Patent Publication Number: US-2005127931-A1

Title: Variable temperature test cell and associated method

Description:
SPONSORSHIP  
      Work on this invention was sponsored by the National Science Fund Grant No. DMF-9870246 and National Aeronautics and Space Administration Grant No. NAG3-2588. The government has certain rights in this invention. 
    
    
     FIELD OF THE DISCLOSURE  
      The present disclosure relates generally to test cells for testing the electrochemical properties of a solid-state specimen such as a polymer or inorganic glass.  
     BACKGROUND OF THE DISCLOSURE  
      Solid-state materials such as clay, ceramic, inorganic glass, silicon, polymers and the like are developed for a large number of industrial applications. During development, it is often desirable to determine the electrochemical properties (e.g., dielectric constant or electrical conductivity) of the material. For example, impedance spectroscopy may be utilized to determine the electrical conductivity of a material.  
      During testing of a solid-state material, it is often necessary to subject the material to a wide range of temperatures to determine the behavior of the material in different conditions. For example, to test a solid-state material at elevated temperatures, the material is placed in a large oven, heated to the desired temperature, and tested. The material may be positioned in a test jig or cell when positioned in the oven. In certain systems, the material may be tested while positioned in the oven.  
     SUMMARY OF THE DISCLOSURE  
      According to one aspect of the disclosure, a test cell includes an integrated heating assembly. In a specific exemplary implementation, a heating element is secured to a housing of the test cell. The heating element may be embodied as a number of resistors which heat the housing, and hence the test specimen positioned therein, upon the application of electrical current to the resistors.  
      The heating element is electrically coupled to a controller which operates the heating element to generate a desired amount of heat. In the case of the heating element embodied as a number of heat generating resistors, the controller may be embodied as an R/G bridge which supplies a controlled current to the resistors to generate the desired amount of heat.  
      The test cell may include a temperature sensor to determine the temperature of the test specimen. The temperature sensor may be used as part of a closed-loop control scheme to maintain the test specimen at a desired temperature. The temperature sensor may be embodied as a platinum resistor secured to a sample support plate on which the test specimen is positioned.  
      The housing of the test cell may be constructed of a thermally conductive metal such as aluminum or copper.  
      The test cell may include a sample support assembly positioned in the housing. In an exemplary implementation, the sample support assembly includes a pair of electrodes, each of which is secured to a metallic support plate. Each of the electrodes is electrically coupled to an impedance meter which is operable to measure electrical characteristics of the test specimen (e.g., electrical impedance).  
      The test cell may also include a number of metallic rods which are secured to the housing&#39;s cap. A portion of the metallic rods is positionable in a cooling bath, such as a nitrogen bath, to cool the test specimen to a desired temperature.  
      According to another aspect of the disclosure, a method of testing a solid-state specimen includes the steps of positioning the solid-state specimen in a housing of a test cell, and applying an electrical current to a number of resistors secured to the housing to heat the housing.  
      The electrical current may be adjusted based on output from a temperature sensor to maintain the specimen at a desired temperature.  
      Electrochemical properties of the specimen may be measured while the specimen is maintained at a desired temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross sectional view of a test cell for testing the electrochemical properties of a solid-state specimen, note that a number of the components are not shown in cross section for clarity of description;  
       FIG. 2  is a top view of the test cell of  FIG. 1  with the cap and upper sample plate removed; and  
       FIG. 3  is a fragmentary view of the test cell of  FIG. 1  showing the test cell in an inverted position with its metallic cooling rods extending into a cooling bath. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      While the concepts of the present disclosure are susceptible to various modifications and alternative forms, specific exemplary embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the disclosure to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives following within the spirit and scope of the invention as defined by the appended claims.  
      Referring now to the drawings, there is shown a test cell  10  for use in testing the electrochemical properties of a solid-state materials such as clay, ceramic, inorganic glass, silicon, polymers and the like. The test cell  10  includes a housing  12  having a cap  14  and a base  16  secured thereto. Illustratively, the cap  14  and the base  16  are secured to the housing  12  with threaded fasteners such as screws  18 . In the exemplary embodiment described herein, the housing  12 , the cap  14 , and the base  16  are each constructed of a metallic material such as aluminum or copper.  
