LOW TEMPERATURE CONDENSATE ADHERENCE METHOD AND APPARATUS

Abstract: In an isotope separation system in which hot, isotopi­ cally separated vapor is deposited on enriched product and depleted tails plates (40, 42) deposit adherence is improved and plate cooling simplified. The tails plate (42) is liquid-­ cooled to a lower temperature which permits the product plate (40) to be radiatively cooled by it simplifying product plate cooling and lowering its temperature to reduce the tendency for the product plate deposit to separate during temperature cycling. In one embodiment plate surfaces are cleaned of impurities in a vacuum by allowing the tem­ perature of the tails plates to reach the relatively low operat­ ing temperature prior to proceeding with the deposition. In another embodiment plate surfaces are provided with rough surfaces to promote air penetration during deposit strip­ ping, which permits the formation of a brittle, more easily severed bond between plate and deposit. In a further em­ bodiment for the deposition of biphasic materials, the op­ erating temperature of the tails plate is set to the lowest temperature at which the lower thermal coefficient of expan­ sion phase of the material will adhere to the tails plate.

Description: The present invention relates to a method and to an apparatus for vapor deposition of the kind referred to in the preamble portion of patent claims 1 and 12, respectively. Such a method and such an apparatus are known, for example, from US-A- 4 155 008. In the field of laser isotope separation, a vapor of a material such as uranium having a mixture of isotopes is photoionized and the ionized vapor components are then extracted from the vapor through the use of substrates in the form of tails plates and product plates. These components are separated due to the isotopic selectivity of the process. In normal practice, the tails plates are cooled, whereas the product plates run hot due to the temperature of the condensing deposited material and the extractive ion current. The deposited vapor represents both an enriched and depleted isotope composition. In a practical system the deposition of vapor particles onto the substrates is permitted to continue until a significant quantity of material has been deposited before the substrates are removed from the process to recover the deposited material. Both the enriched and depleted product are recovered in this manner. As the deposition of material progresses during the isotope separation, the mass of the deposition on the substrate increases which, along with other factors occuring in the process, tends to cause the deposition to flake or fall from the substrate unless good adherence is achieved between the deposited layer and the substrate. It has been typical in the past to operate the tails plates as a substantially elevated temperature, for example, at or above 450 °C in order to insure an adequate adherence of the deposition to the substrate. With the tails plates at 450 °C, the product plates will run substantially higher. In this regard reference is made to US-A- 4, 210 814. The operation of the collecting substrates at such elevated temperatures is undesirable because the elevated temperature increases the warping tendency of the substrates which must be maintained in precise linear alignment in order to insure the proper operation of a laser isotope separation system. More importantly, when operating at high plate-deposition temperatures, large temperature swings occur at the beginning end of the process cycle and during unavoidable mid-process shutdowns. These large temperate swings result in the deposit breaking off the collecting substrates. High operating temperatures for all collecting substrates also result in a high temperature radiative surface for the tails plate. Thus in prior systems the tails plate cannot serve as an efficient heat sink for radiative transfer from the product plates. Poor radiative transfer efficiency necessitates the use of high boiling point fluids as active coolants, and the use of additional cooling pipes or exotic heat pipe technology to remove the large amounts of heat which cannot be completely removed by radiative transfer. US-A- 4 155 008 discloses a method for vapor deposition in a system including a source of material to be vaporized, a first deposition structure, and a second deposition structure, comprising the steps of aligning the first deposition structure with the source of material to be vaporized, diposing the second deposition structure adjacent the first deposition structure in such manner that the first deposition structure is intermediate the second deposition structure and the source of material to be vaporized; vaporizing the material to form a vapor, and depositing components of the vapor on the first deposition structure and the second deposition structure. Said US-A-4 155 008 further discloses an apparatus for vapor deposition, comprising vapor source means for vaporizing a material by heating a surface of the material to form a vapor, a first deposition structure aligned to receive depositions of vapor provided by the vapor source means, a second deposition structure disposed adjacent the first deposition