introduced into the distribution unit 2 to appropriately distribute the respective gases into the inlet of the reaction chamber 10. The distribution unit 2 helps distribute reactive gases throughout the reaction chamber 10 in order to maximize the rate of deposition of material onto the fluidized particles within the reaction chamber while minimizing the deposits on the liner. As used herein, "first gas" is a gas with a different composition than the "second gas" and vice versa. The first gas and second gas can compose a plurality of gaseous compounds as long as the mass composition or molar composition of at least one of the compounds in the first gas is different than the composition of that compound in the second gas. A product withdrawal tube 12 extends through the gas

distribution unit 2. Product particles can be withdrawn from the tube 12 and transported to a product storage unit 15. Spent gas 16 exits the reaction chamber 10 and can be introduced into further processing units 18. In this regard, it should be understood that the reactor 1 shown in FIG. 1 is exemplary and other reactor designs may be used without departing from the scope of the present disclosure.

[00024] Referring now to FIG. 2, the reaction chamber 10 of the FBR 1 is shown according to some aspects of the present disclosure where a source of the first gas 5, the source of second gas 7, the gas distribution unit 2, the spent gas 16, a reaction chamber interior 30 and a reaction liner 32 are depicted. A heating apparatus 34 of the FBR 1 may be maintained within an annular inner chamber 39 formed between the reaction liner 32 and an outer shell 35 of the reactor. The inner chamber 39 (or a portion thereof) may include insulating material (not shown) to prevent heat from being lost through the shell 35. The insulating material used may be any material suitable for insulating at high temperatures (both carbon and inorganic materials) as appreciated by those of skill in the art and may take a variety of forms including insulating blocks, blankets or felts.

[00025] In some optional aspects of the present disclosure, the pressure in inner chamber 39 is equal to or less than the pressure of processes gasses 5 and 7. In such aspects of the present disclosure, the pressure differential between process gasses 5 and 7 and inner chamber 39 (i.e. Pressure _process gasses - Pressure _inn er chamber ) is maintained by control valves to establish desired flow velocities for fluidization .

[00026] In some aspects of the present disclosure, not depicted in FIG. 2, the heating apparatus 34 may be in physical contact with the liner 32 and there may be a void space between the heating apparatus 34 and the outer shell 35. In some other aspects of the present disclosure, not depicted in FIG. 2, the heating apparatus 34 may be in physical contact with the outer shell 35 and there may be a void space between the heating apparatus 34 and the liner 32. In yet other aspects of the present disclosure, not depicted in FIG. 2, the heating apparatus 34 may be in physical contact with the outer shell 35 and the liner 32.

[00027] The heating apparatus 34 is depicted in FIG. 2 as extending along an axis 33 of the FBR 1 less than the entire length of the annular inner chamber 39 as measured from the portion of the gas distribution unit 2 adjacent to the reaction gas inlet section of reaction chamber 10 to the spent gas 16 outlet. However, those skilled in the art can appreciate that the heating

apparatus 34 may be selected to extend any suitable length as required to achieve the objects of any of the various aspects of the present disclosure such as, for instance, about one-third, one-half, two-thirds, or essentially the entire length of the annular inner chamber 39 as measured from the portion of the gas distribution unit 2 adjacent to the reaction gas inlet section of the reaction chamber 10 to the spent gas 16 outlet. Those skilled in the art may readily select the surface area of the heating apparatus 34 and the ratio thereto to the reaction chamber 10 volume and/or liner surface area required to achieve the objects of the present disclosure. The heating apparatus 34 may be an electrical resistance heater or one or more induction coils; however, other types of heating apparatus may be used without limitation (e.g., the heating apparatus 34 may be heated gas such as a combustion gas) . In some optional aspects of the present disclosure, at least a portion of the external surface area of the gas distribution unit 2 may be exposed to a heating apparatus, such as the heating apparatus 34.

[00028] In some optional aspects of the present disclosure, the liner 32 includes a thickness Tl and is used as a heat source in combination with an induction heating system (described in further detail below) such as where an induction coil is positioned around the liner 32 through which a high-frequency alternating current is passed to provide induction power along at least part of the height of the liner 32 to produce a temperature profile. In reference to FIG. 2 therefore, in such aspects, the liner 32 is the heating source and heating apparatus 34 is an induction coil.

[00029] FIG. 3 is a cross -sectional view of the right side of a first example liner 100 that may be used with the FBR system shown in FIG. 1. As such, the liner 32 (shown in FIG. 1) may be liner 100. In the example embodiment, the liner 100 includes an isostatically molded graphite tube 102 extending along the axis 33 (shown in FIG. 2) and having a first end 104 and a second end 106 that define a length LI therebetween.

