阅读说明：本技术 一种恒温法合成无色金刚石的工艺 (Process for synthesizing colorless diamond by constant temperature method ) 是由 刘乾坤 胡来运 易良成 赵鹏 邢志华 申幸卫 于 2021-10-20 设计创作，主要内容包括：本发明属于超硬材料制造工艺技术领域,具体涉及一种恒温法合成无色金刚石的工艺。本发明针对现有技术中温差法合成金刚石存在工艺条件不易实现,产品产量低的问题,通过对合成原料进行调整,并将粉体除氮剂直接均匀混入石墨触媒粉中,并均匀混入种晶压制成芯柱,制备得到无色金刚石,以使合成出来的无色金刚石质量稳定,产出量高。本发明为了弥补合成腔体径向温度场不均衡的问题,通过增加复合加热结构来改善温度场温差过大的问题,最终,本发明所采用的无色金刚石在生长过程中能够保证双面受热,温差减小,腔体内适宜生长区域更大。(The invention belongs to the technical field of superhard material manufacturing processes, and particularly relates to a process for synthesizing colorless diamond by a constant temperature method. Aiming at the problems of difficult realization of process conditions and low product yield in the prior art of synthesizing diamond by a differential temperature method, the invention prepares the colorless diamond by adjusting the synthesis raw materials, directly and uniformly mixing the powder denitrifier into the graphite catalyst powder, uniformly mixing the denitrifier into the seed crystal and pressing the seed crystal into the core column, so that the synthesized colorless diamond has stable quality and high yield. In order to make up for the problem of unbalanced radial temperature field of the synthetic cavity, the problem of overlarge temperature difference of the temperature field is solved by adding the composite heating structure, and finally, the colorless diamond adopted by the invention can ensure that two sides are heated in the growth process, the temperature difference is reduced, and the suitable growth area in the cavity is larger.)

1. A process for synthesizing colorless diamond by a constant temperature method is characterized by comprising the following steps:

(1) metal powder premixing: taking iron powder and nickel powder as catalyst powder, taking titanium powder and aluminum powder as nitrogen removal agents, premixing for 3-5h to obtain metal premixed powder, wherein the metal premixed powder comprises the following components in percentage by mass: 39-40% of nickel powder, 2.7-2.9% of titanium powder, 1.1-1.3% of aluminum powder and the balance of iron powder;

(2) pressing a synthesized core column: carrying out secondary mixing on graphite powder, diamond seed crystals and the metal premixed powder in the step (1) for 5-7h, uniformly mixing to obtain a secondary mixed material, pressing the secondary mixed material, and respectively obtaining a core column and a hollow core column with a cavity; wherein the graphite powder accounts for 66 to 68 percent of the mass sum of the metal premixed powder and the graphite powder, and the metal premixed powder accounts for 32 to 34 percent of the mass sum of the metal premixed powder and the graphite powder;

(3) and (3) carrying out vacuum reduction treatment on the synthesized core column: carrying out vacuum reduction treatment on the hollow core column and the core column obtained in the step (2), wherein the treatment temperature is 950-1050 ℃, the treatment time is 18-24h, and embedding the core column into a cavity in the middle of the hollow core column for assembly after treatment to obtain a synthetic assembly block;

(4) synthesizing diamond at high temperature and high pressure: heating the synthetic assembly block obtained in the step (3) at the temperature of 180-220 ℃ for 15-18h, and then synthesizing diamond at high temperature and high pressure by adopting a secondary pressurization mode; after the synthesis is finished, uniform pressure relief is carried out according to the pressure relief rate of less than or equal to 10MPa/min, and the colorless diamond can be grown.

2. The process according to claim 1, wherein the particle size of the iron powder and the nickel powder in step (1) is 15 to 18 μm, and the particle size of the titanium powder and the aluminum powder is 23 to 26 μm.

3. The process of claim 1, wherein the graphite powder of step (2) has a purity of 99.9% and the diamond seed has a grain size of 70-90 mesh.

