Abstract:
Under a same heating condition as a sample to be measured, a physical property is measured for a reference substance whose temperature dependency of a physical property value is previously known. Reversely, a temperature profile of the reference substance is read so that the temperature profile is applied to a physical property measurement result for the sample. Thus, thermal analysis for the large diameter sample is made accurately without the necessity of arranging a temperature sensor in the vicinity of the sample.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a thermal analysis apparatus and a thermal analysis method to reveal how a physical property of a material varies with temperature. Particularly, the invention relates to a novel improvement for suppressing uncertainty in temperature measurement which inevitably occurs upon measuring a large-sized sample. 
     Thermal analysis is an approach used to analyze a change in a physical or chemical property of a material as a function of temperature, and based on simultaneous measurements of material temperature and physical property. Various thermal analytic methods have so far been developed depending on the kind of physical property to be measured, typical ones of which, if rearranged in relation between technique and physical property, are as follows. 
     Differential scanning calorimetry (DSC): differential heat flow 
     Thermogravimetric measurement (TG): weight 
     Thermomechanical analysis (TMA): dimension 
     Dynamic thermomechanical analysis (DMA): modulus of elasticity 
     Dielectric thermal analysis (DETA): dielectric constant 
     In these thermal analysis methods, sample temperatures have been measured by temperature sensors such as thermocouples and resistor thermometers arranged in the vicinity of samples. 
     In order to accurately measure the temperature of a sample, it is desired to avoid a temperature distribution in the sample. Due to this, it is a general practice in thermal analysis to conduct measurement by decreasing the amount of a sample as much as possible so long as the sensitivity of physical property measurement does not become insufficient. The technique of reducing sample amount is effective for many applications where ingredient distribution in a sample does not cause problems. Particularly, significant effects are available in DSC and TG. Further, in DSC and TG, temperature distribution is suppressed by accommodating a sample in a vessel formed of aluminum or the like as a good heat conductive material. Furthermore, a temperature sensor such as a thermocouple contacted with a sample vessel is used for temperature detection. 
     That is, it is possible in DSC and TG to satisfy comparatively easily three elements of sample amount decrease, homogeneous sample heating by a good heat conductive vessel, and contact between a sample and a temperature sensor. As a result, they greatly contribute to accurate measurement of sample temperature. 
     However, in the case of TMA or DMA, it is difficult to satisfy, such conditions as sample amount decrease and homogeneous heating due to a vessel, of the three conditions to be easily satisfied by DSC and TG. Thus, situations become severe to accurately measure sample temperature. 
     DSC and TG in nature are not concerned with sample shape whereas in TMA information about sample length is important. For example, in expansion coefficient measurement as one of the important applications for TMA, it becomes requisite to know concretely a sample initial length. DMA deals with a modulus of elasticity as a ratio of stress to strain caused in a sample. Then, in order to accurately determine a modulus of elasticity, it is essential to accurately know at least a sample three dimensional shape. 
     That is, in TMA and DMA the sample tends to increase in diameter and hence a technique cannot be used to improve measurement accuracy of sample temperature, such as sample amount decrease and homogeneous heating. 
     Further, because of the restriction that no effect should be caused to reduce high accuracy dimensional measurement or strain measurement, it is difficult to put a temperature sensor in contact with a sample center. Moreover, even if measurement could be conducted with a temperature sensor contacted with a sample, a problem arises due to a temperature distribution caused by diameter increase such that it is uncertain whether a measured sample temperature correctly represents a sample overall temperature. 
     As stated above, in TMA and DMA the sample tends to increase in diameter. Due to this, there has arisen a problem that sample temperature measurement is inaccurate. 
     In particular, when precisely determining the coefficient of thermal expansion, the amount of sample expansion can be determined comparatively accurately. However, it has been a bottleneck to precisely measure a sample temperature. 
     SUMMARY OF THE INVENTION 
     The present invention has been developed in order to effectively solve the above problems. In a thermal analytic technique such as TMA or DMA in which an increase in diameter of a sample is inevitable, first a reference substance having a known temperature dependency of a physical property value and a sample to be measured are measured in physical property at the same time within the same furnace or under the same heating conditions. Next, in place of measuring a sample temperature using a temperature sensor such as a thermocouple arranged in the vicinity of the sample, a physical property change signal obtained by measuring a physical property value of the reference substance is converted into a temperature change signal. At this time, if the reference substance and the sample were separately measured in physical property change under the same heating conditions, the physical change of the reference substance with respect to an elapsing time from a measurement start is converted into a temperature change in the sample with respect to the elapsing time. Hereinafter, the analysis is proceeded similarly to usual thermal analysis. 