structure to receive depositions of vapor provided by the vapor source means, the first deposition structure being intermediate the second deposition structure and the vapor source means. It is the object of the present invention to provide a method and an apparatus which are most effective in increasing the product adherence during deposition. This object is accomplished with a method and an apparatus as claimed in claims 1 and 12, respectively. Dependent claims are directed on features of preferred embodiments of the invention. It is a finding of this invention that the vapor deposit will more readily adhere to the various substrates if deposited at lower temperatures, and that certain plate cleaning, contouring, and preheating techniques increase product adherence during deposition and temperature cycling and aid product stripping when exposed to air. The result is that the entire process can be run at a lower temperature which dramatically increases adherence in circumstances of thermal cycling by preventing cracking of the deposit during deposition. Running at lower temperatures permits the use of simplified collector plate structures which remove the majority of the product plate heat through radiative heat transfer without costly and complex fluid cooling. This obviates the need for exotic heat removal liquids and heat pipe structures. In the present invention, radiative transfer to a relatively cold heat sink in the form of the tails plates allows adherent alpha phase uranium deposits to be laid upon the tails plates and side shields, with the tails and side plates being fluid-cooled to temperature controlled in the temperature range of 150 °C to 170 °C in one embodiment. The ability to use relatively cool surfaces through all the various start-stop procedures of a real production facility allows the use of radiatively cooled main and prestripper product collector plates. Given the collector plate heat fluxes (heat of condensation of uranium vapor plus the ion current times extraction potential) and a radiative sink maintained at 150° to 170°C, the resultant product plate temperatures are about 450° to 600°C. The latter temperature allows uranium to condense as a solid in the low thermal coefficient of expansion alpha phase as opposed to the higher expansion beta phase. A lower deposition temperature also avoids large thermal changes during cycling between operational and stand-by conditions. These combine to significantly reduce the occurence of product peeling or flaking in actual operation. Note that the deposition of lower thermal coefficient expansion phases of biphasic materials is applicable to materials other than uranium. The practical effect is to permit the use of simplified extractor plate structures, avoiding elaborate fluid cooled assemblies. In accordance with one teaching of the present invention, the surface of the substrate to receive depositions of material from a vapor, and in particular uranium vapor, is preconditioned to improve the adherence of deposited atoms by removing impurity particles at the surface. In a preferred embodiment, the surface impurities are removed by a preheating of the substrate to a selected temperature for a selected time which tends to induce vaporization of the impurity particles. The preheat temperature is typically a small fraction of the substrate operating temperature previously required to produce adherence. According to this invention, preheat temperatures are within the range of 150° 170°C, and the preheat duration can range from 15 minutes to 4 hours. After the preheating step, the tails substrate is maintained at least at this temperature, typically only slightly higher if at all, during the vaporization and isotopically selective photoionization phase of uranium isotope enrichment. The product plates range significantly higher in temperature as described above. Throughout this period the environment of the substrates is maintained in a high vacuum condition, such as at least 10⁻⁴Torr. Prior to installation of the substrates within a uranium enrichment chamber, they may also be sandblasted, washed and dried. The purpose of sandblasting is to provide a contoured rough substrate surface. Because the uranium vapor usually arrives at a surface from one direction, it tends to adhere to the "peaks" and does not reach the "valleys". This creates a network of microscopic caverns or tunnels between the condensate and substrate. When the deposit is introduced into a normal atmosphere, oxygen molecules travel down the caverns, attaching to the uranium atoms which turns the interface into a brittle interface that is easy to break, causing separation of the uranium deposit from the substrate. Thus the sandblasting aids deposit release after deposition. Washing and drying is preferably performed to remove dust caused by the sandblasting process. Chemical dissolution techniques may also be employed BRIEF DESCRIPTION OF THE DRAWING These and other features of the present invention are more fully described below in the solely exemplary detailed description and accompanying drawing of which: Fig. 1 is a temperature level diagram useful in explaining the operation of the present invention; Fig. 