阅读说明：本技术 高导热高强度散热基板及其制备方法 (High-heat-conductivity high-strength heat dissipation substrate and preparation method thereof ) 是由 李康 雷君 王彬彬 黄红卫 于 2020-12-10 设计创作，主要内容包括：本发明提供了一种高导热高强度散热基板,它为PCBN复合片,主要由CBN微粉和包覆在所述CBN微粉表面的AlN膜组成的AlN包覆的CBN粉体通过高温高压烧结而成。本发明还提供一种上述高导热高强度散热基板的制备方法。上述高导热高强度散热基板中,由于在所述AlN包覆的CBN粉体中,所述AlN膜均匀包覆在所述CBN微粉表面,同时AlN与CBN之间结合紧密,所述高温高压合成PCBN后AlN与CBN之间的界面热阻小,使得所述PCBN复合片的热导率达到300 W/(m·k)以上,且弯曲强度达到600 MPa以上,从而使得所述PCBN复合片成为一种高导热高强度散热基板的材料。(The invention provides a high-heat-conduction high-strength heat dissipation substrate which is a PCBN composite sheet and is mainly formed by sintering AlN-coated CBN powder consisting of CBN micro powder and an AlN film coated on the surface of the CBN micro powder at high temperature and high pressure. The invention also provides a preparation method of the high-heat-conductivity high-strength heat dissipation substrate. In the high-heat-conductivity high-strength heat dissipation substrate, the AlN film is uniformly coated on the surface of the CBN micro powder in the AlN-coated CBN powder, meanwhile, AlN is tightly bonded with CBN, and the interface thermal resistance between AlN and CBN is small after PCBN is synthesized at high temperature and high pressure, so that the heat conductivity of the PCBN composite sheet reaches more than 300W/(m.k), and the bending strength reaches more than 600 MPa, and the PCBN composite sheet becomes a material of the high-heat-conductivity high-strength heat dissipation substrate.)

1. The high-heat-conduction high-strength heat dissipation substrate is characterized by being a PCBN composite sheet, and mainly formed by sintering AlN-coated CBN powder consisting of CBN micro powder and an AlN film coated on the surface of the CBN micro powder at high temperature and high pressure.

2. The substrate according to claim 1, wherein the CBN fine powder has a particle size of 35 μm or less, and the AlN film has a thickness of 0.8 to 3 μm.

3. The high-thermal-conductivity high-strength heat dissipation substrate as claimed in claim 2, wherein the CBN fine powder comprises 15-35 μm CBN fine powder and 1-6 μm CBN fine powder in a mass ratio of 4-1: 1.

4. A method for preparing a high thermal conductivity and high strength heat dissipation substrate as claimed in any one of claims 1 to 3, comprising the steps of:

preparing AlN-coated CBN powder, and forming an AlN film on the surface of the CBN micro powder to prepare AlN-coated CBN powder;

preparing a heat dissipation substrate, sintering the AlN-coated CBN powder at high temperature and high pressure to form a PCBN composite sheet, and treating the PCBN composite sheet to obtain the high-heat-conduction high-strength heat dissipation substrate.

5. The method according to claim 4, wherein the step of preparing the AlN-coated CBN powder comprises:

plating an Al film on the surface of the CBN micro powder by adopting a vapor deposition method to form CBN powder coated by the Al film;

and nitriding the CBN powder coated by the Al film to enable the Al film to generate a nitriding reaction to form the AlN film so as to obtain the CBN powder coated by the AlN.

6. The method for preparing the high-thermal-conductivity high-strength heat dissipation substrate as claimed in claim 5, wherein the step of plating the Al film comprises plating the Al film on the surface of the CBN micro powder by a vacuum evaporation method or a sputtering evaporation method, and the thickness of the Al film is 0.6-2.4 μm.

7. The method for preparing the high-thermal-conductivity high-strength heat dissipation substrate as claimed in claim 6, wherein the step of plating the Al film comprises the step of uniformly plating the Al film on the surface of the CBN micro powder by using a magnetron sputtering method by using a metal Al plate as a target material to prepare the CBN powder coated by the Al film, wherein the sputtering power is 3-7 kW, and the sputtering time is 40-100 min.

