Abstract:
Crystals having a narrowband transmission window in the UV range and methods for producing such crystals are disclosed. The method comprises the steps of preparing a saturated nutrient solution of a nickel compound and a dopant salt; and incubating the nutrient solution under conditions suitable for crystal growth. The nickel compound is nickel silicon fluoride, nickel fluoroborate, or potassium nickel sulfate. The dopant salt is a salt of cobalt, calcium, barium, strontium, lead, copper, germanium, praseodymium, neodymium, zinc, lithium, potassium, sodium, rubidium, or cesium. The doped nickel compounds crystals have a narrow transmission window in the UV range and can be used as filters for optical sensors in applications such as the passive missile approach warning systems.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to ultraviolet (UV) crystal filters for optical sensors, and more particularly to crystals having high transmission, narrowband windows in the UV range. 
       BACKGROUND OF THE INVENTION 
       [0002]    There are a variety of devices which use ultraviolet (UV) light filters that allow selected wavelengths of light to pass through. For example, such filters are used in passive missile approach warning systems (PMAWS) which locate and track sources of ultra-violet energy, enabling the system to distinguish the plume of an incoming missile from other UV sources that pose no threat. The efficiency of the missile approach warning system depends on the efficiency, stability and quality of the UV filters. 
         [0003]    All UV sensors have finite sensitivity to visible radiations. It is very important for a UV sensor to discriminate against the visible radiation so as to maximize UV sensitivity while minimizing false signals caused by visible light sources. Therefore, the UV filters should have high transmittance in the UV spectral region and have strong absorption at longer wavelengths. Moreover, the filters should have high thermal stability because the UV sensors may be used in environments with high temperatures, such as aircrafts parked in tropical and desert areas. 
         [0004]    It is known that certain transition metal ions, such as Ni 2+  and Co 2+ , absorb visible radiations and transit in certain UV range. These metals have been used in UV filters such as Corning 9863 glass which is a UV transmitting glass doped with Ni 2+  and Co 2+ . The doped glass provide effective blocking of visible radiations. However, there is a significant absorption in 250-300 nm wavelength region that sacrifices in-band transmittance and reduces the sensitivity of the detector. There still exists a need for UV filter materials with filter transmittance in the wavelength range of interests and higher temperature stability. 
       SUMMARY OF THE INVENTION 
       [0005]    One aspect of the present invention relates to a method for producing a crystal with a transmission window in the UV range. The method comprises the steps of (1) preparing a first saturated nutrient solution of a nickel compound and a first dopant salt; and 
         [0006]    (2) incubating the first nutrient solution under conditions suitable for crystal growth, wherein the nickel compound is selected from the group consisting of nickel silicon fluoride, nickel fluoroborate, and potassium nickel sulfate, and wherein said dopant salt is selected from the group consisting of salts of cobalt, calcium, barium, strontium, lead, copper, germanium, praseodymium, neodymium, zinc, lithium, potassium, sodium, rubidium, and cesium. 
         [0007]    In one embodiment, the nickel compound is nickel fluoroborate, the dopant salt is cobalt fluoroborate, and the crystal has a formula of Ni x Co (1-x) (BF 4 ) 2 .6H 2 O, where 0&lt;x&lt;1. 
         [0008]    In another embodiment, the nickel compound is potassium nickel sulfate, the dopant salt is potassium cobalt sulfate, and the crystal has a formula of K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O, where 0&lt;x&lt;1. 
         [0009]    In another embodiment, the method further comprises the steps of (3) preparing a saturated second nutrient solution of a doped nickel compound obtained from step (2) and a second dopant salt; and (4) incubating the second nutrient solution under conditions suitable for crystal growth, wherein said second dopant is different from said first dopant. 
         [0010]    In one embodiment, the doped nickel compound obtained from step (2) is one of Ni x Co (1-x) SiF 6 .6H 2 O and K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O, where 0&lt;x&lt;1, and wherein said second dopant salt is one of PbCO 3  and CaCO 3 . 
         [0011]    Another aspect of the present invention relates to crystals produced by the method of the present invention and UV filters fabricated from the crystals. 
