Abstract:
The present invention relates to an apparatus and method for measuring specific heat using flash. To this end, the specific heat measurement apparatus includes sample fixing means for fixing a sample  50  so that each of both surfaces  52, 55  of the sample  50  is partially exposed, flash irradiation means for irradiating flash to one surface  52  of the sample  50 , which is exposed by the sample fixing means, a light-receiving detector for receiving light irradiated from the other surface  55  of the sample  50 , which is exposed by the sample fixing means, and a calculation unit  74  for calculating specific heat of the sample  50  based on an output signal of the light-receiving detector.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates, in general, to specific heat measurement, and more particularly, to an apparatus and method for measuring specific heat using flash. 
         [0003]    2. Background of the Related Art 
         [0004]    Thermal physical properties (thermal diffusivity, specific heat, thermal conductivity) are unique to a material system, and accurate measurement of the thermal physical properties is important in application techniques in terms of thermal transfer analysis and engineering. In particular, as new materials having a good thermal characteristic and special functional materials are actively developed in line with the development of the industry, accurate measurement for securing rapidness and reliability, which can be applied to new materials, is required. 
         [0005]    Meanwhile, the flash method is a method of eliminating contact resistance at normal state, which was inherent in the existing thermal conductivity measurement, and was developed for thermal diffusivity measurement. The advantages of the laser flash method includes measurement for a short period of time, easy data acquisition, a smaller size of a sample, and measurement with high accuracy up to a wide range of the temperature range. Thus, the laser flash method has recently been widely used along with lots of developments. 
         [0006]    In order to measure the thermal conductivity k using the flash method, the thermal conductivity k can be found from the following Equation 1. 
         [0000]      k=ρC p α  (1) 
         [0007]    where the thermal diffusivity α, the specific heat C p  using a Differential Scanning Calorimetry (DSC) method, and the sample density ρ employing Archimedes&#39; Principle are obtained. The density ρ can be obtained relatively simply, but the specific heat measurement using the DSC method takes lots of time since three-step measurement, including an empty vessel, a standard sample and a test sample, is required. Thus, a method of measuring the thermal diffusivity and the specific heat at the same time using the flash method without additional specific heat measurement equipment has been researched and commercialized, but there has been great error. 
         [0008]    H. Watanabe (Chemical Geology, Watanabe, H. v. 70 no. 1/2, 1988, p. 90) calculated the specific heat by comparing the maximum temperatures at the rear surfaces of a standard sample and a test sample. Shinzato, etc. (J. ther. analy. cal. Shinzato, K. and Baba, T. v. 64 no. 1, 2001, pp. 413-422) compared the specific heat at a temperature at the half time when temperatures at the rear surfaces of a standard sample and a test sample reached the highest. However, it was recommended that the thickness and the thermal physical properties of the standard and test samples were similar, but there has been an error of 10% or higher in reality. Thus, today, the measurement of the specific heat almost depends on the DSC method. 
         [0009]    However, the specific heat using the existing DSC method largely depends on the accuracy of the degree of contact of the sample and bottom surface of the vessel. 
         [0010]    The measurement error by the (DSC) method is large due to the difference in the processed mass of the standard sample and the (measured) sample, which must be the same. Furthermore, the thickness of the test sample must be limited to within 1 mm, and should not be high because of a temperature delay phenomenon. 
         [0011]    Accordingly, the method has lots of difficulties in accurate measurement and is accompanied by many error factors. 
       SUMMARY OF THE INVENTION 
       [0012]    Accordingly, the present invention has been made in an effort to solve the above problems occurring in the prior art, and it is an object of the present invention to provide an apparatus and method for measuring specific heat using flash, in which the thermal diffusivity and the specific heat can be measured at the same time accurately, and the thermal conductivity can be measured further rapidly and accurately by developing a new method of measuring the specific heat from thermal diffusivity data using the flash method. 
         [0013]    The object of the present invention can be accomplished by a specific heat measurement apparatus using flash, including sample fixing means for fixing a sample so that each of both surfaces of the sample is partially exposed, flash irradiation means for irradiating flash to one surface of the sample, which is exposed by the sample fixing means, a light-receiving detector for receiving light irradiated from the other surface of the sample, which is exposed by the sample fixing means, and a calculation unit for calculating specific heat of the sample based on an output signal of the light-receiving detector. 
         [0014]    Further, the sample fixing means may include a sample holder having a first holder hole formed penetratingly therein so that the one surface of the sample is exposed and a second holder hole for accommodating the sample therein, a sample cover placed on the sample holder and having a covering hole formed penetratingly therein so that the other surface of the sample is exposed, and a sample holder plate in which the sample holder is seated. 
         [0015]    In addition, the sample holder plate may include a first sample holder plate formed with a first through-hole for inserting the sample holder thereto, and a second sample holder plate fixed closely to one surface of the first sample holder plate and having a second through-hole formed therein, the second through-hole being smaller than the first through-hole. 
         [0016]    Further, it is favorable that plural pairs of first and second through-holes are formed in the first and second sample holder plates. Further, the flash irradiation means may include a laser oscillation unit or a xenon flash, and the flash irradiated by the flash irradiation means may include a pulse wave. Furthermore, the light-receiving detector may include an infrared detector. It is also favorable that black graphite is coated on the both surfaces of the sample. 
         [0017]    The object of the present invention can be accomplished by a specific heat measurement method using flash, including a sample fixing step of fixing a sample whose specific heat will be measured so that each of both surfaces of the sample is partially exposed, a step of allowing a light-receiving detector to measure light at the other surface of the sample, a first calculation step of calculating a temperature change at the other surface of the sample according to the lapse of time t based on an output signal of the light-receiving detector, a second calculation step of calculating the temperature change by performing the sample fixing step to the first calculation step with respect to a standard sample having a known specific heat C pr , and a third calculation step of calculating a specific heat C ps  of the sample based on the temperature change of the first calculation step and the temperature change of the second calculation step. 
         [0018]    Further, the third calculation step S 600  may include a step S 610  of selecting a maximum temperature rise value ΔT max  and a maximum time t max  (defined as 15 times t 1/2 ) at a rear surface with respect to the measured sample and a standard sample, a step of dividing an elapsed time t of each of the measured sample and the standard sample by the maximum time t max , a step of calculating a temperature rise value ΔT, of the measured sample and a temperature rise value ΔT, of the standard sample by integrating predetermined sections of a non-dimensional time t/t max  based on a temperature rise ΔT with respect to non-dimensional time t/t max , which is divided with respect to the measured sample and the standard sample, and a step of calculating specific heat C ps  of the measured sample from the following equation based on the temperature rise value ΔT, of the measured sample and the temperature rise value ΔT, of the standard sample: 
         [0000]    
       