8. The method for preparing a heat dissipating substrate with high thermal conductivity and high strength as claimed in any one of claims 5 to 7, wherein the step of nitriding comprises: and introducing nitrogen in a vacuum atmosphere, heating the CBN powder coated with the Al film to 600-700 ℃, and directly nitriding the Al film to form the AlN film to obtain the CBN powder coated with the AlN.

9. The method as claimed in claim 8, wherein the step of performing the nitridation process comprises: and introducing nitrogen at the rate of 0.4-0.7L/min in a vacuum atmosphere, heating the CBN powder coated with the Al film to the temperature of 600-700 ℃ at the rate of less than or equal to 5 ℃/min, preserving the heat for 2-5 h, and directly nitriding the Al film to form the AlN film to obtain the AlN-coated CBN powder.

10. The method for preparing a high thermal conductivity and high strength heat dissipating substrate according to claim 9, wherein the step of preparing the heat dissipating substrate comprises: putting the AlN-coated CBN powder into a cubic press, and sintering at 5-9 GPa and 1300-1800 ℃ for 25-40 min to synthesize the PCBN composite sheet; and carrying out post-processing treatment on the PCBN composite sheet to prepare the high-heat-conductivity high-strength heat dissipation substrate with the preset specification.

Technical Field

The invention relates to the technical field of heat dissipation materials, in particular to a high-heat-conductivity high-strength heat dissipation substrate and a preparation method thereof.

Background

With the rapid development of the fields of high-speed rail, aerospace, war industry and the like in China, the demand for high-power electronic devices is increasing in the future, and in order to be suitable for more complex and harsh application conditions, the high-power electronic devices develop towards high temperature, high frequency, modularization and systematization, so that more serious challenges are provided for heat dissipation substrate materials. The commonly used heat dissipation substrate includes AlN substrate and Al₂O₃Substrate and Si₃N₄A substrate, etc. The AlN substrate has high thermal conductivity, but has poor mechanical property, cannot bear multiple thermal cycles and has short service life. Al (Al)₂O₃Substrate and Si₃N₄Although the substrate has excellent mechanical properties, the thermal conductivity is low, and the requirement of a high-power device cannot be met. The heat conductivity and mechanical properties of the existing substrate materials in the market cannot meet the requirements of high-power electronic devices at the same time, and how to make the heat conductivity of the heat dissipation substrate more than 150W/(m.k) and the bending strength more than 500MPa is important in the research field of the heat dissipation substrate.

Cubic Boron Nitride (CBN) has better mechanical property and thermal conductivity, AlN has excellent thermal conductivity, and the composite material of CBN and AlN theoretically has excellent thermal conductivity and mechanical property, thus being an ideal material of a heat dissipation substrate. Therefore, the invention of China invention patent application CN108516836A, filed by Dong xialin on 3/27/2018, discloses a preparation method of an aluminum nitride ceramic substrate, which comprises the steps of dispersing boron nitride powder and a modifier in a first solvent to obtain a boron nitride mixed solution, pouring the mixed solution into a sand mill, carrying out high-speed treatment, drying, crushing and grinding to obtain modified boron nitride; uniformly mixing aluminum nitride ultrafine powder, modified boron nitride, a sintering aid, an adhesive and a second solvent through high polymer mixing equipment to obtain sintering slurry; defoaming the sintering slurry by a vacuum defoaming machine, and then passing through a casting machine to obtain a casting belt with a certain thickness; and punching, laminating, removing glue and sintering the tape at 1000-1500 ℃ for 2-6 h to obtain the compact aluminum nitride ceramic substrate. The patent application of the invention utilizes the characteristics that the thermal conductivity of the cubic boron nitride reaches 600W/mK, and aluminum nitride can promote the transformation of hexagonal boron nitride to the cubic boron nitride when the temperature of the aluminum nitride exceeds 900 ℃, and sinters slurry at the temperature of more than 900 ℃ with the help of sintering aids to obtain a uniform and compact high-thermal-conductivity cubic boron nitride-containing aluminum nitride substrate, and the thermal conductivity can reach 220W/(mK), but the bending strength of the aluminum nitride substrate is not described.

Therefore, the bending strength of the high thermal conductivity aluminum nitride ceramics with the thermal conductivity of more than or equal to 240W/m.k is rarely reported in the prior art, and the high thermal conductivity high strength aluminum nitride substrate with the thermal conductivity of more than or equal to 240W/m.k and the bending strength of more than 500MPa is hardly reported in the prior art.