     
     
       DETAILED DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is a schematic showing a method for producing single-doped, nickel compound crystal filters suitable for narrowband UV sensors. 
           [0013]      FIG. 2  is a schematic showing a method for producing double-doped, nickel compound crystal filters suitable for narrowband UV sensors. 
           [0014]      FIG. 3  is a picture of recrystallized NiSiF 6 .6H 2 O crystals. 
           [0015]      FIG. 4  is a picture of cobalt doped NiSiF 6 .6H 2 O (Ni x Co (1-x) SiF 6 .6H 2 O) crystals. 
           [0016]      FIG. 5  is a picture of recrystallized K 2 Ni(SO 4 ) 2 .6H 2 O crystals. 
           [0017]      FIG. 6  is a picture of cobalt doped K 2 Ni(SO 4 ) 2 .6H 2 O (K 2 Ni x Co (1-x)) (SO 4 ) 2 .6H 2 O) crystals. 
           [0018]      FIG. 7  is a picture of recrystallized Ni(BF 4 ) 2 .6H 2 O crystals. 
           [0019]      FIG. 8  is a picture of cobalt doped Ni(BF 4 ) 2 .6H 2 O (Ni x Co (1-x)) (BF 4 ) 2 .6H 2 O) crystals. 
           [0020]      FIG. 9  is a picture of a disc filter fabricated from Ni x Co (1-x) SiF 6 .6H 2 O crystals. 
           [0021]      FIGS. 10A and 10B  are absorption curves showing spectral characteristics of pure nickel fluorosilicate ( FIG. 10A ) and nickel/cobalt fluorosilicate ( FIG. 10B ). 
           [0022]      FIGS. 11A-11F  are absorption curves showing spectral characteristics of double- and triple-doped nickel fluorosilicate.  FIG. 11A : nickel/cobalt fluorosilicate doped with low concentration Pb 2+ .  FIG. 11B : nickel/cobalt fluorosilicate doped with low concentration Ca 2+ .  FIG. 11C : nickel/cobalt fluorosilicate doped with high concentration Pb 2+ .  FIG. 11D : nickel/cobalt fluorosilicate doped with equal concentrations of Pb 2+ and Ca 2+ .  FIG. 11E : nickel/cobalt fluorosilicate doped with Pb 2+  and Ca 2+  at low Ca 2+  ratio.  FIG. 11F : Potassium nickel sulfate doped with Pb 2+  and Ca 2+ at high Ca 2+  ratio. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The present invention provides narrowband crystals useful for UV sensors and filters. The crystals are nickel fluorosilicate (NiSiF 6 .6H 2 O), nickel fluoroborate (Ni(BF 4 ) 2 .6H 2 O) or potassium nickel sulfate (K 2 Ni(SO 4 ) 2 .6H 2 O) crystals (collectively “the nickel compounds” doped with one, two, or more dopant ions. 
         [0024]      FIG. 1  shows a block diagram of a method  100  for producing a narrow band UV filter using nickel compound crystals doped with a dopant ion (i.e., single-doped nickel compound crystals. The method  100  includes the steps of preparing ( 110 ) a saturated nutrient solution of a nickel compound and a dopant salt; growing ( 120 ) doped crystals from the nutrient solution; and fabricating ( 130 ) narrow band UV filter using the doped crystals. 
         [0025]    The nickel compound is one of nickel fluorosilicate (NiSiF 6 .6H 2 O), nickel fluoroborate (Ni(BF 4 ) 2 .6H 2 O) and potassium nickel sulfate (K 2 Ni(SO 4 ) 2 .6H 2 O), all of which are commercially available. In one embodiment, commercially available NiSiF 6 .6H 2 O, Ni(BF 4 ) 2 .6H 2 O, or K 2 Ni(SO 4 ) 2 .6H 2 O is further purified by re-crystallization before step  110 . 