         
           
             
               C 
               ps 
             
             = 
             
               
                 
                   ρ 
                   r 
                 
                  
                 
                   l 
                   r 
                 
                  
                 
                   C 
                   pr 
                 
                  
                 Δ 
                  
                 
                     
                 
                  
                 
                   T 
                   r 
                 
               
               
                 
                   ρ 
                   s 
                 
                  
                 
                   l 
                   s 
                 
                  
                 Δ 
                  
                 
                     
                 
                  
                 
                   T 
                   s 
                 
               
             
           
         
       
     
         [0019]    where ρ r  is the density of the standard sample, ρ s , is the density of the measured sample, l r , is the thickness of the standard sample, l s  is the thickness of the measured sample, and C pr  is the specific heat of the standard sample. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Further objects and advantages of the invention can be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0021]      FIG. 1  is a plan view of a specific heat measurement apparatus using flash according to the present invention; 
           [0022]      FIG. 2  is a exploded perspective view of the specific heat measurement apparatus illustrated in  FIG. 1 ; 
           [0023]      FIG. 3  is a partial cross-sectional view of the specific heat measurement apparatus illustrated in  FIG. 1 ; 
           [0024]      FIG. 4  is a theoretical graph regarding temperature rise at the rear surface of a sample; and 
           [0025]      FIG. 5  is a graph showing integrated sections in the range of 0.5 to 1.5 times of the half time of the temperature rise curve in a non-dimensional time axis. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    The present invention will now be described in detail in connection with a specific embodiment with reference to the accompanying drawings. 
         [0027]    (Construction) 
         [0028]      FIG. 1  is a plan view of a specific heat measurement apparatus using flash according to the present invention.  FIG. 2  is a exploded perspective view of the specific heat measurement apparatus illustrated in  FIG. 1 .  FIG. 3  is a partial cross-sectional view of the specific heat measurement apparatus illustrated in  FIG. 1 . 
         [0029]    The specific heat measurement apparatus according to the present invention largely includes, as illustrated in  FIGS. 1 to 3 , first and second sample holder plates  12 ,  16 , a sample holder  40 , a sample  50 , a sample cover  30  and surrounding measurement devices. 
         [0030]    The first sample holder plate  12  is made of steel and has a first through-hole  20  of a square shape formed therein. The first through-hole  20  can have a regular square whose one side is 3 cm in length. Four or more through-holes  20  can be formed in one first sample holder plate  12 , as illustrated in  FIG. 1 . 
         [0031]    The second sample holder plate  16  is also made of steel, and is closely adhered and fixed to one side of the first sample holder plate  12  by means of a mating unit  14 , such as the screw. The second sample holder plate  16  has a second through-hole  25  formed therein. The second through-hole  25  has a diameter less than 3 cm. It serves to support the sample holder  40  without falling down when the sample holder  40  is placed horizontally. 
         [0032]    The sample holder  40  is a member to hold the sample  50 . The sample holder  40  is made of steel and has a length less than 3 cm in one side so that it can be inserted into the first through-hole  20 . A second square-shaped holder hole  42  is formed at the center of the sample holder  40 . The thickness of the second square-shaped holder hole  42  is half of that of the sample holder  40 . A first square-shaped holder hole  44  is formed at the center of the second square-shaped holder hole  42 . The thickness of the first square-shaped holder hole  44  is the same as that of the second square-shaped holder hole  42 . 
         [0033]    The second square-shaped holder hole  42  is the space at which the square sample  50  is directly placed. The first square-shaped holder hole  44  forms a path along which a laser beam  65  is directly irradiated on the sample  50 . 
         [0034]    The sample  50  is an object whose specific heat will be measured, and has a regular square cross-section whose one side is at most 8 mm in length. Black graphite coating is coated on the surface of the sample  50  in order to accelerate the absorption of heat by flash and the radiation of infrared rays. 