4. The process of claim 1, wherein in the step (3), after the vacuum reduction treatment and before the assembly of the hollow core column and the core column obtained in the step (2) are respectively carried out, the hollow core column and the core column are respectively coated, and the coating treatment comprises the following specific steps:

a. the core column is coated with an insulating material, and the hollow core column is coated with the insulating material outside and in the cavity of the hollow core column;

b. respectively coating heating materials outside the insulating material of the core column and the insulating material of the hollow core column, and embedding the core column into a cavity in the middle of the hollow core column for assembly to obtain a composite assembly block;

c. and coating a heat-insulating material outside the obtained synthetic assembly block, and then coating, sealing and transmitting pressure material outside the synthetic assembly block to finish coating.

5. The process as claimed in claim 4, wherein the insulating material is magnesium oxide lining material, the heating material is heating carbon tube, the heat insulating material is dolomite heat insulating outer lining, and the sealing pressure-transmitting material is pyrophyllite.

6. The process according to claim 1, wherein the specific high-temperature high-pressure diamond synthesis process conditions in the step (4) are as follows: firstly pressurizing to 1.9-2.1GPa, raising the temperature to 370-380 ℃, then releasing the pressure to 0.3-0.4GPa, keeping the temperature at 370-380 ℃ for 500-520s, then raising the pressure to 5.5-5.7GPa, raising the temperature to 1250-1280 ℃, and maintaining the pressure for 150000-300000 seconds to synthesize the diamond.

7. Colorless diamond prepared by the process of any one of claims 1 to 6.

Technical Field

The invention belongs to the technical field of superhard material manufacturing processes, and particularly relates to a process for synthesizing colorless diamond by a constant temperature method.

Background

The synthetic colorless diamond has a strong market demand due to its excellent quality and suitable price. With the increase of the demand of colorless diamonds, the industry competition is becoming more and more intense, and the colorless diamonds synthesized by the temperature difference method commonly used in the industry can not meet the market demand due to the problems of poor stability, low output and the like.

The synthetic process of the artificial diamond by the constant temperature method (high temperature and high pressure method) is mainly used for synthesizing the industrial diamond, the process technology is relatively mature, and the high output of the artificial diamond can be realized by matching with a domestic cubic press, as shown in figure 1, the synthetic process comprises A, B, C, D and other power-variable stages, the power is continuously increased in the initial A-B stage, and then the initial A-B stage enters a B-C stage with constant temperature and constant power, spontaneous nucleation can be inevitably caused by the influence of the process conditions in the stage, and the granularity of the industrial diamond is dispersed and is thinner and is not easy to grow; because the industrial diamond also contains a large amount of monomer impurity nitrogen atoms, the industrial diamond synthesized by a constant temperature method is easy to present yellow, and the color can not be effectively removed, most of the obtained products can only be used for industrial abrasive material grinding tool materials, and the application of the industrial abrasive material grinding tool materials is limited.

Thus, the production of colorless synthetic diamonds has become a direction of research and development for products in the industry. The mainstream production method of colorless diamond at home and abroad adopts a temperature difference method. When the diamond is synthesized by the temperature difference method, the graphite raw material is used as a carbon source and is positioned at a high temperature end, the diamond seed crystal is positioned at a low temperature end, the metal catalyst used as a solvent is positioned between the carbon source and the diamond seed crystal, the graphite raw material is used as a low-pressure stable phase in the process, the diamond seed crystal is used as a high-pressure stable phase, the solubility of the carbon source at the high temperature end is higher than that of the diamond seed crystal at the low temperature end due to the temperature difference, namely, the solubility difference is generated between the graphite raw material and the diamond seed crystal, the solubility difference becomes a driving force for the diffusion of the carbon source (graphite raw material) from the high temperature end to the low temperature end, simultaneously, the temperature and the pressure between the graphite raw material and the diamond seed crystal gradually tend to be the same in size under the participation of the catalyst, then the carbon source is gradually separated out at the diamond seed crystal to obtain diamond crystals, then the diamond crystals gradually grow up, and finally the product is obtained, and the method, the graphite raw material is converted into diamond, and the temperature and pressure conditions for converting the graphite raw material into the diamond are greatly reduced because the conversion is carried out in the presence of a catalyst.