      As shown in  FIG. 1 , the housing  12  defines a cavity  22 . A sample support assembly  20  is positioned in the cavity  22 . The sample support assembly  20  includes an upper sample plate  24  and a lower sample plate  26 . The sample plates  24 ,  26  are illustratively constructed of aluminum. A nylon or fiberglass washer  28  is secured to each of the sample plates  24 ,  26 .  
      The sample support assembly  20  also includes a pair of electrodes. In the exemplary embodiment described herein, an upper electrode  30  is positioned on the upper sample plate  24 , and a lower electrode  32  is positioned on the lower sample plate  26 . The electrodes  30 ,  32  may be embodied as any type of electrical contact. In the exemplary embodiment described herein, the electrodes  30 ,  32  are embodied as copper discs.  
      The upper electrode  30  is coupled to a female connector  34  via a wire  36  which extends through a bore  38  defined in the upper sample plate  24 . A mating male connector  40  is soldered or otherwise connected to an external connector  42 . In the exemplary embodiment described herein, the connector  42  is embodied as a BNC connector which is threaded into a threaded bore  44  defined in the cap  14 .  
      Likewise, the lower electrode  32  is coupled to a female connector  46  via a wire  48  which extends through a bore  50  defined in the lower sample plate  26 . A mating male connector  52  is soldered or otherwise connected to an external socket  54 . As with the connector  42 , in the exemplary embodiment described herein, the connector  54  is also embodied as a BNC connector. The connector  54  is threaded into a threaded bore  56  defined in the base  16 .  
      The above-described configuration allows for ease of removal of the components of the sample support assembly  20 . For example, to remove the sample support assembly  20  from the housing  12 , the male connector  40  may be disconnected from the female connector  34  thereby allowing the wire  36  to be separated from the BNC connector  42 . Similarly, the wire  48  may be separated from the BNC connector  54  by disconnecting the connectors  46 ,  52  from one another thereby allowing the lower plate  26  to be separated from the housing  12 .  
      As shown in  FIG. 1 , the BNC connectors  42 ,  54  are electrically coupled to an impedance meter  58 . Specifically, one end of a cable  60  is coupled to the BNC connector  42  with the other end of the cable  60  being coupled to the impedance meter  58 . A first end of a cable  62  is coupled to the BNC connector  54 , with the other end being coupled to the impedance meter  58 . In such a way, the impedance meter  58  may be used to measure the electrical conductivity of a test specimen  64  positioned between the electrodes  30 ,  32 . The impedance meter  58  is configured to provide a test voltage to measure the impedance across the specimen  64 . In the case of impedance spectroscopy, the meter  58  may take numerous measurements at various frequencies. One impedance meter which may be used as the impedance meter  58  is a model number HP4192A Impedance Meter which is commercially available from Hewlett-Packard Company of Palo Alto, Calif.  
      The test cell  10  also includes a heating element  66 . As used herein, the term “heating element” is intended to mean any component or assembly that transforms fuel or electricity into heat. As shown in  FIG. 1 , the heating element  66  is coupled to the housing  12 . In the specific exemplary embodiment described herein, the heating element  66  is secured to an outer surface  68  of the housing  12 . It should be appreciated that the heating element  66  may be coupled to the housing by securing it directly to the housing  12 , or, alternatively, a number of thermally conductive spacers (not shown) may be positioned between the heating element  66  and the housing  12 .  
      In the specific exemplary embodiment described herein, the heating element  66  is embodied as a number of wire-wound power resistors  70 . Each of the resistors  70  is secured to the outer surface  68  of the housing. Specifically, as shown in  FIG. 2 , the outer surface  68  of the housing  12  has three flats  72  defined therein. Two of the resistors  70  are secured to each of the flats  72 .  
