阅读说明：本技术 一种热电纤维材料塞贝克值测试装置 (Thermoelectric fiber material Seebeck value testing device ) 是由 阮莉敏 赵艳杰 陈子豪 周贺武 曾玮 王思亮 赵晋陵 梁栋 于 2019-10-22 设计创作，主要内容包括：本发明涉及一种热电纤维材料塞贝克值测试装置,与现有技术相比解决了无法针对热电纤维材料进行Seebeck测试的缺陷。本发明中测试纤维样品的左端安装在左陶瓷片上、右端安装在右陶瓷片上,所述的左陶瓷片为加热陶瓷片,左电压探针台的探针抵在测试纤维样品的左端,右电压探针台的探针抵在测试纤维样品的右端。本发明能够适用于热电纤维材料的塞贝克值测试。(The invention relates to a device for testing a Seebeck value of a thermoelectric fiber material, which overcomes the defect that Seebeck test cannot be performed on the thermoelectric fiber material compared with the prior art. The left end of a test fiber sample is arranged on a left ceramic chip, the right end of the test fiber sample is arranged on a right ceramic chip, the left ceramic chip is a heating ceramic chip, a probe of a left voltage probe station is abutted against the left end of the test fiber sample, and a probe of a right voltage probe station is abutted against the right end of the test fiber sample. The invention can be applied to the Seebeck value test of the thermoelectric fiber material.)

1. The utility model provides a thermoelectric fiber material seebeck value testing arrangement, includes left voltage probe platform (1), right voltage probe platform (2) and is platform (3), its characterized in that:

the left voltage probe station (1) and the right voltage probe station (2) are movably mounted on the presenting table (3) and are in sliding fit with the presenting table (3), a supporting platform (5) is arranged on the presenting table (3), a concave table (6) is arranged in the middle of the supporting platform (5), a sample frame (4) is placed in the concave table (6), the left side of the sample frame (4) is wrapped on the sample frame (4) through a PI film (27) to form a left test table (7), the right side of the sample frame (4) is wrapped on the sample frame (4) through the PI film (27) to form a right test table (8), a cavity (9) is arranged between the left test table (7) and the right test table (8), a left shaft support (10) is arranged on one side of the left test table (7) on the supporting platform (5), a right shaft support (11) is arranged on one side of the right test table (8), a left ceramic wafer auxiliary mounting assembly (12) is mounted on the left shaft support (10) and the left ceramic wafer auxiliary mounting assembly and the left and, the right shaft support (11) is provided with a right ceramic chip auxiliary mounting component (26) which is in running fit with the right shaft support, and the left ceramic chip auxiliary mounting component (12) and the right ceramic chip auxiliary mounting component (26) have the same structure and are in mirror image correspondence based on the cavity (9);

when the ceramic plates are installed, the left ceramic plate (13) is clamped on the left ceramic plate auxiliary installation component (12), the right ceramic plate (14) is clamped on the right ceramic plate auxiliary installation component (26), and the left ceramic plate (13) and the right ceramic plate (14) are respectively placed on the left test bed (7) and the right test bed (8) through the left ceramic plate auxiliary installation component (12) and the right ceramic plate auxiliary installation component (26); the left end of test fiber sample (15) is installed on left ceramic wafer (13), and the right-hand member is installed on right ceramic wafer (14), left ceramic wafer (13) for heating the ceramic wafer, the probe of left voltage probe platform (1) supports at the left end of test fiber sample (15), the probe of right voltage probe platform (2) supports at the right-hand member of test fiber sample (15).

2. The device for testing the seebeck value of the thermoelectric fiber material as claimed in claim 1, wherein: the left ceramic piece (13) is wrapped and fixed on the left test bed (7) through a PI film (27).

3. The device for testing the seebeck value of the thermoelectric fiber material as claimed in claim 1, wherein: the left end of the test fiber sample (15) is bonded on the left ceramic piece (13) through silver adhesive.

4. The device for testing the seebeck value of the thermoelectric fiber material as claimed in claim 1, wherein: the left ceramic chip auxiliary mounting assembly (12) comprises a ceramic chip support (16) and a ceramic chip clamp (17), the ceramic chip clamp (17) is mounted on the side portion of the ceramic chip support (16), and a pressure spring (18) is mounted between the ceramic chip support (16) and the ceramic chip clamp (17).