         [0026]    The dopant salt is preferably a salt that matches the nickel compound, e.g., a fluorosilicate salt for NiSiF 6 .6H 2 O, a fluoroborate salt for Ni(BF 4 ) 2 .6H 2 O, and a potassium sulfate salt for K 2 Ni(SO 4 ) 2 .6H 2 O. Examples of the dopant ions include, but are not limited to, Co ++ , Ca ++ , Ba ++ , Sr ++ , Pb ++ , Cu ++ , Ce +3 , Pr +3  , Nd +3 , Zn ++ , Li + , K + , Na + , Rb + , and Cs + . The ratio between the nickel compound and the dopant salt is determined based on the desired absorption characteristics of the doped crystals grown out of the solution. 
         [0027]    The nutrient solution is prepared at an elevated temperature, preferably in the range of 35° C. to 45° C., and then cooled at a controlled cooling rate. A seed crystal is added to initiate the crystallization process. Crystals are harvested when they reach desired sizes. In one embodiment, the cooling rate is 0.1° C.-5° C./100 hour. In another embodiment, an acid is added to the nutrient solution to keep the pH of the solution in the range of 1-3. The quality of the crystals is controlled by the temperature, the cooling rate, the size of the bath containing the nutrient solution, the quality of seed, and the purity of the starting materials. 
         [0028]    In step  120 , grown crystals of doped nickel fluorosilicate (NiSiF 6 .6H 2 O), doped nickel fluoroborate (Ni(BF 4 ) 2 .6H 2 O) or doped potassium nickel sulfate (K 2 Ni(SO 4 ) 2 .6H 2 O) are fabricated into filters using conventional methods. Typically, the crystals are cut into desired sizes, mounted on a support, and shaped into filters of desired shapes. The filters are polished using non-aqueous lubricants such as Linde powder and ethylene glycol. In one embodiment, the narrowband UV filters produced by the method  100  have a transmission window between 200 nm and 300 nm. The transmission window may be further modified by a second dopant as described below. 
         [0029]      FIG. 2  shows a method  200  for producing a narrow band UV filter using nickel fluorosilicate, nickel fluoroborate or potassium nickel sulfate crystals doped with two metal ions (i.e., double-doped nickel compound crystals). The method  200  comprises the steps of producing ( 210 ) single-doped nickel compound crystals with fluorosilicate, nickel fluoroborate or potassium nickel sulfate crystals and a first dopant salt by a first solution growth procedure, producing ( 220 ) double-doped nickel compound crystals with the single-doped nickel compound crystals and a second dopant salt by a second solution growth procedure, and fabricating ( 230 ) narrowband UV filter using double-doped crystals obtained from step  220 . One skilled in the art would understand that additional solution growth steps may be added to the method  200  to produce nickel fluorosilicate, nickel fluoroborate or potassium nickel sulfate crystals doped with more than two dopant ions. 
         [0030]    The single-doped nickel fluorosilicate, nickel fluoroborate or potassium nickel sulfate crystals in step  210  is produced using procedures similar to that described in Method  100 . Examples of the first dopant ion include, but are not limited to, Co ++ , Ca ++ , Ba ++ , Sr ++ , Pb ++ , Cu ++ , Ce +3 , Pr +3 , Nd +3 , Zn ++ , Li + , K + , Na + , Rb + , and Cs + . 
         [0031]    The second solution growth procedure is carried out under conditions similar to that of the first solution growth procedure. Briefly, a saturated solution of single-doped nickel compounds (product of step  210 , i.e., nickel fluorosilicate, nickel fluoroborate or potassium nickel sulfate crystals doped with a first dopant) is mixed with a saturated solution of the second dopant (the doping solution) at an elevated temperature (e.g., 35° C. to 45° C.) to form a crystallization mixture. A small pre-grown seed crystal was added to the crystallization mixture for the nucleating. The temperature of the crystallization mixture was then lowered gradually (e.g., at a rate of 0.1° C.-5° C./100 hour) to allow crystallization of double-doped nickel compounds. Examples of the dopant metal ions include, but are not limited to, Ca 2+ , Ba 2+ , Sr 2+ , Pb 2+ , Cu 2+ , Ce 3+ , Pr 3+ , Nd 3+ , Zn 2+ , Li + , K + , Na + , Rb + , and Cs + . The ions can be provided in the form of a salt, such as a carbonate salt, sulfate salt, nitrate salt, chloride salt, chlorate salt, or phosphoric salt. The transmission spectra of the crystallization mixture is determined. The amount of the doping solution in the crystallization mixture can be adjusted until a desired transmission spectra is achieved. 