[00030] The tube 102 may suitably be formed from a single isostatically molded graphite monolith or from two or more co-joined isostatically molded graphite segments or rings. In some aspects of the disclosure, the tube 102 may be formed from multiple isostatically molded rings along the axial length thereof wherein the rings are connected at abutment points or locations such as by an interlocking connection or by a connection wherein at least one surface of a first segment or ring overlaps at least one surface of a second segment or ring. Such an

interlocking assembly facilitates increasing tube 102 mechanical strength.

[00031] Methods of preparing isostatic graphite are known in the art. In general, isostatic graphite molded bodies are formed from particulate graphite wherein a compacting force is applied uniformly in all directions. In any of the various aspects of the present disclosure, the isostatically molded graphite has a density of between 1.7 grams per cubic centimeter (g/cm ³ ) and 1.95 g/cm ³ . The isostatically molded graphite further comprises less than 1000 parts per million by weight (ppmw) ash. The particulate graphite utilized to form the isostatically molded graphite may be characterized by particle or grain size wherein a plurality of the graphite particles exhibit an average particle size of less than 100 microns. Such particles sizes are typically smaller than the particle size of the fluidized polysilicon particles and, it is believed without being bound to any particular theory, contribute to the properties of the present liner including (i) low porosity due to high pressure formation, (ii) high strength and reduced chemical reaction due to a multitude of closely packed small grains, (iii) matched coefficient of thermal expansion with silicon carbide coating.

[00032] The tube 102 also includes an inner cylindrical surface 108 and an outer cylindrical surface 110 that define a tube thickness T2 therebetween. In the example embodiment, surfaces 108 and 110 are substantially parallel such that thickness T2 is substantially constant along the tube length LI between ends 104 and 106. In the example embodiment, thickness T2 is between approximately 50mm and approximately 140mm.

[00033] In one embodiment, the inner tube surface 108 bounds the fluidization reaction chamber interior 30. As used herein, the fluidization reaction chamber is defined as a fluidized bed in which silicon particles are suspended by an upward flow of the fluidizing gas in the reactor and is the region wherein silicon particles are contacted with the thermally decomposable silicon compound to cause silicon to deposit onto the silicon particles and thereby produce the high purity polysilicon particles. The fluidization reaction chamber may suitably include any portion of the fluidized bed wherein the silicon compound thermally decomposes and may include the interior section of gas distributor 2.

[00034] In the example embodiment, the liner 100 also includes at least one layer of a pre-tensioned wrap 112 wound about the outer surface 110 of tube 102 and configured to prevent radial expansion of the tube 102. The wrap 112 is made from at least one of a graphite fiber and a carbon fiber and is in the form of at least one of a plurality of fibers, a tape, and a rope. The wrap 112 includes a first end 114 and a second end 116 that define a wrap length LI therebetween. In the example embodiment, the wrap first end 114 is aligned with the tube first end 104 and the wrap second end 116 is aligned with the tube second end 106 such that the wrap length LI is

substantially similar to the tube length LI. [00035] The wrap 112 also includes an inner surface 118 and an outer surface 120 that define a wrap thickness T3 therebetween. In the example embodiment, surfaces 118 and 120 are substantially parallel such that thickness T3 is substantially constant along the wrap length LI between ends 114 and 116. In the example embodiment, thickness T3 is between approximately 10mm and approximately 100mm. As such, wrap thickness T3 plus tube thickness T2 is equal to overall thickness Tl of liner 100, which is less than 150mm. In one embodiment, tube

thickness T2 is greater than wrap thickness T3.

Alternatively, wrap thickness T3 is greater than tube thickness T2 , or thicknesses T2 and T3 are substantially similar. As shown in FIG. 3, the tube inner surface 108, the tube outer surface 110, the wrap inner surface 118, and the wrap outer surface 120 are all substantially parallel to one another and to the axis 33. In any of the various aspects of the present disclosure, wrap outer surface 120 is in contact with annular inner chamber 39, the heating apparatus 34 or a combination thereof.