5. The device for testing the seebeck value of the thermoelectric fiber material as claimed in claim 1, wherein: the probe platform is characterized in that a left guide rail (19) and a right guide rail (20) are arranged on the platform (3), the left voltage probe platform (1) is installed on the left guide rail (19) in a sliding mode, and the right voltage probe platform (2) is installed on the right guide rail (20) in a sliding mode.

6. The device for testing the seebeck value of the thermoelectric fiber material as claimed in claim 1, wherein: still include left temperature sensor (21) and right temperature sensor (22), left temperature sensor (21) glue on left ceramic wafer (13), the left end interval of left temperature sensor (21) and test fiber sample (15) is less than 2mm, right temperature sensor (22) glue on right ceramic wafer (14), the right-hand member interval of right temperature sensor (22) and test fiber sample (15) is less than 2 mm.

7. The device for testing the seebeck value of the thermoelectric fiber material as claimed in claim 1, wherein: and a fiber mounting groove (25) is formed in the left ceramic piece (13).

8. The device for testing the seebeck value of the thermoelectric fiber material as claimed in claim 6, wherein: the temperature-sensing device is characterized by further comprising an MCU (23), a voltage output module (24) is connected to a serial port of the MCU (23), an output end of the voltage output module (24) is connected with a voltage input end of the left ceramic wafer (13), data output ends of the left temperature sensor (21) and the right temperature sensor (22) are respectively connected with a temperature data input end of the MCU (23), and voltage data output ends of the left voltage probe station (1) and the right voltage probe station (2) are respectively connected with a voltage data input end of the MCU (23).

Technical Field

The invention relates to the technical field of thermoelectric material testing, in particular to a thermoelectric fiber material Seebeck value testing device.

Background

In 1821, Thomas Johann Seeback, German scientist, discovered that when two different conductors or semiconductors come into contact, a voltage difference, called Seebeck effect, is formed between the two substances caused by the difference in temperature difference. The thermoelectric material is a functional material capable of converting heat energy and electric energy into each other, and the heat energy in the material is directly converted into the electric energy. The main reason for the Seebeck effect of semiconductors is the result of diffusion of carriers from the hot side to the cold side under the action of the thermal field. For a p-type semiconductor, because a hot-end hole has higher temperature, the hole diffuses from a high-temperature end to a low-temperature end under the action of a thermal field, and when the diffusion effect in the material and the drift effect of an electric field are mutually offset, a stable state is achieved, and at the moment, an electric field is generated in the semiconductor, namely, temperature difference electromotive force; the thermoelectromotive force is the electromotive force caused by the temperature gradient at the cold end and the hot end of the semiconductor. In the case of an n-type semiconductor, the thermoelectromotive force is formed by the accumulation of electrons at the cold end due to the diffusion of electrons from high temperature to low temperature. Obviously, the direction of the thermoelectric electromotive force of the p-type semiconductor is from the low-temperature end to the high-temperature end, the Seebeck coefficient is negative at the moment, and conversely, the direction of the thermoelectric electromotive force of the n-type semiconductor is from the high-temperature end to the low-temperature end, and the Seebeck coefficient is positive at the moment, so that the conductivity type of the semiconductor can be judged by utilizing the direction of the thermoelectric electromotive force. The thermoelectric power of the semiconductor is large, so that the thermoelectric power generator can be manufactured, the heat energy can be converted into the electric energy by utilizing the waste heat of the environment, and the thermoelectric power generator is a novel green and environment-friendly energy technology.

The Seebeck coefficient (Seebeck coefficient), also known as the rate of thermo-electromotive force, is related only to the properties of the material itself and is an inherent physical property of the material. The thermoelectric figure of merit (ZT) of a material determines the thermoelectric conversion efficiency of the material, and generally speaking, the greater the thermoelectric figure of merit of the material, the higher the thermoelectric conversion efficiency, and the formula is that ZT is S²σ T/k, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and k is the thermal conductivity. So to evaluate aThe thermoelectric effect of the material is good or bad, and the S of the material needs to be known. The seebeck coefficient of the material to be measured can be calculated from S ═ Δ V/Δ T, and thus, to obtain the seebeck coefficient of the material, only the temperature difference Δ T and the electromotive force Δ V generated with respect to the temperature difference need to be measured. The sample' S absolute seebeck coefficient S requires the removal of Sref, an additional contribution of wires and thermocouples to the seebeck coefficient, from the obtained relative seebeck coefficient, which is low for metals and negligible Sref if the copper-plated probe is used to measure the seebeck coefficient of the material.