         [0032]    Typically, the amount of the doping solution is in the range of 0.1-5% (v/v), more preferably in the range of 0.5-3% (v/v) of the saturated solution of the single-doped nickel compounds. As used hereinafter, a “low concentration” of the second dopant generally refers to an amount of doping solution in the range of 0-3% (v/v), and a “high concentration” of the second dopant generally refers to an amount of doping solution in the range of 3-5% (v/v). 
         [0033]    The doping solution may be a saturated solution of two or more dopants. The total amount of dopants and the ratio among the different dopants may be adjusted to achieve the desired transmission spectra. 
         [0034]    In one embodiment, a saturated solution of Ni x Co (1-x) SiF 6 .6H 2 O or K 2 Ni x Co (1-x)(SO   4 ) 2 .6H 2 O is prepared and mixed with a doping solution of PbCO 3 , CaCO 3  or a mixture of PbCO 3  and CaCO 3  to form a crystallization mixture. 
         [0035]    In step  230 , the grown, double-doped nickel compound crystals are fabricated into filters using conventional methods. Similar to step  130  in Method  100 , the crystals are cut into desired sizes, mounted on a support, and shaped into filters of desired shapes. The filters may be polished using non-aqueous lubricants such as Linde powder and ethylene glycol. 
       EXAMPLES 
     Example 1 
     Preparation of Ni x Co (1-x) SiF 6 .6H 2 O, Crystals 
       [0036]    Ni x Co (1-x) SiF 6 .6H 2 O crystals are grown in a saturated solution of NiSiF 6  and CoSiF 6 . The ratio between the NiSiF 6  and CoSiF 6  affects the absorption characteristics of the Ni x Co (1-x) SiF 6 .6H 2 O crystals grown out of the solution. In one embodiment, the NiSiF 6 :CoSiF 6  ratio in the solution is between 2:1 and 6:1, preferably between 3:1 and 5:1, and more preferably between 3:1 and 4:1. 
         [0037]    NiSiF 6  and CoSiF 6  are synthesized by reactions between their corresponding carbonate salts and hydrofluorosilicic acid. The reactions can be given as follows: 
         [0000]      NiCO 3 +H 2 SiF 6 ═NiSiF 6 +H 2 O+CO 2    (1) 
         [0000]      CoCO 3 +H 2 SiF 6 ═CoSiF 6 +H 2 O+CO 2    (2) 
         [0038]    The reaction mixtures are heated to 80° C. to accelerate the reactions. The reactions are preferably carried out in plastic containers because hydrofluorosilicic acid is erosive to glass containers. After their synthesis, NiSiF 6 .6H 2 O and CoSiF 6 .6H 2 O are purified by recrystallizing from water.  FIG. 3  is a picture of recrystallized NiSiF 6 .6H 2 O crystals. 
         [0039]    The crystallization of Ni x Co (1-x) SiF 6 .6H 2 O is carried out under conditions suitable for growing NiSiF 6 .6H 2 O crystals. The conditions are described in detail in the U.S. Pat. No. 5,837,054, which is hereby incorporated by reference. In one embodiment, a saturated NiSiF 6 /CoSiF 6  solution is prepared at an elevated temperature of 35° C. to 45° C., preferably at about 40° C. The temperature of the solution is then lowered gradually (e.g., at a rate of 0.2° C.-5° C./100 hour) to allow the formation of Ni x Co (1-x)SiF   6 .6H 2 O crystals. 
         [0040]    H 2 SiF 6  may be added to the NiSiF 6 /CoSiF 6  solution to keep the pH of the solution in the range of 1-3, preferably at pH 2. The low pH environment improves the quality of crystals by stopping nucleation.  FIG. 4  is a picture of cobalt doped NiSiF 6 .6H 2 O (Ni x Co (1-x) SiF 6 .6H 2 O) crystals. 