         [0035]    The sample cover  30  is made of steel and has a circular disk shape. The external diameter of the sample cover  30  is within 3 cm so that it can be inserted into the first through-hole  20 . A covering hole  35  is formed at the center of the sample cover  30 . The covering hole  35  forms a path along which infrared rays emitted from the sample  50  passes. The covering hole  35  has an inside diameter of 6 to 8 mm. The sample cover  30  and the covering hole  35  serve to prevent excessive flash and apply constant energy. 
         [0036]    A laser oscillation unit  60  can include any kinds of xenon flash, other light, etc. only if it is flash emitting light. The laser oscillation unit  60  is disposed vertically under the sample  50 . The surface of the sample  50  is exposed so that the laser beam  65  can be directly irradiated on it. 
         [0037]    An infrared detector  70  is placed vertically over the sample  50 , and is a member for measuring infrared rays emitted from the sample  50  and outputting it as an electrical signal (voltage or resistance). 
         [0038]    A signal processor  72  is connected to the infrared detector  70  and a calculation unit  74 . The signal processor  72  includes an amplification unit (not illustrated), a bandpass filter (not illustrated) and an A/D converter (not illustrated) in order to amplify and filter an output signal of the infrared detector  70  and convert the resulting signal into a discrete signal. 
         [0039]    The calculation unit  74  is a member that calculates a specific heat value of the sample  50  by performing an operation according to a predetermined operation method based on a signal input from the signal processor  72 . The calculation unit  74  can include a computer, a microcomputer, a CPU or the like. 
         [0040]    (Experiment Method) 
         [0041]    A measurement method of the specific heat measurement apparatus using flash is described in detail below. One surface  52  of the sample  50  whose specific heat will be measured is heated by instant light of flash. Thereafter, a temperature on the other surface  55  of the sample  50  rises as time goes by. 
         [0042]      FIG. 4  is a theoretical graph regarding the temperature rise on the rear surface of the sample. In order to measure the thermal diffusivity, the maximum time t max , which is 15 times of the half time, is required. The specific heat was obtained by dividing an elapsed time by the maximum time t max  to have non-dimension and comparing the temperature rises between a standard sample and the measured sample  50  depending on the non-dimension time axis t/t max . 
         [0043]    It was found that the specific heat had the reproducibility and accuracy of within 2% without regard to the thickness and properties of material in the integrated sections between 0.5 and 1.5 times of the half time on the curve of the non-dimensional time axis versus the temperature rise. This is a method of measuring the specific heat accurately within a short period of time in comparison with the conventional specific heat measurement method having the error of 10% or more and the DSC specific heat measurement method having the error of 5%. 
         [0044]    In the specific heat measurement apparatus using flash according to the present invention, the specific heat can be obtained by comparing the temperature rises at the standard sample and the other surface  55  of the sample  50 . If one surface  52  of the sample  50  is heated by flash (for example, the laser beam  65 ), the temperature rise at the other surface  55  of the sample  50  at any time can be written as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       Δ 
                        
                       
                           
                       
                        
                       T 
                     
                     
                       Δ 
                        
                       
                           
                       
                        
                       
                         T 
                         max 
                       
                     
                   
                   = 
                   
                     1 
                     + 
                     
                       2 
                        
                       
                         [ 
                         
                           
                             ∑ 
                             
                               n 
                               = 
                               1 
                             
                             ∞ 
                           
                            
                           
                             
                               