Disclosure of Invention

Accordingly, the present invention is directed to a high thermal conductivity and high strength heat dissipation substrate and a method for manufacturing the same, which overcome the above-mentioned problems.

Therefore, the invention provides a high-heat-conduction high-strength heat-dissipation substrate, which is a PCBN (polycrystalline cubic boron nitride) composite sheet, and is formed by sintering AlN-coated CBN powder mainly composed of CBN micro powder and an AlN film coated on the surface of the CBN micro powder at high temperature and high pressure.

Preferably, the high-thermal-conductivity and high-strength heat dissipation substrate is a square or rectangle with a side length of 20-40 mm, and the thickness of the heat dissipation substrate is 0.64-2.0 mm.

Based on the above, the granularity of the CBN micro powder is less than or equal to 35 microns, and the thickness of the AlN film is 0.8-3 microns. Preferably, the AlN film has a thickness of 0.8 to 2.6 μm.

Based on the above, the CBN micro powder comprises CBN micro powder with the particle size of 15-35 mu m and CBN micro powder with the particle size of 1-6 mu m in a mass ratio of 4-1: 1.

The invention also provides a preparation method of the high-heat-conductivity high-strength radiating substrate, which comprises the following steps:

preparing AlN-coated CBN powder, and forming an AlN film on the surface of the CBN micro powder to prepare AlN-coated CBN powder;

preparing a heat dissipation substrate, sintering the AlN-coated CBN powder at high temperature and high pressure to form a PCBN composite sheet, and treating the PCBN composite sheet to obtain the high-heat-conduction high-strength heat dissipation substrate.

Based on the above, the step of preparing AlN-coated CBN powder includes:

plating an Al film on the surface of the CBN micro powder by adopting a vapor deposition method to form CBN powder coated by the Al film;

and nitriding the CBN powder coated by the Al film to enable the Al film to generate a nitriding reaction to form the AlN film so as to obtain the CBN powder coated by the AlN.

Based on the above, the step of plating the Al film comprises plating the Al film on the surface of the CBN micro powder by a vacuum evaporation method or a sputtering evaporation method, wherein the thickness of the Al film is 0.6-2.4 μm. Preferably, the thickness of the Al film is 0.6-2.0 μm.

Based on the above, the step of plating the Al film comprises the step of uniformly plating the Al film on the surface of the CBN micro powder by using a metal Al plate as a target material and adopting a magnetron sputtering method to prepare the CBN powder coated by the Al film, wherein the sputtering power is 3-7 kW, and the sputtering time is 40-100 min.

In view of the above, the step of nitriding includes: and introducing nitrogen in a vacuum atmosphere, heating the CBN powder coated with the Al film to 600-700 ℃, and directly nitriding the Al film to form the AlN film to obtain the CBN powder coated with the AlN.

In view of the above, the step of nitriding includes: and introducing nitrogen at the rate of 0.4-0.7L/min in a vacuum atmosphere, heating the CBN powder coated with the Al film to the temperature of 600-700 ℃ at the rate of less than or equal to 5 ℃/min, preserving the heat for 2-5 h, directly nitriding the Al film to form the AlN film, wherein the thickness of the AlN film is 0.8-3 mu m, and thus obtaining the AlN-coated CBN powder. Preferably, the heating reaction temperature in this step is 600 ℃ to 670 ℃.

In the step of nitriding treatment, the flow rate of nitrogen is limited to be 0.4-0.7L/min, mainly because the nitrogen is a reaction gas for nitriding the Al film and the flow rate of the nitrogen is too low, the Al powder cannot be completely nitrided; the nitrogen flow is too high, and the powder can be blown away by the airflow. In the step, the heating rate is less than 5 ℃/min mainly because the nitriding temperature of the Al film is between 600 ℃ and 700 ℃, the surface generates a hard AlN film due to the excessively fast temperature rise in the reaction temperature range, and the inner layer Al film can not completely react.

Based on the above, the step of preparing the heat dissipation substrate includes: putting the AlN-coated CBN powder into a cubic press, and sintering at 5-9 GPa and 1300-1800 ℃ for 25-40 min to synthesize the PCBN composite sheet; and carrying out post-processing treatment on the PCBN composite sheet to prepare the high-heat-conductivity high-strength heat dissipation substrate with the preset specification.