[00036] In the example embodiment, the liner 100 also includes a silicon carbide coating 122 uniformly applied to inner surface 108 of tube 102. In embodiments having the silicon carbide coating 122, the coating 122 bounds the fluidization reaction chamber interior 30 and is configured and positioned to protect the tube inner surface 108 from diffusion of reactive chemicals and mechanical abrasion of the hard polysilicon granules within the chamber interior 30. Alternatively, the liner 100 does not include silicon carbide coating 122. The silicon carbide coating 122 has a density within a range of between approximately 3.05 to approximately 3.2 g/cm ³ . The silicon carbide coating 122 further comprises less than 1 ppmw of each of the elements boron, sodium, magnesium, aluminum, phosphorus, sulfur, potassium, calcium, titanium, vanadium, chromium, manganese, iron, copper, zinc, gallium,

germanium, arsenic, indium, tin, antimony, tungsten, tellurium, lead and bismuth. The silicon carbide coating 122 is further characterized by essentially zero porosity. The silicon carbide coating 122 may be still further characterized by particle or grain size wherein a plurality of silicon carbide particles exhibit an average particle size of less than 100 microns. As with isostatically molded graphite, the above described silicon carbide particle size contributes to a reduction in the tendency of the polysilicon particles to abrade the liner surface and provides for higher polysilicon purity and mechanical damage-resistant liner.

[00037] The silicon carbide coating 122 may be suitably applied to the tube inner surface 108 by any of various methods known in the art such as by chemical vapor deposition, evaporation, ion sputtering, RF sputtering, or DC magnetron sputtering. The silicon carbide layer 122 is desirably deposited so as to be nearly defect-free and stress-free after application to the tube inner surface 108. In conventional chemical vapor deposition, the coating 122 is deposited by passing a stream of the desired precursor gases over a heated substrate. When the

precursor gases contact the hot surface, they react and deposit the silicon carbide coating. Substrate

temperatures in the range of about 100-1000°C are generally sufficient to form these coatings 122 in several minutes to several hours, depending on the precursors and the

thickness of the coating desired. Any of the various silicon carbide deposition processes within the scope of the present disclosure produce a dense coating with essentially zero porosity.

[00038] FIG. 4 is a cross -sectional view of a second example liner 200 that may be used with the FBR system 1 (shown in FIG. 1) . As such, the liner 32 (shown in FIG. 1) may be liner 200. As shown in FIG. 4, liner 200 includes an isostatically molded graphite tube 202

extending along the axis 33 (shown in FIG. 2) and having a first end 204 and a second end 206 that define a length LI therebetween. The tube 202 also includes an inner surface 208 and an outer surface 210 that define a tube thickness T4 therebetween. In the example embodiment, inner surface 208 is substantially parallel to axis 33 and outer surface 210 includes at least one curved surface such that

thickness T4 varies along the tube length LI between ends 204 and 206. For example, the thickness T4 is greater towards ends 204 and 206 and smaller at a midpoint 205 along length LI. In the example embodiment, thickness T4 is between approximately 50mm and approximately 140mm.

[00039] Similar to the tube 102 (shown in FIG. 3) , the tube 202 may suitably be formed from a single isostatically molded graphite monolith or from two or more co-joined isostatically molded graphite segments or rings. In some aspects of the disclosure, the tube 202 may be formed from multiple isostatically molded rings along the axial length thereof wherein the rings are connected at abutment points or locations such as by an interlocking connection or by a connection wherein at least one surface of a first segment or ring overlaps at least one surface of a second segment or ring. Such an interlocking assembly facilitates increasing tube segment alignment and sealing.

[00040] As shown in FIG. 4, the liner 200 also includes at least one layer of a pre-tensioned wrap 212 wound about the outer surface 210 of tube 202 and disposed to prevent radial expansion of the tube 202. Similar to the wrap 112 (shown in FIG. 3) , the wrap 212 is made from at least one of a graphite fiber and a carbon fiber and is in the form of at least one of a plurality of fibers, a tape, and a rope. The wrap 212 includes a first end 214 and a second end 216 that define a wrap length LI

therebetween. In the example embodiment, the wrap first end 214 is aligned with the tube first end 204 and the wrap second end 216 is aligned with the tube second end 206 such that the wrap length LI is substantially similar to the tube length LI.