       Example 2 
     Preparation of K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O Crystals 
       [0041]    K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O crystals were grown in a saturated solution of K 2 Ni(SO 4 ) 2  and K 2 CO(SO 4 ) 2 . Commercially available K 2 Ni(SO 4 ) 2  and K 2 CO(SO 4 ) 2  were further purified by recrystallization. The recrystallization was carried out in a temperature controlled thermostat from a water based solution. The pH of the water based solution was kept around 2 by adding H 2 SO 4  to the solution. The recrystallization temperature started at 40° C. and was gradually decreased to about 25° C. during crystallization with constant stirring.  FIG. 5  is a picture of recrystallized K 2 Ni(SO 4 ) 2 .6H 2 O crystals. 
         [0042]    The crystallization of K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O was carried out under conditions suitable for growing NiSiF 6 .6H 2 O crystals. The conditions are described in detail in the U.S. Pat. No. 5,837,054, which is hereby incorporated by reference. In one embodiment, a saturated K 2 Ni(SO 4 ) 2 /K 2 Co(SO 4 ) 2  solution was prepared at an elevated temperature of 35° C. to 45° C., preferably at about 40° C. The temperature of the solution is then lowered gradually (e.g., at a rate of 0.2° C.-5° C./100 hour) to allow the formation of K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O crystals. 
         [0043]    H 2 SO 4  may be added to the K 2 Ni(SO 4 ) 2 /K 2 Co(SO 4 ) 2  solution to keep the pH of the solution in the range of 1-3, preferably at pH 2, to improve the quality of crystals by stopping nucleation.  FIG. 6  is a picture of cobalt doped K 2 Ni(SO 4 ) 2 .6H 2 O (K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O) crystals. 
       Example 3 
     Preparation of Ni x Co (1-x) (BF 4 ) 2 .6H 2 O Crystals 
       [0044]    Ni x Co (1-x) (BF 4 ) 2 .6H 2 O crystals were grown in a saturated solution of Ni(BF 4 ) 2  and Co(BF 4 ) 2 . The starting materials, i.e., Ni(BF 4 ) 2  and Co(BF 4 ) 2 , were individually purified by recrystallization. The recrystallization was carried out in a temperature controlled thermostat from a water based solution. The pH of the water based solution was kept around 2 by adding HF to the solution. The recrystallization temperature started at 40° C. and was gradually decreased to about 25° C. during crystallization with constant stirring.  FIG. 7  is a picture of recrystallized Ni(BF 4 ) 2 .6H 2 O crystals. 
         [0045]    The crystallization of Ni x Co (1-x) (BF 4 ) 2 .6H 2 O was carried out under conditions suitable for growing NiSiF 6 .6H 2 O crystals. The conditions are described in detail in the U.S. Pat. No. 5,837,054, which is hereby incorporated by reference. In one embodiment, a saturated K 2 Ni(SO 4 ) 2 /K 2 CO(SO 4 ) 2  solution was prepared at an elevated temperature of 35° C. to 45° C., preferably at about 40° C. A small pre-grown seed crystal was added to the saturated solution for the nucleation. The temperature of the solution was then lowered gradually (e.g., at a rate of 0.2° C.-5° C./100 hour) to allow crystallization. The crystal grew on the seed, to a size which would allow a filter with a diameter of greater than three centimeters to be fabricated.  FIG. 8  is a picture of cobalt doped Ni(BF 4 ) 2 .6H 2 O (Ni x Co (1-x) (BF 4 ) 2 .6H 2 O) crystals. 
       Example 4 
     Fabrication of Filters from Ni x Co (1-x) SiF 6 .6H 2 O Crystals 
       [0046]    Grown crystals of Ni x Co (1-x) SiF 6 .6H 2 O were cut by a string saw into desired sizes. The cylindrical disc filter was fabricated by mounting the crystal on a prefabricated precise circular rod. Crystals were mounded on the rod with wax. The steel rod was then rotated to shape the crystal into desired radius size. Crystal disc was demounted and polished by using a nan-aqueous lubricant, such as Linde powder or ethylene glycol. The doped crystals (Ni x Co (1-x) SiF 6 .6H 2 O) showed superior fabricability (in both cutting and polishing) to that of pure crystals (NiSiF 6 .6H 2 O). A 20 mm diameter and 8 mm thick disc filter fabricated from Ni x Co (1-x) SiF 6 .6H 2 O is shown in  FIG. 9 . 