      The power resistors  70  may be embodied as any type of resistor which generates suitable amounts of heat when a current is applied thereto. One such resistor which may be used as the power resistor  70  is a part number RER60F127 OM, 5 Watt, 127 Ω, 1% tolerance wire-wound power resistor which is commercially available from Vishay Intertechnology, Inc. of Malvern, Pa. In an exemplary arrangement, six (6) of such resistors are arranged in a parallel combination of two sets of three resistors in series to yield a total resistance of 190.5 Ω.  
      The resistors  70  may be utilized to heat the test specimen  64  to a desired temperature. In particular, when an electrical current is applied to the resistors  70 , the resistors generate significant amounts of thermal energy (i.e., heat). Such heat is transferred to the outer surface  68  of the housing  12  to heat the housing and hence the test specimen  64  positioned in the sample support assembly  20  via a thermal path which includes the housing  12  and the sample plates  24 ,  26 .  
      The resistors  70  are electrically coupled to a current source  74  via a number of twisted pair cables  76 . In the exemplary embodiment described herein, the current source  74  is embodied as an R/G bridge. As such, a desired electrical current may be applied and maintained on the resistor circuit. On such R/G bridge which may be used is a model number 1802 Digital R/G Bridge which is commercially available from Quantum Design of San Diego, Calif.  
      A personal computer (PC)  78  is interfaced with the R/G bridge  74 . The PC  78  is operable to execute a desired control routine for testing the specimen at desired parameters. For example, the PC  78  may operate the R/G bridge  74  to generate a desired current to produce a desired specimen temperature profile. Specifically, by varying the current applied to the resistors  70 , the amount of heat generated by the resistors  70  may be varied. The PC  78  may be programmed to execute predetermined test routines which allow the test specimen  64  to be tested at various temperature profiles.  
      The test cell  10  includes a temperature sensor  80 . The temperature sensor  80  is utilized to determine the temperature of the test specimen  64  positioned between the electrodes  30 ,  32 . In the exemplary embodiment described herein, the temperature sensor  80  is secured to the lower sample plate  26 . As such, the temperature sensor  80  senses the temperature of the sample plate  26  which is indicative of, or may be correlated to, the temperature of the test specimen  64 . It should be appreciated that the temperature sensor  80  may be located in any number of locations to determine the temperature of the test specimen  64 , with the embodiment described herein (i.e., secured to the sample plate  26 ) being merely exemplary in nature.  
      As described herein, the temperature of the test specimen  64  is determined indirectly. In particular, as described above, the temperature of the sample plate  26  is sensed by the temperature sensor  80  and used as an indicator of the temperature of the test specimen  64 . Calculations may be performed to account for the use of such an indirect temperature measurements, if desired. Alternatively, the output from such an indirect temperature measurement may be extrapolated to a corresponding direct specimen temperature or otherwise adjusted. In other words, the herein described methods and systems may be configured to accommodate for the use of indirect temperature measurements of the test specimen  64 .  
      It should be appreciated that a temperature sensor could be utilized to sense the temperature of the test specimen  64  directly, if desired. As used herein, references to “determining” or “to determine” the temperature of the test specimen are intended to mean the use of either direct or indirect temperature measurements.  
      In the exemplary embodiment described herein, the temperature sensor  80  is embodied as a platinum resistor. The platinum resistor  80  is electrically coupled to the R/G bridge  74  via a connector assembly  82 . Specifically, a wire  84  is soldered on one end to a first lead  86  of the platinum resistor  80 , whereas a second end of the wire  84  is soldered to a first pin  88  of a male connector  90  of the connector assembly  82 . A wire  92  is soldered on one end to the first lead  86  of the platinum resistor  80 , with a second end of the wire  92  being soldered to a second pin  94  of the male connector  90 . A wire  96  is soldered on one end to a second lead  98  of the platinum resistor  80 , whereas a second end of the wire  96  is soldered to a third pin  100  of the male connector  90 . A wire  102  is soldered on one end to the second lead  98  of the platinum resistor  80 , with a second end of the wire  102  being soldered to a fourth pin  104  of the male connector  90 .  