[00041] The wrap 212 also includes an inner surface 218 and an outer surface 220 that define a wrap thickness T5 therebetween. Specifically, the wrap 212 includes a plurality of layers that vary in number along the length LI of the wrap 212 according to a predetermined stress profile such that the overall wrap thickness T5 varies. In the example embodiment, inner surface 218 includes at least one curved surface corresponding to tube outer surface 210 and wrap outer surface 220 is

substantially parallel to axis 33 and to tube inner surface 208 such that thickness T5 varies along the wrap length LI between ends 214 and 216. For example, the thickness T5 increases towards a midpoint 215 along length LI and decreases towards ends 204 and 206. As such, the

additional layers of the wrap 212 proximate the midpoint 215 are thereby configured to provide additional

compression at that specific location on the tube 202 than the wrap 212 applies proximate the tube ends 204 and 206. In the example embodiment, thickness T5 is between

approximately 10mm and approximately 100mm. As such, the wrap thickness T5 plus tube thickness T4 is equal to overall thickness Tl of liner 200 which is less than 150mm.

[00042] Similar to liner 100, liner 200 also includes a silicon carbide coating 122 applied to the tube inner surface 208. Alternatively, liner 200 does not include the silicon carbide coating 122. Liner 200 is substantially similar to liner 100 in operation and composition, with the exception that both the graphite tube 202 and the wrap 212 include a thickness that varies along a length of the liner 200.

[00043] FIG. 5 is a cross -sectional view of a third liner 300 that may be used with the FBR system 1 (shown in FIG. 1) . As such, the liner 32 (shown in FIG. 1) may be liner 300. As shown in FIG. 5, the liner 300 includes an isostatically molded graphite tube 302

extending along the axis 33 (shown in FIG. 2) and having a first end 304 and a second end 306 that define a length LI therebetween. The tube 302 also includes an inner surface 308 and an outer surface 310 that define a tube thickness T2 therebetween. In the example embodiment, surfaces 308 and 310 are substantially parallel such that thickness T2 is substantially constant along the tube length LI between ends 304 and 306. In the example embodiment, thickness T2 is between approximately 50mm and approximately 140mm. As such, the tube 302 of the liner 300 may be the same graphite tube 102 used in the liner 100. [00044] As shown in FIG. 5, the liner 300 also includes at least one layer of a pre-tensioned wrap 312 wound about the outer surface 310 of tube 302 and

positioned to prevent radial expansion of the tube 302. Similar to the wraps 112 and 212 (shown in FIGS. 3 and 4) , the wrap 312 is made from at least one of a graphite fiber and a carbon fiber and is in the form of at least one of a plurality of fibers, a tape, and a rope. The wrap 312 includes a first end 314 and a second end 316 that define a wrap length LI therebetween. In the example embodiment, the wrap first end 314 is aligned with the tube first end 304 and the wrap second end 316 is aligned with the tube second end 306 such that the wrap length LI is

substantially similar to the tube length LI.

[00045] The wrap 312 also includes an inner surface 318 and an outer surface 320 that define a wrap thickness T6 therebetween. Specifically, the wrap 312 includes a plurality of layers that vary in number along the length LI of the wrap 312 according to a predetermined stress profile such that the overall wrap thickness T6 varies. The temperature difference and the process operating conditions build stress between the wrap 312 and a silicon carbide coating 122. The graphite liner 300 also includes stresses due to axial loading to maintain seals, radial loading to process pressure and radial thermal gradients due to process heat transfer. The stress profile is suitably a mathematically predicted stress profile for the entire process condition, plus the relative thermal expansions of the graphite liner 300 and the carbon fiber wrap 312 from room temperature to operating temperature. [00046] In the example embodiment, inner surface 318 is substantially parallel to axis 33 and to tube surfaces 308 and 310 and wrap outer surface 320 includes at least one curved surface such that thickness T6 varies along the wrap length LI between ends 314 and 316. For example, the thickness T6 increases towards a midpoint 315 along length LI and decreases towards ends 314 and 316. As such, the additional layers of the wrap 312 proximate the midpoint 315 are positioned to provide additional compression at that specific location on the tube 302 than the wrap 312 applies proximate the tube ends 304 and 306. In the example embodiment, thickness T6 is between

approximately 10mm and approximately 100mm. As such, the wrap thickness T6 plus tube thickness T2 is equal to overall thickness T7 of liner 200, which is less than 150mm. The wrap thickness T6 of liner 300 may be

substantially similar to wrap thickness T5 of liner 200. Alternatively, wrap thicknesses T5 and T6 are different. Similarly, the liner thickness T7 is substantially similar to the liner thickness Tl (shown in FIGS. 3 and 4) .

Alternatively, the liner thicknesses T7 and Tl are

different .

[00047] Similar to liners 100 and 200, liner 300 also includes a silicon carbide coating 122 applied to the tube inner surface 308. Alternatively, liner 300 does not include the silicon carbide coating 122.