However, the main problems of the synthesis of the colorless diamond by the temperature difference method at present are that the temperature high-quality growth region of the colorless diamond synthesized by the temperature difference method is narrow (only 30 ℃), the influence of structure and process fluctuation is serious, and the difficulty in accurately controlling the temperature region in the synthesis process is large, so that the problems of insufficient growth stability, low single yield, unstable chromaticity and the like of the colorless diamond synthesized by the temperature difference method are caused. In addition, as the synthesis cavity in the technological process of synthesizing the colorless diamond by the temperature difference method is gradually enlarged, the radial temperature difference of the synthesis cavity is gradually increased, and the temperature difference in the radial direction of the synthesis cavity also affects the growth quality of the colorless diamond (for example, causes the problems of deflection of the growth direction and the like). Although the synthetic colorless diamond can also adopt a chemical vapor deposition method (CVD method), the synthetic process of the method is complex and is not easy to popularize in industrial production.

Based on the current situation, the invention aims to develop the process for synthesizing the colorless diamond by the constant temperature method, the conditions of synthesis temperature and pressure are stable, the product yield is high, the process is convenient to operate and easy to apply industrially, and the problems that the process conditions of the constant temperature method are difficult to realize and the product yield is low in the prior art are solved.

Disclosure of Invention

The invention aims to provide a process for synthesizing colorless diamond by a constant temperature method, which is characterized in that the process method comprises the steps of adjusting synthetic raw materials, directly and uniformly mixing a powder denitrifier into graphite catalyst powder, uniformly mixing the denitrifier into seed crystals, pressing the seed crystals into a core column, and preparing the colorless diamond, so that the synthesized colorless diamond has stable quality and high yield.

In order to achieve the technical purpose, the invention adopts the following technical scheme:

a process for synthesizing colorless diamond by a constant temperature method comprises the following steps:

(1) metal powder premixing: taking iron powder and nickel powder as catalyst powder, taking titanium powder and aluminum powder as nitrogen removal agents, premixing for 3-5h to obtain metal premixed powder, wherein the metal premixed powder comprises the following components in percentage by mass: 39-40% of nickel powder, 2.7-2.9% of titanium powder, 1.1-1.3% of aluminum powder and the balance of iron powder;

(2) pressing a synthesized core column: carrying out secondary mixing on graphite powder, diamond seed crystals and the metal premixed powder in the step (1) for 5-7h, uniformly mixing to obtain a secondary mixed material, pressing the secondary mixed material, and respectively obtaining a core column and a hollow core column with a cavity; wherein the graphite powder accounts for 66 to 68 percent of the mass sum of the metal premixed powder and the graphite powder, and the metal premixed powder accounts for 32 to 34 percent of the mass sum of the metal premixed powder and the graphite powder;

(3) and (3) carrying out vacuum reduction treatment on the synthesized core column: carrying out vacuum reduction treatment on the hollow core column and the core column obtained in the step (2), wherein the treatment temperature is 950-1050 ℃, the treatment time is 18-24h, and embedding the core column into a cavity in the middle of the hollow core column for assembly after treatment to obtain a synthetic assembly block;

(4) synthesizing diamond at high temperature and high pressure: placing the synthetic assembly block obtained in the step (3) into a high-temperature oven at 180-220 ℃ for baking for 15-18h for preheating, and placing the preheated synthetic assembly block on a cubic press for high-temperature high-pressure synthesis of diamond by adopting a secondary pressurization mode; after the synthesis is finished, uniform pressure relief is carried out according to the pressure relief rate of less than or equal to 10MPa/min, and the colorless diamond can be grown.

Preferably, the particle size of the iron powder and the nickel powder in the step (1) is 15-18 μm, and the particle size of the titanium powder and the aluminum powder is 23-26 μm.

Preferably, the purity of the graphite powder in the step (2) is 99.9%, the grain size of the diamond seed crystals is 70-90 meshes, and the density range of the diamond seed crystals is 1-6 grains/cm for carrying out thin film year.

Preferably, in the step (3), after the vacuum reduction treatment and before the assembly of the hollow core column and the core column obtained in the step (2) are respectively performed, the hollow core column and the core column are respectively subjected to a coating treatment, and the coating treatment specifically comprises the following steps:

a. coating a magnesium oxide lining material (namely an insulating material) outside the core column, and coating the magnesium oxide lining material (namely the insulating material) outside the hollow core column and in the cavity of the hollow core column;

b. respectively coating heating carbon tubes (namely heating materials) outside the magnesium oxide lining material of the core column and the magnesium oxide lining material of the hollow core column, namely forming a composite heating structure by the insulating materials and the heating materials outside the core column and the hollow core column, embedding the core column into a cavity in the middle of the hollow core column for assembly, and obtaining a composite assembly block with the composite heating structure;

c. the obtained synthetic assembly block is coated with a dolomite thermal insulation outer lining (namely thermal insulation material), the shaft ends of the synthetic assembly block are respectively provided with a carbon sheet (used for conducting heat when the synthetic assembly block is heated), the synthetic assembly block is coated with pyrophyllite (namely sealing pressure transmission material), and the carbon sheet is provided with a conductive ring, so that the coating is completed.