                                 ( 
                                 
                                   - 
                                   1 
                                 
                                 ) 
                               
                               n 
                             
                             · 
                             
                               exp 
                                
                               
                                 ( 
                                 
                                   
                                     - 
                                     
                                       n 
                                       2 
                                     
                                   
                                    
                                   2 
                                    
                                   
                                     π 
                                     2 
                                   
                                    
                                   α 
                                    
                                   
                                       
                                   
                                    
                                   
                                     tl 
                                     
                                       - 
                                       2 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0045]    where α and l are the thermal diffusivity and thickness of the sample  50  respectively, ΔT is temperature rise at the other surface  55  of the sample  50  depending on time, ΔT max  is the maximum temperature value at the other surface  55  of the sample  50 , and t is time after the heating of flash (i.e. pulse wave). 
         [0046]    Designating the time when the temperature rise ΔT at the other surface  55  of the sample  50  reaches half of the maximum temperature value ΔT max  after the heating of flash (i.e. pulse wave) as the half time t 1/2 , the thermal diffusivity a can be obtained from Equation 3 as follows. 
         [0000]    
       
         
           
             
               
                 
                   α 
                   = 
                   
                     
                       0.138785 
                        
                       
                         l 
                         2 
                       
                     
                     
                       t 
                       
                         1 
                         / 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0047]    The specific heat measurement method according to the present invention is described below. When the flash energy Q is irradiated uniformly on the standard sample and the measured sample  50  at a rate per unit area and time, the temperature rises of the standard sample and the measured sample  50  can be expressed by the following Equations 4 and 5. 
         [0000]    
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       T 
                       r 
                     
                   
                   = 
                   
                     
                       [ 
                       
                         Q 
                         
                           ρ 
                            
                           
                               
                           
                            
                           
                             lC 
                             p 
                           
                         
                       
                       ] 
                     
                     r 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     
                       T 
                       s 
                     
                   
                   = 
                   
                     
                       [ 
                       
                         Q 
                         
                           ρ 
                            
                           
                               
                           
                            
                           
                             lC 
                             p 
                           
                         
                       
                       ] 
                     
                     s 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where the suffixes r and s are the standard sample and the measured sample  50  respectively, and ρ is the sample density. The specific heat C ps  of the sample  50  can be determined by comparing the temperature rise of the measured sample  50  to that of the standard sample with known specific heat based on Equation 6. 
         [0000]    
       
         
           
             
               
                 
                   
                     C 
                     ps 
                   
                   = 
                   
                     
                       
                         ρ 
                         r 
                       
                        
                       
                         l 
                         r 
                       
                        
                       
                         C 
                         pr 
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       
                         T 
                         r 
                       
                     
                     
                       
                         ρ 
                         s 
                       
                        
                       
                         l 
                         s 
                       
                        
                       Δ 
                        
                       
                           
                       
                        
                       
                         T 
                         s 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0048]    In the specific heat measurement method according to an embodiment of the present invent, after the heating of flash (i.e. pulse wave) in order to measure the thermal diffusivity a of the sample  50 , the maximum time t max  at the other surface  55  of the sample  50  was measured as 15 times of the half time t 1/2  so as to obtain a data correction coefficient. The temperature change ΔT at the other surface  55  of the sample  50  after the heating of flash (i.e. pulse wave) depends on the properties and the thickness of material, and the maximum time t max  also differs from material to material. 
         [0049]    In the present invention, the specific heat of the test sample can be obtained by dividing each elapsed time t by the maximum time t max  to have non-dimension, and comparing the temperature rises between the standard sample and the measured sample  50  depending on the non-dimensional time axis t/t max  through integration of predetermined sections of the non-dimensional time axis. This process is shown in the graph of  FIG. 5 .  FIG. 5  is a graph showing the integrated sections in the range of 0.5 to 1.5 times of the half time of the temperature rise curve in the non-dimensional time axis. 
         [0050]    Table 1 compares the measurement of NIST standard sample (Pyrex 7790, Polycrystalline Alumina, Copper RM 5) at normal temperature by using the specific heat measurement apparatus and method according to the present invention. As can be seen from Table 1, the measured sample  50  was material having a different thermal conductivity (Pyrex: 1.098 W/mk, Alumina: 30.92 W/mk, Copper: 404.2 W/mk) and having a thickness of 1 to 3 mm. The measured sample  50  was compared with the standard sample. It was found that there are accuracy and reproducibility of within 2%. 
         [0000]    
       
         
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
             
          
         
       
     
         [0051]    As described above, according to an embodiment of the present invention, the thermal diffusivity and the specific heat can be measured at the same time accurately, and the thermal conductivity can be measured further rapidly and accurately. 
         [0052]    While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiment but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.