[00048] Liner 300 is substantially similar to liners 100 and 200 in operation and composition, with the exception that the graphite tube 302 includes a constant thickness and the wrap 312 include a thickness that varies along a length of the liner 300. [00049] FIG. 6 is a side view of any of the liners 100, 200, and 300 shown in FIGS. 3-5 illustrating that the wraps 112, 212, and 312 extend a full length LI of the tubes 102, 202, and 302.

[00050] FIG. 7 is a cross -sectional view of a fourth liner 400 that may be used with the FBR system 1 shown in FIG. 1. As such, the liner 32 (shown in FIG. 1) may be liner 400. In the example embodiment, the liner 400 includes an isostatically molded graphite tube 402

extending along the axis 33 (shown in FIG. 2) and having a first end 404 and a second end 406 that define a length LI therebetween .

[00051] The tube 402 also includes an inner surface 408 and an outer surface 410 that define a tube thickness Tl therebetween. In the example embodiment, outer surface 410 includes a notch or groove 405 formed therein such that the tube 402 includes a first flange 407 at the first end 404 and a second flange 409 at the second end 406. The notch 405 includes an outer surface 411 that is radially spaced from tube outer surface 410 such that each flange 407 and 409 includes a thickness T8. In the example embodiment, surfaces 408, 410, and 411 are substantially parallel such that thickness T8 is between approximately 10mm and approximately 100mm.

[00052] In the example embodiment, the liner 400 also includes at least one layer of a pre-tensioned wrap 412 wound about tube 402 and configured to strengthen and minimize radial expansion of the tube 402. Similar to the wrap 112 (shown in FIG. 3) , the wrap 412 is made from at least one of a graphite fiber and a carbon fiber and is in the form of at least one of a plurality of fibers, a tape, and a rope. The wrap 412 includes a first end 414 and a second end 416 that define a wrap length L2

therebetween. In the example embodiment, the wrap 412 is positioned within the notch 405 formed in the tube outer surface 410 such that tube length LI is greater than wrap length L2. As such, first and second flanges 407 and 409 do not include the wrap 412.

[00053] The wrap 412 also includes an inner surface 418 and an outer surface 420 that define a wrap thickness T8 therebetween. In the example embodiment, surfaces 418 and 420 are substantially parallel such that thickness T8 is substantially constant along the wrap length L2 between the ends 414 and 416. Specifically, the wrap inner surface 418 contacts the notch outer surface 411 and the wrap outer surface 420 is substantially flush with the tube outer surface 410 such that the wrap thickness is substantially similar to the thickness of the flanges 407 and 409. Furthermore, in one embodiment, the wrap 412 is substantially centered along the length LI of the tube 402 such that the first and second flanges 407 and 409 have a substantially similar length L3. Alternatively, the wrap 412 is positioned along the length LI of the tube 402 at any location according to a predetermined stress profile that facilitates operation of the liner 400 as described herein .

[00054] Similar to the liners 100, 200, and 300 as described above, the liner 400 also includes a silicon carbide coating 122 applied to the inner surface 408 of the tube 402. Alternatively, the liner 400 does not include the coating 122. [00055] FIG. 8 is a cross -sectional view of a fifth example liner 500 that may be used with the FBR system 1 (shown in FIG. 1) . As shown in FIG. 8, the liner 500 includes an isostatically molded graphite tube 502 extending along the axis 33 (shown in FIG. 2) and having a first end 504 and a second end 506 that define a length LI therebetween. The tube 502 also includes an inner surface 508 and an outer surface 510 that define a tube thickness T9 therebetween. In the example embodiment, inner surface 508 is substantially parallel to axis 33 and outer surface 210 includes a notch 505 including at least one curved outer surface 511 such that thickness T9 varies along the tube length LI between ends 504 and 506. For example, the thickness T9 is greater towards ends 504 and 506 and smaller at a midpoint along length LI. In the example embodiment, thickness T10 is between approximately 10mm and approximately 100mm. At its maximum, thickness T10 is substantially similar to overall liner thickness T10.