Preferably, the specific process conditions for synthesizing diamond at high temperature and high pressure in step (4) are as follows: firstly pressurizing to 1.9-2.1GPa, raising the temperature to 370-380 ℃, then releasing the pressure to 0.9-1.1GPa, keeping the temperature at 370-380 ℃ for 500-128s, then raising the pressure to 5.5-5.7GPa, raising the temperature to 1250-1280 ℃, maintaining the pressure for 150000-300000 seconds (realizing the growth of the diamond seed crystal by adopting a pressure-maintaining growth mode), and synthesizing the diamond.

According to the method, the colorless diamond is prepared by catalyst powder, graphite powder and diamond seed crystal in a specific ratio at high temperature and high pressure, the synthesized diamond is in a complete octahedral crystal form, the particle size (1.2-2.0) mm of the synthesized colorless diamond is obtained by detecting according to the national standard GB/T16554 plus 2003, the color grade can reach H color, and the single synthesis yield can reach (33-40) ct.

Compared with the prior art, the invention has the beneficial effects that:

1. the method changes the technological mode of removing nitrogen after synthesizing the colorless diamond by the traditional temperature difference method, adopts the mode of uniformly mixing the powdery nitrogen removing agent into the synthetic raw materials to realize the arrangement of the sheet metal nitrogen removing agent layer between the carbon source and the diamond seed crystal, and removes nitrogen in the process of carbon atom infiltration. The powder of the denitrifier has larger specific surface area than the flake powder, and the denitrifier can be uniformly mixed into the synthetic material, so that the denitrifier can better realize the denitrogenation effect in the synthetic process, thereby solving the problems of small growth interval, harsh growth conditions, lower yield and the like in the process of synthesizing the colorless diamond by adopting a temperature difference method in the prior art.

2. According to the diamond growth V-shaped graph (fig. 2), it can be known that the nucleation region of diamond is related to the pressure increase path in the diamond nucleation stage, and if the pressure and temperature are high, under the condition of keeping other parameters unchanged, the equivalent point C moves to the left, causing the diamond to deviate to the region II rich crystal region, resulting in increased diamond nucleation. Therefore, the invention adopts the processes of secondary pressure rise, primary pressure relief and primary intermediate stop temperature (370 degrees), firstly, the pressure is increased by 2GPa, then the pressure is relieved to 1GPa, and the period (pre-synthesis period) after the pressure rise and the pressure reduction adopts the mode of no continuous intermediate stop pressure and low intermediate stop temperature. The invention reasonably avoids the spontaneous nucleation area in the stop stage of industrial diamond synthesis by adjusting the technological parameters of colorless diamond synthesis, and directly enters the synthesis growth area after the transition to the pressure maintaining stage. The secondary pressure boosting and pressure relieving process has the advantages that the early-stage temperature is low, partial phase change of the pyrophyllite due to high temperature is avoided, and the sealing performance of the sealing edge of the pyrophyllite is improved.

3. Because the industrial diamond is synthesized by adopting a powder constant-temperature catalyst method, the phenomena of continuous crystals, polycrystal, fine growth granularity and the like are easily caused by fine granularity of the seed crystal or the mother powder in the mixing and synthesizing processes. According to the invention, the diamond with coarse granularity (granularity of 70-90 meshes) is used as the seed crystal, so that the deposition area of the diamond in the initial growth stage is increased, and the initial growth quality of the colorless diamond by a constant temperature method can be effectively improved. By adjusting the number of the seed crystals, the aim of controlling the production granularity and quality can be better achieved.