The high-heat-conductivity high-strength heat dissipation substrate is mainly a PCBN (polycrystalline cubic boron nitride) composite sheet formed by sintering AlN-coated CBN powder at high temperature and high pressure, wherein in the AlN-coated CBN powder, an AlN film is uniformly coated on the surface of the CBN micro powder, AlN is tightly combined with CBN, and after PCBN is synthesized at high temperature and high pressure, the interface thermal resistance between AlN and CBN is small, so that the heat conductivity of the PCBN composite sheet reaches more than 300W/(m.k), and the bending strength reaches more than 600 MPa, and the PCBN composite sheet becomes a material of the high-heat-conductivity high-strength heat dissipation substrate.

In addition, the preparation method of the high-heat-conductivity high-strength radiating substrate provided by the invention is simple and easy to operate, and is easy for industrial production. Further, when uniformly mixed CBN micro powder with different particle sizes is used as a raw material, in the process of synthesizing PCBN at high temperature and high pressure, the small-particle-size particles fill the pores of the large-particle-size particles, and the densification degree of the PCBN composite sheet is improved, so that the thermal conductivity and the bending strength of the PCBN composite sheet are improved, and further the thermal conductivity and the bending strength of the high-thermal-conductivity high-strength heat dissipation substrate using the PCBN composite sheet as the raw material are improved.

Drawings

Fig. 1 is a photograph of a high thermal conductivity and high strength heat dissipation substrate provided in embodiment 1 of the present invention.

Fig. 2 is an XRD spectrum of the high thermal conductivity and high strength heat dissipation substrate provided in example 1 of the present invention.

Detailed Description

The technical solution of the present invention is further described in detail by the following embodiments.

Example 1

Referring to fig. 1, the present embodiment provides a high thermal conductivity and high strength heat dissipation substrate, which is a square with a side length of 30 mm and a thickness of 0.8-1 mm. The preparation method of the high-heat-conductivity high-strength heat dissipation substrate comprises the following three steps.

(1) Preparation of AlN-coated CBN powder

The method specifically comprises the following steps:

selecting CBN micro powder with the granularity of 20-30 mu m for the Al-plated film, putting the CBN micro powder into a storage bin of a magnetron sputtering film plating machine, taking a high-purity metal Al plate as a target material, vacuumizing and introducing argon, starting mechanical vibration, starting a magnetron sputtering power supply, wherein the sputtering power is 4 kW, the sputtering time is 60 min, and uniformly plating an Al film with the thickness of 0.6 mu m on the surface of the CBN micro powder to obtain CBN powder with the surface uniformly coated by the Al film;

and (2) performing nitriding treatment, namely putting the CBN powder uniformly coated with the Al film into an alumina crucible, placing the alumina crucible into a tubular furnace, vacuumizing, filling high-purity nitrogen at the nitrogen flow rate of 0.4L/min, heating to 650 ℃ at the nitrogen flow rate of 5 ℃/min, and preserving heat for 3 hours to ensure that the Al film on the surface is fully nitrided into an AlN film with the thickness of 0.8 mu m, so as to prepare the AlN-coated CBN powder.

(2) Composite PCBN sheet

And (3) dry-pressing and molding the AlN-coated CBN powder, putting the assembled CBN powder into pyrophyllite, and synthesizing PCBN by using a cubic press at 5 GPa and 1350 ℃ for 25 min under pressure maintaining to obtain a PCBN composite sheet blank.

(3) And (3) cutting, grinding and polishing the blank PCBN composite sheet to obtain the high-heat-conductivity and high-strength heat dissipation substrate with the required size, as shown in figure 1.

And (4) detecting the PCBN blank by adopting a diffractometer, wherein the detection result is shown in figure 2. As can be seen from fig. 2: the Al film is completely nitrided, and the PCBN compact blank phase includes a CBN phase and an AlN phase, so that the high thermal conductivity and high strength heat dissipation substrate provided in this embodiment is formed by sintering AlN-coated CBN powder at high temperature and high pressure, where the AlN-coated CBN powder includes CBN fine powder and an AlN film coated on the surface of the CBN fine powder.