[00056] Similar to the tube 102 (shown in FIG. 3) , the tube 502 may suitably be formed from a single isostatically molded graphite monolith or from two or more co-joined isostatically molded graphite segments or rings. In some aspects of the disclosure, the tube 502 may be formed from multiple isostatically molded rings along the axial length thereof wherein the rings are connected at abutment points or locations such as by an interlocking connection or by a connection wherein at least one surface of a first segment or ring overlaps at least one surface of a second segment or ring. Such an interlocking assembly facilitates increasing tube 502 mechanical strength. [00057] As shown in FIG. 8, the liner 500 also includes at least one layer of a pre-tensioned wrap 512 wound about the tube 502 and configured to strengthen and minimize radial expansion of the tube 502. Similar to the wrap 112 (shown in FIG. 3) , the wrap 512 is made from at least one of a graphite fiber and a carbon fiber and is in the form of at least one of a plurality of fibers, a tape, and a rope. The wrap 512 includes a first end 514 and a second end 516 that define a wrap length L2 therebetween. In the example embodiment, the wrap 512 is positioned within the notch 505 formed in the tube outer surface 510 such that tube length LI is greater than wrap length L2. As such, the tube outer surface proximate the ends 504 and 506 do not include the wrap 512.

[00058] The wrap 512 also includes an inner surface 518 and an outer surface 520 that define a wrap thickness T10 therebetween. Specifically, the wrap 512 includes a plurality of layers that vary in number along the length L2 of the wrap 512 according to a predetermined stress profile such that the overall wrap thickness T10 varies. In the example embodiment, inner surface 518 includes at least one curved surface corresponding to and contacting the notch outer surface 511 and the wrap outer surface 520 is substantially parallel to axis 33 and to tube inner surface 508 such that thickness T10 varies along the wrap length L2 between ends 514 and 516. For example, the thickness T10 increases towards a midpoint 515 along length L2 and decreases towards ends 514 and 516. As such, the additional layers of the wrap 512 proximate the midpoint 515 are configured to provide additional

compression at that specific location on the tube 502 than the wrap 512 applies proximate the tube ends 504 and 506. In the example embodiment, thickness T10 is between approximately 10mm and approximately 100mm. In the example embodiment, the wrap thickness T8 is substantially similar to the wrap thickness T10 (shown in FIG. 7) .

Alternatively, the wrap thickness T10 is not similar to the wrap thickness T8.

[00059] Similar to the liners 100, 200, 300, and 400 as described above, the liner 500 also includes a silicon carbide coating 122 applied to the inner surface 508 of the tube 502. Alternatively, the liner 500 does not include the coating 122.

[00060] FIG. 9 is a cross -sectional view of a sixth example liner 600 that may be used with the FBR system 1 (shown in FIG. 1) . As such, the liner 32 (shown in FIG. 1) may be liner 600. In the example embodiment, the liner 600 includes an isostatically molded graphite tube 602 extending along the axis 33 (shown in FIG. 2) and having a first end 604 and a second end 606 that define a length LI therebetween.

[00061] The tube 602 also includes an inner surface 608 and an outer surface 610 that define a tube thickness Til therebetween. In the example embodiment, outer surface 610 includes a notch or groove 605 formed therein such that the tube 602 includes a first flange 607 at the first end 604 and a second flange 608 at the second end 606. The notch 605 includes an outer surface 611 that is radially spaced from tube outer surface 610 such that each flange 607 and 609 includes a thickness T12. In the example embodiment, surfaces 608, 610, and 611 are

substantially parallel such that thickness Til is between approximately 50mm and approximately 140mm and thickness T12 is between approximately 0mm and approximately 50mm.

[00062] In the example embodiment, the liner 600 also includes at least one layer of a pre-tensioned wrap 612 wound about tube 602 and configured to strengthen and minimize radial expansion of the tube 602. Similar to the wrap 112 (shown in FIG. 3) , the wrap 612 is made from at least one of a graphite fiber and a carbon fiber and is in the form of at least one of a plurality of fibers, a tape, and a rope. The wrap 612 includes a first end 614 and a second end 616 that define a wrap length L2

therebetween. In the example embodiment, the wrap 612 is positioned within the notch 605 formed in the tube outer surface 610 such that tube length LI is greater than wrap length L2. As such, first and second flanges 607 and 609 do not include the wrap 612.

[00063] The wrap 612 also includes an inner surface 618 and an outer surface 620 that define a wrap thickness T13 therebetween. Specifically, the wrap 612 includes a plurality of layers that vary in number along the length L2 of the wrap 612 according to a predetermined stress profile such that the overall wrap thickness T13 varies. Specifically, the wrap inner surface 618 contacts and is substantially parallel to the notch outer surface 611 and the wrap outer surface 620 includes at least one curved surface such that thickness T13 varies along the wrap length L2 between ends 614 and 616. For example, the thickness T13 increases towards a midpoint 615 along length L2 and decreases towards ends 614 and 616. Specifically, wrap outer surface 620 extends beyond tube outer surface 610 by a distance Dl such that the overall thickness of liner 600 is thickness Til plus the distance Dl. As such, the additional layers of the wrap 612 proximate the midpoint 615 are configured to provide additional

compression at that specific location on the tube 602 than the wrap 612 applies proximate the wrap ends 614 and 616. In the example embodiment, thickness Til is between approximately 50mm and approximately 140mm, thickness T12 is between approximately 0mm and approximately 70mm, and the overall wrap thickness of thickness Til plus distance Dl is less than 150mm.