Because thermoelectric materials are in various forms, block materials, fibers and films are common, most of existing Seebeck coefficient measuring instruments are expensive and long in test time consumption, and only the block materials and the films with large volumes are tested, the fiber materials cannot be tested, and the tiny fiber materials are difficult to install on the traditional Seebeck coefficient measuring instrument, so that Seebeck test cannot be carried out on the fiber materials.

Therefore, how to develop a set of special devices for the Seebeck test of the thermoelectric fiber material has become an urgent technical problem to be solved.

Disclosure of Invention

The invention aims to solve the problems that the prior art can not carry out the Seebeck test on the thermoelectric fiber material and the defect that special equipment for the Seebeck coefficient test of the thermoelectric fiber material is lacked, and provides a Seebeck value testing device for the thermoelectric fiber material to solve the problems.

In order to achieve the purpose, the technical scheme of the invention is as follows:

a thermoelectric fiber material Seebeck value testing device comprises a left voltage probe station, a right voltage probe station and a display station,

the left voltage probe station and the right voltage probe station are movably arranged on the presenting table and are in sliding fit with the presenting table, a supporting platform is arranged on the presenting table, a concave table is arranged in the middle of the supporting platform, a sample frame is placed in the concave table, the left side of the sample frame is coated on the sample frame through a PI film to form a left test table, the right side of the sample frame is coated on the sample frame through the PI film to form a right test table, a cavity is arranged between the left test table and the right test table, a left shaft support is arranged on one side of the left test table on the supporting platform, a right shaft support is arranged on one side of the right test bed, a left ceramic chip auxiliary mounting assembly is mounted on the left shaft support and forms a rotating fit with the left ceramic chip auxiliary mounting assembly, a right ceramic chip auxiliary mounting assembly is mounted on the right shaft support and forms a rotating fit with the right ceramic chip auxiliary mounting assembly, and the left ceramic chip auxiliary mounting assembly and the right ceramic chip auxiliary mounting assembly are identical in structure and correspond to each other in a mirror image mode based on a cavity;

when the (left and right) ceramic plates are installed, the left ceramic plate is clamped on the left ceramic plate auxiliary installation component, the right ceramic plate is clamped on the right ceramic plate auxiliary installation component, and the left ceramic plate and the right ceramic plate are respectively placed on the left test bed and the right test bed through the left ceramic plate auxiliary installation component and the right ceramic plate auxiliary installation component; the left end of the testing fiber sample is arranged on the left ceramic wafer, the right end of the testing fiber sample is arranged on the right ceramic wafer, the left ceramic wafer is a heating ceramic wafer, the probe of the left voltage probe station is abutted against the left end of the testing fiber sample, and the probe of the right voltage probe station is abutted against the right end of the testing fiber sample.

The left ceramic wafer is fixedly wrapped on the left test bed through the PI film.

The left end of the test fiber sample was bonded to the left ceramic plate by means of a silver adhesive.

The auxiliary mounting assembly for the left ceramic wafer comprises a ceramic wafer support and a ceramic wafer clamp, the ceramic wafer clamp is mounted on the lateral portion of the ceramic wafer support, and a pressure spring is mounted between the ceramic wafer support and the ceramic wafer clamp.

The left voltage probe station is arranged on the left guide rail in a sliding mode, and the right voltage probe station is arranged on the right guide rail in a sliding mode.

Still include left temperature sensor and right temperature sensor, left temperature sensor glue on left ceramic chip, left side temperature sensor is less than 2mm with the left end interval of test fiber sample, right temperature sensor glue on right ceramic chip, right side temperature sensor is less than 2mm with the right-hand member interval of test fiber sample.

And a fiber mounting groove is formed in the left ceramic piece.

The temperature control device is characterized by further comprising an MCU, a voltage output module is connected to a serial port of the MCU, an output end of the voltage output module is connected with a voltage input end of the left ceramic wafer, data output ends of the left temperature sensor and the right temperature sensor are respectively connected with a temperature data input end of the MCU, and voltage data output ends of the left voltage probe station and the right voltage probe station are respectively connected with a voltage data input end of the MCU.