Example 2

The present embodiment provides a high thermal conductivity and high strength heat dissipation substrate, which is rectangular with a length of 25 mm and a width of 30 mm, and the thickness of the heat dissipation substrate is 0.9-1.4 mm. The structure of the heat dissipation substrate provided in this embodiment is substantially the same as that of the heat dissipation substrate provided in embodiment 1, and the heat dissipation substrate is formed by sintering AlN-coated CBN powder at high temperature and high pressure, and the main difference is that the CBN fine powder therein has different particle sizes.

The embodiment also provides a preparation method of the high-thermal-conductivity high-strength heat dissipation substrate, which includes the steps of:

(1) preparation of AlN-coated CBN powder

Al film plating this step is essentially the same as the corresponding step in example 1, with the main difference that: in the embodiment, CBN micro powder with the particle size of 25 mu m is used as a raw material, the magnetron sputtering power is 6 kw, and the thickness of an Al film is 0.9 mu m;

nitridation process this step is substantially the same as the corresponding step in example 1, with the main differences being: in the embodiment, the nitrogen flow rate is 0.6L/min, the heating temperature is 660 ℃, and the AlN film thickness is 1.2 mu m;

(2) composite PCBN sheet

This step is essentially the same as the corresponding step in example 1, with the main differences: the applied pressure in the cubic apparatus press of the embodiment is 6 GPa, and the sintering temperature is 1500 ℃;

(3) the procedure for the preparation of the finished product was substantially the same as the corresponding procedure in example 1.

Example 3

The embodiment provides a high-thermal-conductivity and high-strength heat dissipation substrate, wherein the heat dissipation substrate is a square with a side length of 30 mm, and the thickness of the heat dissipation substrate is 0.8-1.2 mm. The structure of the heat dissipation substrate provided in this embodiment is substantially the same as that of the heat dissipation substrate provided in embodiment 1, and the heat dissipation substrate is formed by sintering AlN-coated CBN powder at high temperature and high pressure, and the main difference is that the CBN fine powder therein has different particle sizes.

The embodiment also provides a preparation method of the high-thermal-conductivity high-strength heat dissipation substrate, which includes the steps of:

(1) preparation of AlN-coated CBN powder

Al film plating this step is essentially the same as the corresponding step in example 1, with the main difference that: in the embodiment, CBN micro powder with the particle sizes of 25 mu m and 5 mu m, which is uniformly mixed according to the mass ratio of 2:1, is used as a raw material, the magnetron sputtering time is 80 min, and the thickness of an Al film is 0.8 mu m;

nitridation process this step is substantially the same as the corresponding step in example 1, with the main differences being: the heating temperature in this example was 670 ℃ and the AlN film thickness was 1 μm;

(2) composite PCBN sheet

This step is essentially the same as the corresponding step in example 1, with the main differences: the applied pressure in the cubic apparatus press of the embodiment is 5.5 GPa, and the sintering temperature is 1600 ℃;

(3) the procedure for the preparation of the finished product was substantially the same as the corresponding procedure in example 1.

Example 4

The present embodiment provides a high thermal conductivity and high strength heat dissipation substrate, which is rectangular with a length of 40 mm and a width of 30 mm, and the thickness of the heat dissipation substrate is 0.64-0.9 mm. The structure of the heat dissipation substrate provided in this embodiment is substantially the same as that of the heat dissipation substrate provided in embodiment 1, and the heat dissipation substrate is formed by sintering AlN-coated CBN powder at high temperature and high pressure, and the main difference is that the CBN fine powder therein has different particle sizes.

The embodiment also provides a preparation method of the high-thermal-conductivity high-strength heat dissipation substrate, which includes the steps of:

(1) preparation of AlN-coated CBN powder

Al film plating this step is essentially the same as the corresponding step in example 1, with the main difference that: in the embodiment, CBN micro powder with the particle size of 20 mu m and 2 mu m which is uniformly mixed according to the mass ratio of 3:1 is used as a raw material, the magnetron sputtering power is 7 kw, the sputtering time is 80 min, and the thickness of an Al film is 1.5 mu m;

nitridation process this step is substantially the same as the corresponding step in example 1, with the main differences being: in the embodiment, the nitrogen flow rate is 0.7L/min, the heating temperature is 670 ℃, the heat preservation time is 4h, and the AlN film thickness is 1.9 mu m;

(2) composite PCBN sheet

This step is essentially the same as the corresponding step in example 1, with the main differences: the applied pressure in the cubic apparatus press of the embodiment is 8 GPa, the sintering temperature is 1750 ℃ and the pressure maintaining time is 30 min;

(3) the procedure for the preparation of the finished product was substantially the same as the corresponding procedure in example 1.