      A female connector  106  is coupled to the male connector  90 . The interface pins of the male connector  90  and the corresponding receptacles of the female connector  106  electrically couple the pins  88 ,  94 ,  100 ,  104  to the first end of a number of wires  108 ,  110 ,  112 ,  114 , respectively. A second end of each of the wires  108 ,  110 ,  112 ,  114  is coupled to the R/G bridge  74 .  
      The R/G bridge uses one of the electrical connections to the first lead  86  of the platinum resistor  80  and one of the electrical connections from the second lead  98  of the platinum resistor  80  to measure the resistance of the resistor  80 . For example, the R/G bridge  74  may be used to provide a test voltage of a given value across the platinum resistor  80  to determine the resistance thereof. The platinum resistor&#39;s resistance changes as a function of the temperature of the resistor  80 . As such, by monitoring the resistance of the platinum resistor  80 , the temperature of the sample plate  26  may be determined which, as described above, is indicative of the temperature of the test specimen  64 .  
      It should be appreciated that use of the platinum resistor  80  provides the feedback portion of a closed-loop control scheme. In particular, the electrical current applied to the power resistors  70  may be varied in response to output from the platinum resistor  80 . Indeed, as described above, the amount of heat generated by the power resistors  70  may be varied by varying the current applied thereto. As such, the sensed temperature from the platinum resistor  80  may be used by the control routine to adjust the current supplied to the power resistors  70  to achieve and maintain a desired temperature within the housing  12 .  
      The R/G bridge uses the second electrical connection to the first lead  86  of the platinum resistor  80  and the second electrical connection from the second lead  98  of the platinum resistor  80  to selectively apply current to the platinum resistor  80 . In certain embodiments, it may be desirable to warm the test specimen “locally.” It should be appreciated that if a given design of a test cell does not utilize such “local” warming of the test specimen, only a single pair of wires (for measuring resistance of the resistor  80  as described above) need be connected to the platinum resistor  80 .  
      As shown in  FIG. 3 , the test cell  10  also includes a number of cold-sink members  116 . In the exemplary embodiment described herein, the cold-sink members  116  are embodied as a number of metallic rods which are threaded into the cap  14 . Illustratively, the metallic rods  116  may be constructed with copper, although other metallic material may be used as well. One end  118  of the metallic rods  116  is threaded into the cap  14 , whereas the other end  120  extends outwardly from the cap  14 .  
      As shown in  FIG. 3 , the outer end  120  of the metallic rods  116  may be positioned in a cooling bath  122  such as a dewar flask of liquid nitrogen. In such a way, the metallic rods  116  provide a thermal pathway from the cooling bath  122  to the housing  12 . Use of the metallic rods  116  and the cooling bath  122  allows the test specimen  64  to be cooled to relatively low temperatures (e.g., −100° C.).  
      In operation, the test cell  10  may be used to measure the electrical properties of the solid-state test specimen  64 . To do so, the test specimen  64  is first positioned in the sample support assembly  20 . Specifically, the test specimen  64  is positioned between the electrodes  30 ,  32 . Both of the sample plates  24 ,  26  may be removed from the housing  12  during such assembly. Alternatively, the lower sample plate  26  may remain in the cavity  22  of the housing  12  during specimen loading. In such a case, the specimen  64  is positioned on the lower electrode  32 . Thereafter, the upper plate  24  is lowered into the sample cavity  22  such that the upper electrode  30  is urged into contact with the sample specimen  64  thereby sandwiching the specimen  64  between the electrodes  30 ,  32 . A number of screws  124  are then installed to secure the sample plates  24 ,  26  to one another. Use of the screws enhances contact between the electrodes  30 ,  32  and the specimen  64 .  