4. The existing colorless diamond production mainly adopts a temperature difference method and adopts an indirect heating mode to carry out heating synthesis. The radial temperature difference of the synthesis cavity is caused by the conduction of the heat of the heating body from outside to inside, and the radial temperature difference of the synthesis cavity is gradually increased along with the expansion of the cavity, so that the problem that the diamond crystal is difficult to uniformly grow is caused. In order to make up for the problem of unbalanced radial temperature field of the synthetic cavity, the problem of overlarge temperature difference of the temperature field is solved by adding the composite heating structure, and finally, the colorless diamond adopted by the invention can ensure that two sides are heated in the growth process, the temperature difference is reduced, and the suitable growth area in the cavity is larger.

Drawings

FIG. 1 is a schematic view of process conditions of a constant temperature method synthetic diamond process in the prior art;

FIG. 2 is a V-shaped curve diagram of diamond growth in a constant temperature method artificial diamond synthesis process in the prior art;

fig. 3 is a schematic structural diagram of the case where the hollow core column and the core column are separately subjected to the coating treatment in step (3) in example 1;

fig. 4 is a schematic cross-sectional structure diagram of the hollow core column and the core column after being subjected to cladding treatment and assembled in step (3) in example 1;

fig. 5 is a photograph of the colorless diamond obtained in example 1.

Detailed Description

In order to make the objects, technical solutions and effects of the present invention clearer and clearer, the present invention is described in further detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The graphite powder used in the specific implementation mode of the invention is pure graphite powder with the model number of G5 and the granularity of 30-35 meshes, and the diamond seed crystal adopts ZND2280 grade 70-90 meshes complete octahedral diamond produced by Zhongnan diamond limited company as the seed crystal.

Example 1

A process for synthesizing colorless diamond by a constant temperature method specifically comprises the following steps:

(1) metal powder premixing: taking iron powder and nickel powder with the granularity of 15-18 mu m as catalyst powder, taking titanium powder and aluminum powder with the granularity of 23-26 mu m as nitrogen removal agents, premixing for 3.5 hours to obtain metal premixed powder, wherein the metal premixed powder comprises the following components in percentage by mass: 39% of nickel powder, 2.7% of titanium powder, 1.1% of aluminum powder and the balance of iron powder;

(2) pressing a synthesized core column: carrying out secondary mixing on high-purity graphite powder with the purity of 99.9%, diamond seed crystals with the grain size of 70-90 meshes and the metal premixed powder obtained in the step (1), mixing for 5 hours, uniformly mixing to obtain a secondary mixed material, pressing the secondary mixed material, and respectively obtaining a core column and a hollow core column with a cavity; wherein the graphite powder accounts for 66 percent of the mass sum of the metal premixed powder and the graphite powder, and the metal premixed powder accounts for 34 percent of the mass sum of the metal premixed powder and the graphite powder; wherein the diamond seed density range is 4 grains/cm for thin and long; the specific diamond seed density range is determined by the growth grain size of the diamond and the size of the distributed growth space;

(3) and (3) carrying out vacuum reduction treatment on the synthesized core column: respectively carrying out vacuum reduction treatment on the hollow core column and the core column obtained in the step (2), wherein the treatment temperature is 950 ℃, the treatment time is 18h, and respectively carrying out coating treatment on the hollow core column and the core column after treatment, wherein the coating treatment specifically comprises the following steps as shown in fig. 3 and fig. 4:

a. coating magnesium oxide lining materials (namely, insulating materials in figure 3) outside the hollow core column and coating the magnesium oxide lining materials (namely, insulating materials in figure 3) outside the hollow core column and in the cavity of the hollow core column;

b. respectively coating heating carbon tubes (namely heating materials in figure 3) outside the magnesium oxide lining material of the core column and the magnesium oxide lining material of the hollow core column, namely forming a composite heating structure by the insulating materials and the heating materials outside the core column and the hollow core column, embedding the core column into a cavity in the middle of the hollow core column for assembly, and obtaining a composite assembly block with the composite heating structure (as shown in figure 4);

c. coating a dolomite thermal insulation outer lining (namely thermal insulation material in figure 3) outside the obtained synthetic assembly block, respectively installing carbon sheets at the shaft ends of the synthetic assembly block, coating the synthetic assembly block with a sealed pressure transmission pyrophyllite (namely sealed pressure transmission material in figure 3), and installing a conductive ring on the carbon sheets to finish coating;

embedding the coated core column into the cavity of the coated hollow core column for assembly to obtain a synthetic assembly block;