Advantageous effects

Compared with the prior art, the device for testing the Seebeck value of the thermoelectric fiber material can be suitable for testing the Seebeck value of the thermoelectric fiber material; according to the invention, the fiber material is fixed on the ceramic heating sheet, and the ceramic heating sheet is utilized to realize rapid heating, so that the elastic contact surface formed by the PI film can enable the fiber material to be carried more stably and conveniently, and the set temperature difference can be rapidly reached. The invention has the advantages of small heat loss, low voltage (the maximum loading voltage is only 5 volts), electric insulation of each test component and high safety, is a quick and efficient Seebeck test system suitable for the thermoelectric fiber device, and can expand and be compatible with the measurement of the Seebeck coefficient of the thin film thermoelectric device.

The PI film is adopted to carry the ceramic heating sheet, so that the testing substrate has certain elasticity when in contact with the probe, the thermoelectric fiber can be ensured to realize effective ohmic contact with the probe, and the measuring result is accurate and reliable.

Drawings

FIG. 1 is a schematic structural view of the present invention;

FIG. 2 is an exploded view of the structure of the present invention;

FIG. 3 is a schematic structural diagram of the present invention when ceramic sheets are mounted;

FIG. 4 is a schematic structural view of a sample frame according to the present invention;

FIG. 5 is a schematic structural diagram of a left ceramic wafer and a right ceramic wafer placed on a left test bed and a right test bed in the invention;

FIG. 6 is a schematic structural diagram of a left ceramic wafer and a right ceramic wafer wrapped on a left test bed and a right test bed through PI films;

FIG. 7 is a schematic structural view of a left ceramic wafer according to the present invention;

FIG. 8 is a block diagram of the circuit connection structure of the present invention;

wherein, 1-left voltage probe station, 2-right voltage probe station, 3-presenting station, 4-sample frame, 5-supporting platform, 6-concave station, 7-left test station, 8-right test station, 9-cavity, 10-left shaft bracket, 11-right shaft bracket, 12-left ceramic chip auxiliary mounting component, 13-left ceramic chip, 14-right ceramic chip, 15-testing a fiber sample, 16-a ceramic wafer support, 17-a ceramic wafer clamp, 18-a pressure spring, 19-a left guide rail, 20-a right guide rail, 21-a right temperature sensor, 22-a left temperature sensor, 23-an MCU, 24-a voltage output module, 25-a fiber mounting groove, 26-a right ceramic wafer auxiliary mounting component and 27-a PI film.

Detailed Description

So that the manner in which the above recited features of the present invention can be understood and readily understood, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings, wherein:

the general structure diagram is shown in fig. 1, fig. 2 and fig. 3, the device for testing the seebeck value of the thermoelectric fiber material comprises a left voltage probe station 1, a right voltage probe station 2 and a display station 3, wherein the left voltage probe station 1 and the right voltage probe station 2 are traditional voltage detection devices in the prior art, and are respectively connected with a lead wire, and the lead wires are connected to a voltmeter and used for measuring the voltage values of the left end and the right end of a test fiber sample 15.

Left voltage probe platform 1, the equal movable mounting of right voltage probe platform 2 are on being platform 3 and all constitute sliding fit with being platform 3, and left voltage probe platform 1, right voltage probe platform 2 all can be being slided on being platform 3 promptly, can be setting up left guide rail 19 and right guide rail 20 on being platform 3, and left voltage probe platform 1 slidable mounting is on left guide rail 19, and right voltage probe platform 2 slidable mounting is on right guide rail 20 to realize the slip of left voltage probe platform 1, right voltage probe platform 2.

As shown in fig. 2, a supporting platform 5 is arranged on the platform 3, a concave platform 6 is arranged in the middle of the supporting platform 5, and the sample frame 4 is placed in the concave platform 6. As shown in fig. 4, the left side of the sample frame 4 is wrapped on the sample frame 4 through the PI film to form a left test bed 7, the right side of the sample frame 4 is wrapped on the sample frame through the PI film to form a right test bed 8, a cavity 9 is arranged between the left test bed 7 and the right test bed 8, and a length space is provided for a test fiber sample 15 through the cavity 9. An elastic contact surface is formed by wrapping the PI film 27, so that when the probe is contacted with a sample, a good elastic contact can be realized, the sample can be effectively prevented from being damaged, and the probe is favorably and closely contacted with the sample; the PI film can also prevent the temperature diffusion of the ceramic wafer during heating, and has good temperature control effect; the PI film can also play an insulating role, and the accuracy of voltage measurement data is ensured.