       Example 5 
     Thermal and Spectroscopic Characterization of Ni x Co (1-x) SiF 6 .6H 2 O Filters 
       [0047]    The short and long term stability of Ni x Co (1-x) SiF 6 .6H 2 O crystals were studied by differential thermal analysis. The crystals were tested at the rate of 5K/minute heating and were stable well above 100° C. The long term stability was tested by placing the crystals in an oven at 95° C. for 60 hours. No decomposition was detected. As shown in  FIGS. 10A and 10B , the spectral transmission of discs prepared from pure nickel NiSiF 6 .6H 2 O ( FIG. 10A ) is quite different from the spectral transmission of discs prepared from Ni x Co (1-x) SiF 6 .6H 2 O ( FIG. 10B ). The doped crystal filter blocks the unwanted transmission in the 400-600 nm and 800-1000 nm ranges, and hence increases the efficiency of the filter. 
       Example 6 
     Preparation of Filters Doped with in Multiple Ions 
       [0048]    Approximately 50 ml of saturated Ni x Co (1-x) SiF 6 .6H 2 O or K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O solution was mixed with 0.5 ml of saturated PbCO 3 , CaCO 3 , or a mixture of PbCO 3 , CaCO 3  solution prepared in HCl. The solutions were prepared at an elevated temperature of 35° C. to 45° C., preferably at about 40° C. A small pre-grown seed crystal was added to the saturated solution for the nucleation. The temperature of the solution was then lowered gradually (e.g., at a rate of 0.2° C.-5° C./100 hour) to allow crystallization. 
       Example 7 
     Thermal and Spectroscopic Characterization of Pb 2+ — and Ca 2+ -Doped Ni x Co (1-x) SiF 6 .6H 2 O and K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O Filters 
       [0049]      FIGS. 11A-11F  show the effect of Pb 2+  and/or Ca 2+  doping on the transmission spectra of Ni x Co (1-x) SiF 6 .6H 2 O and K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O. Compared to the spectral transmission of Ni x Co (1-x) SiF 6 .6H 2 O ( FIG. 10B ), Ni x Co (1-x) SiF 6 .6H 2 O further doped with low concentration of Pb 2+  (0.1-3%, v/v) showed a shift of the transparency window towards the high wave length region ( FIG. 11A ). In addition, the transparency window was significantly narrowed from 250-350 nm to 330-370 nm. Similarly, Ni x Co (1-x) SiF 6 .6H 2 O doped with low concentration of Ca 2+  (0.1-3%, v/v) shows a narrow window of transparency between 250 and 350 nm with diminishing absorbance in 300 nm region ( FIG. 11B ); and Ni x Co (1-x) SiF 6 .6H 2 O doped with high concentration of Ca 2+  (3-5%, v/v) shows a narrow window of transparency between 250 and 320 nm ( FIG. 11C ). The transmission spectra may be further modified by using a combination of ions as the second dopant. For example, Ni x Co (1-x) SiF 6 .6H 2 O doped with equal amounts of Ca 2+  and Pb 2+  shows a window of transparency between 250 and 350 nm ( FIG. 11D ). Ni x Co (1-x) SiF 6 .6H 2 O doped with Ca 2+  and Pb 2+  at a low Ca 2+  ratio (&lt;0.5) shows a narrow window of transparency between 255 and 275 nm, and a large window of transparency at 350 nm and above ( FIG. 11E ). K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O doped with Ca 2+  and Pb 2+  at a high Ca 2+  ratio (&gt;0.5) shows a narrow window of transparency between 260 and 280 nm ( FIG. 11F ). These data clearly demonstrate that the transmission/absorbance spectra of single-doped Ni x Co (1-x) SiF 6 .6H 2 O and K 2 Ni x Co (1-x) (SO 4 ) 2 .6H 2 O can be further tuned to desired ranges by doping with additional ions. 
         [0050]    The foregoing discussion discloses and describes many exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.