[00064] Furthermore, in one embodiment, the wrap 612 is substantially centered along the length LI of the tube 602 such that the first and second flanges 607 and 609 have a substantially similar length L3. Alternatively, the wrap 612 is positioned along the length LI of the tube 602 at any location according to a predetermined stress profile that facilitates operation of the liner 600 as described herein.

[00065] Similar to liners 100, 200, 300, 400, and 500, the liner 600 also includes a silicon carbide coating 122 applied to the tube inner surface 608.

Alternatively, the liner 600 does not include coating.

[00066] FIG. 10 is a side view of any of the liners 400, 500, and 600 shown in FIGS. 7-9 illustrating that the wraps 412, 512, and 612 extend a length L2 that is shorter than the length LI of the tubes 402, 502, and 602. In one embodiment, the notches 405, 505, and 605 in the tubes 402, 502, and 602 are machined into the tube outer surfaces 410, 510, and 610 after production.

Alternatively, the notches 405, 505, and 605 are formed during production of the tubes 402, 502, and 602, such that the notches 405, 505, and 605 are not machined during a post -production process.

[00067] In accordance with any of the various aspects of the present disclosure, polycrystalline silicon purity and throughput for thermal decomposition processes in fluidized bed reactors (FBRs) may be improved relative to conventional FBRs and associated processes by the use of a liner comprising an isostatic graphite tube having a pre- tensioned wrap that applies continuous compressive force onto the graphite tube. The liner includes a graphite fiber wrap wound on an outer liner surface to apply a compressive force on the graphite liner and to prevent radial expansion of the graphite liner. Specifically, the wrap is pre-tensioned to offset predetermined tensile hoop stresses on the inner surface of the liner.

[00068] The liners of the present disclosure are characterized as having, at operating temperatures of from 500°C to 1500°C, among other features, (i) an

essentially non-contaminating surface, (ii) high

compressive strength, (iii) a surface of sufficient hardness to provide high erosion resistance, (iv) optimal differences in the coefficient of thermal expansion between the graphite tube and the pre-tensioned wrap, and (v) high thermal conductivity.

[00069] The liners should have sufficiently high mechanical strength to function without failure under the high temperature and pressure differential conditions presented by the process of the present disclosure. High mechanical strength is required to withstand high axial compressive forces that are generated by maintaining a positive pressure in the annular inner chamber 39 in order to prevent or inhibit the fluidizing gases and product silicon from migrating through the liner. In addition, high mechanical strength is also required to withstand high thermal gradients, either axially or radially that could be generated due to high heat flux requirement for the process. Further, a high strength liner is required to withstand high thermal gradients with time arising due to changes in process heat requirements, silicon deposit buildup on the liner wall, or due to heating system mis- operation or in emergency loss of power.

[00070] The liners of this embodiment have a thermal conductivity that is sufficient to minimize resistance to heat transfer and to minimize thermal gradients along the length thickness and length of the liner. In any of the various aspects of the present disclosure, the thermal conductivity is greater than 115 watts per meter kelvin (W/m»K) at ambient temperature and at least 50 W/m»K at 1500°C.

[00071] In aspects of the present disclosure utilizing an induction heating system, the liner may have an electrical resistivity sufficient to allow the liner to be employed as the heating source. In such aspects, the electrical resistivity is greater than 1,100 microohm centimeters. In some other aspects, wherein only a portion of the liner is used as the heating source, the electrical resistivity is less than 1,600 microohm centimeters.

[00072] Furthermore, after the pre -tensioned wrap is wound about the tube, the newly formed carbon fiber reinforced graphite (CFRG) liner assembly could further be infused with a carbon-rich material and then re-fired to burn-out the binder and to bond the wrap fibers to each other and to the graphite cylinder. After re- firing, the steps of purification, final machining, and silicon carbide coating are performed. The tube and wrap are designed such that the firing process does not relax or recrystallize the carbon fibers, which may counteract the expected positive benefits of strong fibers and the high tension wrap.