A left shaft support 10 is arranged on one side of the left test bed 7 and a right shaft support 11 is arranged on one side of the right test bed 8 on the supporting platform 5, and the left shaft support 10 and the right shaft support 11 are used for assisting in mounting and using the ceramic chip. The left shaft support 10 is provided with a left ceramic auxiliary mounting assembly 12 which forms a rotary fit, the right shaft support 11 is provided with a right ceramic auxiliary mounting assembly 26 which forms a rotary fit, the left ceramic auxiliary mounting assembly 12 can rotate on the left shaft support 10, and the right ceramic auxiliary mounting assembly 26 can rotate on the right shaft support 11. The left ceramic chip auxiliary mounting assembly 12 comprises a ceramic chip support 16 and a ceramic chip clamp 17, the ceramic chip clamp 17 is mounted on the side portion of the ceramic chip support 16, a compression spring 18 is mounted between the ceramic chip support 16 and the ceramic chip clamp 17, and the ceramic chip is squeezed on the ceramic chip support 16 through the elastic force of the compression spring 18 by the ceramic chip clamp 17.

The left ceramic wafer auxiliary mounting component 12 and the right ceramic wafer auxiliary mounting component 26 are identical in structure and correspond in mirror image based on the cavity 9, so that during mounting, the position of the left ceramic wafer 13 and the position of the right ceramic wafer 14 are just opposite to each other, dislocation is not generated, position dislocation when two ceramic wafers are manually placed at each time is prevented, the problem that a test fiber sample 15 is bent when the left ceramic wafer 13 and the right ceramic wafer 14 are dislocated is avoided, data comparison cannot be performed between a plurality of test fiber samples 15 is solved, the fiber samples mounted at each time are accurately and unmistakably located at the same position (the length is guaranteed to be uniform when the plurality of fiber samples are tested), and the accuracy of sample measurement is guaranteed.

Mounting the ceramic plates: when the ceramic plates are installed, the left ceramic plate 13 is clamped on the left ceramic plate auxiliary installation component 12, the right ceramic plate 14 is clamped on the right ceramic plate auxiliary installation component 26, and the left ceramic plate 13 and the right ceramic plate 14 are respectively placed on the left test bed 7 and the right test bed 8 through the left ceramic plate auxiliary installation component 12 and the right ceramic plate auxiliary installation component 26. The left ceramic piece 13 and the right ceramic piece 14 can be directly placed on the left test bed 7 and the right test bed 8 formed by the PI films, and can be used for conveniently taking a plurality of test fiber samples 15 during batch test analysis. In order to fix the left ceramic plate 13, as shown in fig. 6, the left ceramic plate 13 may be further fixed on the left test bed 7 by PI film wrapping, and the right ceramic plate 14 is the same.

Installation of the fiber sample: as shown in fig. 5, the left end of the test fiber sample 15 is mounted on the left ceramic plate 13, and the right end is mounted on the right ceramic plate 14, wherein the left ceramic plate 13 is a heating ceramic plate, so as to realize the temperature difference test requirement of hot (the left ceramic plate 13) and cold (the right ceramic plate 14). The probe of the left voltage probe station 1 abuts against the left end of the test fiber sample 15, and the probe of the right voltage probe station 2 abuts against the right end of the test fiber sample 15. The probe of the left voltage probe station 1 and the probe of the right voltage probe station 2 are abutted to the left end and the right end of the test fiber sample 15, the left end of the test fiber sample 15 can be bonded on the left ceramic piece 13 through the silver colloid, the probes can be accurately pressed at the two ends of the sample, good ohmic contact is realized, and accurate measurement of voltage is ensured.

The PI film has good heat insulation effect, and elastic force generated by the PI film can be formed, because the test samples are diverse, different in test fiber thickness and different in shape, when the probes are abutted to the two ends of the test fiber sample 15, the probes of the voltage probe station are difficult to be tightly pressed on the two ends of the test fiber sample 15. After the two ends of the test fiber sample 15 are wrapped by the PI film, the two ends of the test fiber sample 15 can be matched with probes of the voltage probe station to be abutted tightly by using the elastic force of the PI film, so that the contact between the two ends is ensured.