2 is a sectional diagram of a typical laser enrichment chamber including the apparatus of the present invention; and Fig. 3 is a flowchart illustrating the series of steps of the method of the present invention. DETAILED DESCRIPTION The present invention contemplates a method and apparatus for vapor deposition and collection at low operating temperatures with simplified collection apparatus. The system is particularly useful in laser isotope separation of uranium, for collecting various isotopically differing deposits of uranium. With reference to Fig. 1, the principle of the present invention is shown with relation to a system for uranium isotope separation. As currently practiced, a melt of uranium metal is vaporized by a high flux electron beam focused onto a line at the surface of the uranium. The uranium vapor thus generated is directed into a region of aligned, thin electrodes which permit the majority of the vapor to pass but collect certain components of the vapor, typically those which have been or are ionized in the process of vaporization or isotopically selective photoionization. In Fig. 1 from an arbitrary time point 10, designated time 0, the melt is energized and its temperature raises according to a curve 12 to a vaporization temperature point in the vicinity of 3,000° C. Simultaneously with the heating of the melt to produce surface vaporization, tails plates used for extracting ions from the vapor increase in temperature according to a curve 14, typically reaching a temperature of 450°C. At some time 16 in the heating process, vapor will begin to generate and condense upon the structure being heated in accordance with curve 14. The final steady state tails plates temperature at which the curve 14 stabilizes, typically approximately 450°C, had been previously determined as the lowest temperature at which adherence to tails plates could be achieved. Tails plates are typically fabricated of tungsten or stainless steel. It has been discovered that operating the tails plates at a relatively low temperature according to a temperature profile represented by curve 17, typically 150° to 170°C, produces adequate deposit adherence, not only to the tails plates, but also to the product plates which can now be limited to operate at a maximum of 600°C by radiative cooling to the tails plate. At these temperatures, the deposition of uranium is at the alpha phase as opposed to the higher temperature beta phase. The latter phase has a higher thermal coefficient of expansion The combination of a lower deposition temperature, which assures a lower temperature difference between stand-by and operating conditions, along with a lower expansion coefficient for the deposit, reduces the cracking and peeling forces that impair adherence. In a preferred embodiment, preconditioning comprises substrate preheating for a predetermined time at a selected temperature, is applied to at least the tails plate during an interval 18 in order to clear the substrate surface of impurities which otherwise would impair the low temperature adherence of the deposition. The surface may be preconditioned in other ways including ion bombardment. The interval 18 typically spans a period of 15 minutes to 4 hours, intervals which have been found satisfactory in operation. In one embodiment, the predetermined temperature during the interval 18 is between 150° to 170°C and is maintained by elevating the temperature of the fluid used to cool the tails plate to 150° to 170°C. Typical methods of temperature control include passing preheated fluids such as a gas (air, helium, nitrogen) or a liquid (saturated steam, oil, mercury or sodium) through passages in the tails plates especially designed for the purpose. During the preheat period 18, the vapor source may be turned on to aid in bringing the substrates to the preheat temperature. The source is maintained below the vaporization temperature during this period of time. At the time point 10, the termination of the preheat interval 18, the vapor source is energized to full vaporization energy with the result that vapor generation commences as its surface temperature increases toward the final steady state temperature level in the region of 3000°C. The temperature of the tails collection structure is controlled so as not to exceed the preheat temperature substantially but also so as not to fall below the preheat temperature of interval 18. The temperature of the tails plate is maintained by a control mechanism acting in a manner known in the art. With a surface temperature of, for example, 180°C assured at the tails plate, an effective radiative sink is available to cool the product plates. The latter may be at a temperature of 450°-600°C. Since large quantities of heat are transferred to the product plates via metal condensation and extractive ion current, an effective cooling mechanism is needed and is provided by the radiative transfer of the subject system. Assuming a 180°C (453°K) tails plate and a 600°C (873°K) product plate surface temperature, the driving radiative heat transfer potential is ( 873 ⁴ - 453 ⁴). It is essential that the condensates remain intact on the tails and product plates during start-up or termination either