      Once the specimen  64  has been secured between the electrodes  30 ,  32 , the cap  14  may be installed. To do so, the female connector  34  is first connected to the mating male connector  40  to electrically couple the upper electrode  30  (and hence the specimen  64 ) to the BNC connector  42 . Thereafter, the threaded holes in the cap  14  are aligned with the corresponding holes in the housing  12  and the screws  18  are inserted therein to secure the cap  14  to the housing  12 . If the test procedure to be performed requires cooling of the test specimen  64 , the metallic rods  116  are threaded into their respective holes in the cap  14 .  
      If not already done, the BNC connector  42  is then coupled to the impedance meter  58  by use of the cable  60 , and the BNC connector  54  is coupled to the impedance meter  58  by use of the cable  62 . Likewise, if not already done, the power resistors  70  are coupled to the appropriate connectors of the R/G bridge  74  by use of the twisted pair cables  76 . Moreover, if not already done, the female connector  106  is then connected to the male connector  90  to electrically couple the platinum resistor  80  to the R/G bridge  74 .  
      Once the test cell  10  has been assembled and coupled to the impedance meter  58  and the R/G bridge  74 , testing of the electrochemical properties of the test specimen  64  may commence. For example, the electrical conductance of the test specimen  64  may be measured as the specimen is subjected to wide temperature range such as −40° C. to 40° C., although other temperature ranges (e.g., −100° C. to 80° C.) are possible. To do so, the test cell  10  is first inverted, and the metallic rods  116  are inserted into the liquid nitrogen cooling bath  122  thereby cooling the specimen  64 . The output of the platinum resistor  80  may be monitored to determine the temperature of the specimen  64 . Once the specimen  64  reaches a desired temperature (e.g., −40° C.), the impedance meter  58  may be operated to obtain conductance measurements of the specimen  64  across a wide range of frequencies.  
      The temperature of the cell  10  and hence the test specimen  64  may then be raised at a predetermined rate. The temperature of the test specimen  64  may be raised by reducing the exposure of the test cell  10  to the cooling bath  122 , actuation of the heating element  66 , or a combination of both. During such raising of the temperature of the specimen  64 , the impedance meter  58  begins to develop a spectrum by continuing to measure conductance of the test specimen  64  across a wide range of frequencies.  
      If not already actuated, the heating element  66  is used to elevate the temperature of the specimen to a desired endpoint (e.g., 40° C.). Specifically, the R/G bridge  74  applies a current to the power resistors  70  thereby causing the resistors  70  to generate heat. The heat is transferred to the housing  12  and hence the specimen  64 . The output from the platinum resistor  80  is indicative of the temperature of the specimen  64 , and is used as a feedback mechanism to allow the temperature of the specimen  64  to be raised in a controlled manner. As the temperature increases, the impedance meter  58  continues to develop the spectrum by measuring the conductance of the test specimen  64  across a wide range of frequencies.  
      It should be appreciated that the above procedure may be used to test the specimen  64  across a relatively wide temperature range. Time vs. temperature profiles may be developed based on the design of a specific procedure. Such profiles may be programmed using appropriate software resident on the PC  78  to operate the R/G bridge  74  to produce a desired environment within the test cell  10 .  
      While the concepts of the present disclosure have been illustrated and described in detail in the drawings and foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only the illustrative embodiments have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected.  
      There are a plurality of advantages of the concepts of the present disclosure arising from the various features of the apparatus and methods described herein. It will be noted that alternative embodiments of each of the apparatus and methods of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of an apparatus and method that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the invention as defined by the appended claims.  
      For example, although the power resistors  70  are herein described as being secured to the outer surface  68  of the housing  12 , it should be appreciated that the power resistors  70  could be arranged in any desired arrangement that allows for the transfer of heat to the test specimen  64 . For instance, the power resistors  70  may be secured to an inner surface of the housing  12 . Alternatively, the power resistors  70  may be secured to the sample plates  24 ,  26 . Moreover, the test cell  10  could be constructed with additional structures to which the resistors  70  may be secured. The power resistors  70  may be secured to any structure that can provide a thermal path (or a portion of a thermal path) to the test specimen  64 .