In order to ensure the testing accuracy, the left temperature sensor 21 is adhered on the left ceramic piece 13, the distance between the left temperature sensor 21 and the left end of the testing fiber sample 15 is less than 2mm, the right temperature sensor 22 is adhered on the right ceramic piece 14, and the distance between the right temperature sensor 22 and the right end of the testing fiber sample 15 is less than 2 mm. Meanwhile, in order to ensure the accuracy of testing different samples, the fiber mounting groove 25 can be formed in the left ceramic piece 13, the left end of the testing fiber sample 15 is specially placed in the fiber mounting groove, and the accuracy of the measuring instrument during batch testing of multiple samples is ensured by the right ceramic piece 14 in the same way.

And (3) data output: as shown in fig. 1 and 8, an integrated circuit control structure may be mounted on the stage 3 in order to facilitate direct reading of the seebeck value. The circuit control structure of the heating ceramic chip heating circuit is designed by using a traditional circuit, a voltage output module 24 is connected to a serial port of the MCU23, an output end of the voltage output module 24 is connected with a voltage input end of the left ceramic chip 13, and heating of the left ceramic chip 13 (heating ceramic chip) is controlled by the MCU 23. The data output ends of the left temperature sensor 21 and the right temperature sensor 22 are respectively connected with the temperature data input end of the MCU23, the temperature data are output to the MCU23 for calculation, the voltage data output ends of the left voltage probe station 1 and the right voltage probe station 2 are respectively connected with the voltage data input end of the MCU23, the voltage data are output to the MCU23 for calculation, a display module can be additionally connected to the MCU23, the MCU23 is used for performing traditional mathematical calculation, and the measured Seebeck value is directly output.

Other descriptions: in practical use, the left ceramic plate 13 is clamped on the left ceramic plate auxiliary mounting assembly 12, the right ceramic plate 14 is clamped on the right ceramic plate auxiliary mounting assembly, and the left ceramic plate auxiliary mounting assembly 12 and the right ceramic plate auxiliary mounting assembly are moved downwards to enable the left ceramic plate 13 to be placed on the left test bed 7 and the right ceramic plate 14 to be placed on the right test bed 8. Because the left test bed 7 and the right test bed 8 are formed by PI films, certain elastic force exists, the position deviation of the ceramic plates is easy to generate due to manual placement, the ceramic plates fall on the films to generate displacement and the like, and the fixture can be loosened after the ceramic plates are placed stably by utilizing the left ceramic plate auxiliary mounting assembly 12 and the right ceramic plate auxiliary mounting assembly. Meanwhile, when a plurality of samples are tested in batch, the position among each sample can be ensured to be the same, the possibility of influence of test data caused by the change of the test positions of the samples is avoided, and the requirement of unification of the length sizes of the samples is met (the lengths of a plurality of thermoelectric fibers are not unified, and the micromotion change influence is generated on the test result during the test). After the left ceramic piece 13 and the right ceramic piece 14 are placed stably, two ends of the testing fiber sample 15 are placed on the left ceramic piece 13 and the right ceramic piece 14 respectively, and the thimbles of the left voltage probe station 1 and the right voltage probe station 2 are propped against two ends of the testing fiber sample 15 respectively. Similarly, because the left test bed 7 and the right test bed 8 are formed by PI films, the elastic force exists, the thimble of the left voltage probe station 1 and the thimble of the right voltage probe station 2 are pressed down, and the left test bed 7 and the right test bed 8 formed by the PI films form an upward reaction force, so that the thermoelectric fibers and the probes are in effective ohmic contact, and the measurement result is accurate and reliable. And finally, the left temperature sensor 21 and the right temperature sensor 22 transmit temperature data to the MCU23, the left voltage probe station 1 and the right voltage probe station 2 transmit voltage data to the MCU23, and the MCU23 calculates and then measures the Seebeck value of the fiber material.

The foregoing shows and describes the general principles, essential features, and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are merely illustrative of the principles of the invention, but that various changes and modifications may be made without departing from the spirit and scope of the invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.