at the beginning and end of a process cycle or during an unavoidable process upset. With the subject system, the thermal cycling range is limited by the low operating temperatures to prevent disengagement of condensates. The low temperature system provides effective radiative cooling of the product plates. Without effective radiative cooling to the relatively cool tails plate, the extractive substrates must include complicated heat transfer structures basically involving cooling tubes or heat pipes, or they must operate at temperatures where the deposits are molten. Apparatus for practicing the present invention is illustrated in Fig. 2 showing a chamber 20 which is evacuated by a vacuum pump 22 to a very high vacuum condition without oil backflow contamination from the pump. Within the chamber 20, a crucible 24 of uranium 26 is positioned such that an electron beam 28 from a filamentary source 30 can be focused by a magnetic field 32 from coils, not shown, typically surrounding chamber 20. Electron beam 28 is focused to a point 33 on the surface of uranium 26 in a long line running axially in the direction perpendicular to the plane of the drawing thereby forming a melt. Above crucible 24 is typically arrayed a set of prestripper collection plates 34 which are electrically energized by an electronics system 36 to collect preionized particles from the vapor generated from the melt of uranium 26 by electron beam 28. Above electrode plates 34 is an extraction zone 38 containing a set of product plates 40 to which particles ionized with isotopic selectivity are attracted Un-ionized particles continue through extraction zone 38 and are collected on a tails plate 42 as material that is depleted of product material. Other configurations for extraction zone electrodes and preionization extracting electrodes may be utilized, that shown above being a representative scheme as illustrated in the prior art. Lining the interior of the chamber 20 are first and second right and left hand baffles or plates 44 and 46 and tails plate 42, all of which are cooled by cooling fluid carried in ports 50 and temperature is regulated by a system 48. All plates are subjected to the same preconditioning steps thereby providing excellent thermal bonding of the deposit to the appropriate cooled substrate assuring high thermal conduction from the uranium surface for a good radiative cooling. Importantly, these plates provide a cool surface for radiative cooling of electrodes 34 and 40. These electrodes may be further cooled or regulated in temperature by the flow controlled temperature medium, either gaseous or liquid, such as helium, air, nitrogen, saturated steam, oil, mercury or sodium as an auxilliary cooling system of simplified nature. The operation of the present invention is illustrated by the flowchart of Fig. 3. Fig. 3 illustrates an optional initial step 52, typically implemented prior to insertion of electrode plates 34, 40 and 42 into chamber 20. The surfaces of these plates are initially partially freed of contaminants by sandblasting, washing and drying. This pretreatment also aids in the removal of later deposited material in air as well as increasing adherence of the deposited material in vacuum. Subsequent to this pretreatment step, the electrodes are installed within the chamber either through vacuum ports or by bringing chamber 22 back to room pressure. Thereafter, in step 54, the chamber is evacuated to a desired low pressure operating point. In step 56, after evacuation, the preheating sequence is initiated for the interval 18. The tails and side electrodes are driven to the temperature indicated by curve 17 by passing a coolant that is preheated to the proper temperature through passages in the substrate especially designed to accept the coolant to uniformly maintain the substrate at the proper temperature above 150°C, and to maintain the substrates at the appropriate temperature for time interval 18. Typical temperatures and time intervals include, in the case of uranium deposition on stainless steel, a preheat at 170°C for from 5 minutes to 4 hours to produce an adherent deposition. A preheat at 150°C for 15 minutes will produce adequate adherence. Lower temperatures were found not to be effective. Subsequent to preheating step 54, uranium separation continues for an interval represented by step 58 during which isotopically separated vapor deposits on plates 34, 40, and 42 in accordance with the principles of laser enrichment. During this step, the temperature represented by curve 14 for plates 34 and 40 is maintained principally by radiative cooling to plates 42,44, and 46. After a desired accumulation of deposition is achieved, the plates may be removed through vacuum interlocks or by repressurizing chamber 20. The removed plates are exposed to an oxidizing environment in a step 60 which results in their becoming brittle, especially in channels formed by shadows in the depositions so that separation of the deposit in subsequent step 62, including chemical acceleration of the separation, can occur more rapidly. The above description is of a preferred embodiment for practicing the concept of the present invention as expressed in the following claims.