Example 5

The present embodiment provides a high thermal conductivity and high strength heat dissipation substrate, which is rectangular with a length of 25 mm and a width of 30 mm, and the thickness of the heat dissipation substrate is 1.5-1.8 mm. The structure of the heat dissipation substrate provided in this embodiment is substantially the same as that of the heat dissipation substrate provided in embodiment 1, and the heat dissipation substrate is formed by sintering AlN-coated CBN powder at high temperature and high pressure, and the main difference is that the CBN fine powder therein has different particle sizes.

The embodiment also provides a preparation method of the high-thermal-conductivity high-strength heat dissipation substrate, which includes the steps of:

(1) preparation of AlN-coated CBN powder

Al film plating this step is essentially the same as the corresponding step in example 1, with the main difference that: in the embodiment, CBN micro powder with the particle sizes of 30 microns and 3 microns, which is uniformly mixed according to the mass ratio of 4:1, is used as a raw material, the magnetron sputtering power is 7 kw, the sputtering time is 100 min, and the thickness of an Al film is 1.6 microns;

nitridation process this step is substantially the same as the corresponding step in example 1, with the main differences being: in the embodiment, the nitrogen flow rate is 0.7L/min, the heating temperature is 680 ℃, the heat preservation time is 4 hours, and the thickness of the obtained AlN film is 2 mu m;

(2) composite PCBN sheet

This step is essentially the same as the corresponding step in example 1, with the main differences: the applied pressure in the cubic apparatus press of the embodiment is 9 GPa, the sintering temperature is 1800 ℃ and the pressure maintaining time is 40 min;

(3) the procedure for the preparation of the finished product was substantially the same as the corresponding procedure in example 1.

Example 6

The embodiment provides a high-thermal-conductivity and high-strength heat dissipation substrate, which is a square with a side length of 40 mm, and the thickness of the heat dissipation substrate is 1.6-2 mm. The structure of the heat dissipation substrate provided in this embodiment is substantially the same as that of the heat dissipation substrate provided in embodiment 1, and the heat dissipation substrate is formed by sintering AlN-coated CBN powder at high temperature and high pressure, and the main difference is that the CBN fine powder therein has different particle sizes.

The embodiment also provides a preparation method of the high-thermal-conductivity high-strength heat dissipation substrate, which includes the steps of:

(1) preparation of AlN-coated CBN powder

Al film plating this step is essentially the same as the corresponding step in example 1, with the main difference that: in the embodiment, CBN micro powder with the particle sizes of 35 mu m and 6 mu m, which is uniformly mixed according to the mass ratio of 1:1, is used as a raw material, the magnetron sputtering power is 7 kw, the sputtering time is 100 min, and the thickness of an Al film is 1.9 mu m;

nitridation process this step is substantially the same as the corresponding step in example 1, with the main differences being: in the embodiment, the nitrogen flow rate is 0.7L/min, the heating temperature is 700 ℃, the heat preservation time is 5 hours, and the thickness of the obtained AlN film is 2.6 mu m;

(2) composite PCBN sheet

This step is essentially the same as the corresponding step in example 1, with the main differences: the applied pressure in the cubic apparatus press of the embodiment is 9 GPa, the sintering temperature is 1800 ℃ and the pressure maintaining time is 40 min;

(3) the procedure for the preparation of the finished product was substantially the same as the corresponding procedure in example 1.

Performance testing

The heat dissipation substrates provided in examples 1 to 6 were used as objects, and the thermal conductivity and the bending strength were measured, and the results are shown in table 1. Wherein, the heat conductivity detection is carried out by adopting a laser flash method and utilizing a German Chinesis-resistant LFA 447 instrument according to the reference standard GB/T22588-; the bending strength test method comprises the following steps: and (3) detecting the bending strength by adopting a three-point bending resistance method and using a universal testing machine according to the standard GB/T6569-2006.

Table 1 table of the results of testing the performance of the heat sink in each example

It should be noted later that: the above examples are only intended to illustrate the technical solution of the present invention and not to limit it; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art will understand that: modifications to the specific embodiments of the invention or equivalent substitutions for parts of the technical features may be made; without departing from the spirit of the present invention, it is intended to cover all aspects of the invention as defined by the appended claims.