Additionally, the difference in the coefficient of thermal expansion (CTE) between the wrap and the isostatically molded graphite is designed to be a minimum over an operating temperature range of from 500°C to 1500°C.

Specifically, the CTE of the wrap is designed to be similar to or less than the graphite tube to prevent or inhibit loosening of the wrap under the liner operating

temperatures .

[00073] Still further, the compressive, flexural and tensile strength of monolithic isostatic graphite tubes known in the art may not be sufficient to allow for construction of liners for large FBRs that would enable for improved polycrystalline silicon throughput and production rates as compared to reactors known in the art. Specifically, such monolithic isostatic graphite liners are rigid structures that endure tensile stresses on the exterior thereof in a silicon FBR reactor. These stresses can lead to axial fracture of the graphite cylinder segments that line the reactor and serve as the process inner liner. Axial pressure for maintaining seals and radial pressure for maintaining process purity can create axial cracks or propagate existing graphite cracks axially leading to liner fracture or breakage and run failure. Wrapping the exterior of the isostatic graphite liners with a carbon fiber wrap facilitates reducing or eliminating radial expansion of the liner and the resulting stresses and fractures .

Process for Producing Polycrystalline Silicon

[00074] As generally depicted in FIG. 1, the process of the present disclosure includes introducing a feed gas including a gaseous silicon compound capable of being thermally decomposed into a reactor. A thermally decomposable compound 7 and carrier gas 5 are fed from their respective source to the reactor system via gas distribution unit 2. The carrier gas 5 may include hydrogen or a noble gas such as argon or helium and mixtures thereof. Thermally decomposable silicon compounds include compounds generally capable of being thermally decomposed to produce silicon. Additional products may be produced from the decomposition process, without departing from the scope of the present disclosure, as long as it provides a source of silicon to grow the polysilicon particles to form polysilicon granules. Thermally

decomposable silicon compound gases include all gases containing silicon, that can be deposited by chemical vapor deposition, such as silicon tetrahydride (commonly referred to as silane) , trichlorosilane and other silicon halides, wherein one or more of the hydrogen atoms of silane is substituted with a halogen such as chlorine, bromine, fluorine and iodine. The thermally decomposable compound may be introduced into the reactor without dilution or the gas may be diluted with a carrier gas such as hydrogen, argon, helium or combinations thereof. During

decomposition, by-product hydrogen is produced that may be recycled for use as a carrier gas for additional quantities of thermally decomposable feed gas in the operation of the reactor system, if needed.

[00075] Various gas distribution unit 2 designs are known in the art and are suitable for the practice of the present disclosure.

[00076] In any of the various aspects of the present disclosure, the first gas 5 may comprise a source of decomposable silicon precursor compounds and the second gas 7 may comprise a carrier gas wherein the first gas 5 and second gas 7 are supplied to distinct reaction chamber regions. For example, first gas 5 (decomposable silicon precursor compounds) may be supplied to a center region of distributor 2 and second gas 7 (carrier gas) may be supplied to peripherally around the center region, and polycrystalline silicon product is withdrawn from the reactor via tube 12.

[00077] The feed gas is heated in the reaction chamber to cause at least a portion of the silicon in the silicon compound to deposit, by chemical vapor deposition, onto the silicon seed particles in the reaction chamber, thereby growing the silicon particles into larger particles typically referred to as granular polysilicon. Another portion of the thermally decomposable silicon compound decomposes to form among other things, silicon vapor.

Suitable heating apparatuses include, without limitation, induction heating and radiant heating, such as a resistive radiant element .

[00078] As shown in FIG. 1, particulate polycrystalline silicon is withdrawn from the product withdrawal tube 12. Particulate polycrystalline silicon may be withdrawn from the reactor intermittently as in batch operations; however, it is generally more efficient to withdraw the particulate product continuously.

Regardless of whether batch or continuous withdrawal of silicon product is used, it has been found that the size of the product particles when withdrawn from the reactor influences the reactor productivity. For instance, it has been found that generally increasing the size of the withdrawn silicon particulate results in increased reactor productivity; however if the product particles are allowed to grow too large, contact between the gas and solid phases in the reactor may be reduced thereby reducing

productivity. Accordingly, in various aspects of the present disclosure, the mean diameter of the particulate polycrystalline silicon that is withdrawn from the reactor is from about 600 μπι to about 2000 μπι, from about 800 μπι to about 1500 μιη, or from about 900 μπι to about 1300 μπι.

[00079] When introducing elements of the present invention or the embodiment (s ) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[00080] As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.