(4) synthesizing diamond at high temperature and high pressure: putting the synthetic assembly block obtained in the step (3) into a high-temperature oven at 180 ℃ for baking for 15h for preheating, and putting the preheated synthetic assembly block on a cubic press for high-temperature high-pressure synthesis of diamond by adopting a secondary pressurization mode; after the synthesis is finished, carrying out uniform pressure relief according to the pressure relief rate of less than or equal to 10MPa/min, and growing the colorless diamond;

the specific process conditions for synthesizing the diamond at high temperature and high pressure in the step (4) are as follows: the diamond was synthesized by first pressurizing to 1.9GPa, raising the temperature to 370 ℃, then depressurizing to 0.9GPa, while maintaining the temperature at 370 ℃ for 500s, then raising the pressure to 5.5 GPa, while raising the temperature to 1250 ℃, and maintaining the pressure for 184000 seconds (synthesis time).

As shown in FIG. 5, which is a photograph of the colorless diamond obtained by the process of example 1, the synthesized diamond is a complete octahedral crystal form, and the detection is performed by using the national standard "diamond classification GB/T16554-2003" to obtain the colorless diamond with the granularity of (1.2-1.6) mm, the interior of the colorless diamond is clean, the color grade can reach H color, and the single synthesis yield can reach (35-40) ct.

Example 2

A process for synthesizing colorless diamond by a constant temperature method specifically comprises the following steps:

(1) metal powder premixing: taking iron powder and nickel powder with the granularity of 15-18 mu m as catalyst powder, taking titanium powder and aluminum powder with the granularity of 23-26 mu m as nitrogen removal agents, premixing for 5 hours to obtain metal premixed powder, wherein the metal premixed powder comprises the following components in percentage by mass: 40% of nickel powder, 2.9% of titanium powder, 1.3% of aluminum powder and the balance of iron powder;

(2) pressing a synthesized core column: carrying out secondary mixing on high-purity graphite powder with the purity of 99.9%, diamond seed crystals with the grain size of 70-90 meshes and the metal premixed powder obtained in the step (1), mixing for 7 hours, uniformly mixing to obtain a secondary mixed material, pressing the secondary mixed material, and respectively obtaining a core column and a hollow core column with a cavity; wherein the graphite powder accounts for 68 percent of the mass sum of the metal premixed powder and the graphite powder, and the metal premixed powder accounts for 32 percent of the mass sum of the metal premixed powder and the graphite powder; wherein the diamond seed density range is 2 grains/cm for thin and long;

(3) and (3) carrying out vacuum reduction treatment on the synthesized core column: respectively carrying out vacuum reduction treatment on the hollow core column and the core column obtained in the step (2), wherein the treatment temperature is 1050 ℃, the treatment time is 24 hours, and respectively carrying out coating treatment on the hollow core column and the core column after treatment, wherein the specific steps of the coating treatment are the same as those of the embodiment 1;

embedding the coated core column into the cavity of the coated hollow core column for assembly to obtain a synthetic assembly block;

(4) synthesizing diamond at high temperature and high pressure: putting the synthetic assembly block obtained in the step (3) into a high-temperature oven at 220 ℃ for baking for 18h for preheating, and putting the preheated synthetic assembly block on a cubic press for high-temperature high-pressure synthesis of diamond by adopting a secondary pressurization mode; after the synthesis is finished, carrying out uniform pressure relief according to the pressure relief rate of less than or equal to 10MPa/min, and growing the colorless diamond;

the specific process conditions for synthesizing the diamond at high temperature and high pressure in the step (4) are as follows: firstly pressurizing to 2.1GPa, raising the temperature to 380 ℃, then releasing the pressure to 1.1GPa, keeping the temperature at 380 ℃ for 520s, then raising the pressure to 5.7GPa, raising the temperature to 1280 ℃, and maintaining the pressure for 261000 s (synthesis time) to synthesize the diamond.

The synthesized diamond is a complete octahedral crystal form, partial impurities (metal, graphite and bubbles are detected as components) are contained in the diamond, the particle size (1.6-2.0) mm of the synthesized colorless diamond is obtained by adopting the national standard 'diamond grading GB/T16554-one 2003', the color grade can reach H color, and the single synthesis yield can reach (33-38) ct.

While specific embodiments of the present invention have been described above, it should be understood that the present invention is not limited to the specific embodiments described above. Various changes or modifications may be made by those skilled in the art within the scope of the claims without departing from the spirit of the invention.