Claims: A method for vapor deposition in a system including a source of material (26) to be vaporized, a first deposition structure (40), and a second deposition structure (42), comprising the steps of: aligning the first deposition structure (40) with the source of material (26) to be vaporized; disposing the second deposition structure (42) adjacent the first deposition structure (40) in such manner that the first deposition structure (40) is intermediate the second deposition structure (42) and the source of material (26) to be vaporized; vaporizing the material (26) to form a vapor; and depositing components of the vapor on the first deposition structure (40) and the second deposition structure (42); characterised by the steps of: maintaining the second deposition structure (42) at a temperature no lower than the lower temperature limit at which components of the vapor will adhere to the second deposition structure (42); and cooling the first deposition structure (40) by radiative heat transfer from the first deposition structure (40) to the second deposition structure 42. The method of claim 1 characterised in that the material (26) to be vaporized is a multiphasic material. The method of claim 1 characterised in that the material to be vaporized (26) is uranium. The method of claim 3 further characterised in that: the temperature of the second deposition structure (42) is maintained no lower than the lower temperature limit for the alpha phase deposition of uranium, the lower temperature limit having a range of about 150°C to about 180°C; and the first deposition structure (40) is cooled to a temperature of about 450°C to about 600°C by radiative heat transfer to the second deposition structure (42). The method of claim 1 further characterised by the step of: preconditioning the first deposition structure (40) and the second deposition structure (42) prior to vaporizing the material (26) to promote adherence of components of the vapor deposited on the first deposition structure (40) and the second deposition structure (42). The method of claim 5 characterised in that the preconditioning step comprises: heating at least the second deposition structure (42) in a vacuum for a predetermined preheat interval at a selected temperature to remove surface impurities therefrom to promote adherence of components of the vapor deposited on the second deposition structure (42). The method of claim 6 characterised in that the selected temperature at which the second deposition structure (42) is heated is no lower than the lower temperature limit at which components of the vapor will adhere to the second deposition structure (42). The method of claim 5 characterised in that the preconditioning step comprises: roughening surfaces of the first deposition structure (40) and the second deposition structure (42) to provide product plates (40) and tails plates (42) having rough, irregular surfaces to promote adherence of components of the vapor deposited on the first deposition structure (40) and the second deposition structure (42). The method of claim 8 characterised in that the surfaces of the first deposition structure (40) and the second deposition structure (42) may be roughened by sandblasting, ion bombardment, grinding, or chemical etching. The method of claim 8 characterised in that components of the vapor are deposited on the rough, irregular surfaces of the first deposition structure (40) and the second deposition structure (42) in a manner to create microscopic voids between the rough, irregular surfaces of the first deposition structure (40) and the second deposition structure (42) and the components deposited thereon. The method of claim 10 further characterised by the step of: exposing the components of the vapor deposited on the rough, irregular surfaces of the first deposition structure (40) and the second deposition structure (42) to an oxidizing environment to promote separation of the deposited components from the first deposition structure (40) and the second deposition structure (42). An apparatus for vapor deposition, comprising: vapor source means (24) for vaporizing a material (26) by heating a surface of the material (26) to form a vapor; a first deposition structure (40) aligned to receive depositions of vapor provided by the vapor source means (24); a second deposition structure (42) disposed adjacent the first deposition structure (40) to receive depositions of vapor provided by the vapor source means (24), the first deposition structure (40) being intermediate the second deposition structure (42) and the vapor source means (24); characterised in that: a cooling means (50) is disposed in operative combination with the second deposition structure (42) for controlling the temperature of the second deposition structure (42); the cooling means (50) is operated to maintain the second deposition structure (42) at an operating temperature no lower than the lower temperature limit at which components of the vapor adhere to the second deposition structure (42); and the operating temperature of the second deposition structure (42) causes radiative heat transfer from the first deposition structure (40) to the second deposition structure (42) to cool the first deposition structure (40). The apparatus of claim 12 characterized in that; the first deposition structure (40) and the second deposition structure (42) have rough, irregular surfaces that promote adherence of components of the vapor deposited on the first deposition structure (40) and the second deposition structure (42). The apparatus of claim 12 characterized in that: at least the second deposition structure (42) is preconditioned to remove surface impurities to promote adherence of components of the vapor deposited on the second deposition structure (42).

Inventors: ARENDT, PAUL N., BAYNE, MICHAEL A., FINCH, LESTER M.
Applicants: JERSEY NUCLEAR-AVCO ISOTOPES, INC.
Application year: 1984
Main classification: B01D59
IPC codes: B01D7, C23C14, B01D59