     In this process, the physical property change of the reference substance is used as a temperature sensor. Even in a large diameter sample, reduction in temperature measurement accuracy does not occur as a result of an arrangement or contactability of a temperature sensor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view partly having a block diagram showing one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described in detail based on the drawings. 
     In FIG. 1, reference character  1  is a sample to be measured, and  2  is a reference substance having a known coefficient of thermal expansion. The sample  1  and the reference substance  2  are formed in a columnar shape. The sample  1  and the reference substance  2  are rested symmetrically on left and right sides on the bottom surface of a sample holder  3  formed in a bottomed tubular form of quartz glass as a low expansion material. On top surfaces of the sample  1  and reference substances  2 , a sample side probe  5  and a reference side probe  6  are respectively rested which are made of quartz glass in a branched rod form. 
     The vertical displacements of the sample side probe  5  and the reference side probe  6  are detected as relative displacements by differential transformers  7   b ,  8   b  of cores  7   a ,  8   a  provided on a branch section, and measured by a sample expansion measuring circuit  27  and a reference expansion measuring circuit  28 . 
     The sample holder  3  at its upper portion is fixed by a micrometer  4 , for vertical movement relative to a housing  23 . The differential transformers  7   b ,  8   b  are respectively fixed to the housing by holding members  9 ,  10 . 
     The sample side probe  5  and the reference side probe  6  at top ends are respectively supported at probe fulcrums  11   a ,  12   a  of balance arms  11 ,  12  in rotatable manner. 
     The balance arm  11  has a main fulcrum  11   b  rotatably supported to the housing  23  and a coil fulcrum  11   c  supported with a coil holder  13 . A coil  14  wound around a coil holder  13  is placed in a radial magnetic field created by a permanent magnet  15 . The permanent magnet  15  is fixed to the housing  23  through a table  19 . 
     Similarly, the balance arm  12  is rotatably supported at the main fulcrum  12   b  to the housing  23 , while at the coil fulcrum  12   c  a coil holder  16  is supported. A coil  17  wound around the coil holder  16  is placed in a radial magnetic field created by a permanent magnet  18 . The permanent magnet  18  is also fixed to the housing  23  through a table  20 . 
     Load generating circuits  25 ,  26  respectively connected to the coils  14 ,  17  control current values flowing through the coils  14 ,  17  to control outputs of force generators formed by both the coil  14  and permanent magnet  15 , and both the coil  17  and permanent magnet  18 , respectively. 
     The differential transformer  7   b  is connected with the sample expansion measuring circuit  27  while the differential transformer  8   b  is connected with the reference expansion measuring circuit  28 . The reference expansion measuring circuit  28  is connected with a reference expansion/temperature converter  29  to determine a temperature of a reference substance from an expansion amount of the reference substance. The sample expansion measuring circuit  27  and the reference expansion measuring circuit  28  are connected to a subtractor  30 . The subtractor  30  determines a difference in expansion amount between the sample and the reference substance. 
     A furnace  21  is arranged around the sample holder  3 . The furnace  21  can be vertically moved by a movement mechanism  22 . The furnace  21  is controlled in temperature as a function of elapsing time from a measurement start time point by a temperature controller  24 . 
     Hereinafter, the operation of an apparatus according to the present embodiment will be explained. 
     First, an operator operates the movement mechanism  22 , and lowers the furnace  21  and sets a sample  1  and a reference substance  2  between the sample holder  3  and the sample-side probe  5  and reference-side probe  6 . The operator sets loads to be applied to the sample  1  and reference substance  2  being measured to the load generating circuits  25 ,  26 . As a result of this, a proper current flows through the coil  14 ,  17  due to operation of the load generating circuit  25 ,  26  so that vertical forces are delivered to the coil fulcrums  11   c ,  12   c  through the coil holders  13 ,  16 . The forces applied to the coil fulcrums  11   c ,  12   c  are respectively conveyed to the probe fulcrums  11   a,    12   a  through main fulcrums  11   b ,  12   b  of the balance arms  11 ,  12 , and further applied to top ends of the sample  1  and reference substance  2  through the sample-side probe  5  and reference-side probe  6 . 
     Then, the operator sets a desired temperature program to the movement mechanism  22 , and runs a measurement, thereby changing the temperature of the sample  1  and reference substance  2  due to temperature scanning of the furnace  21 . Before temperature scanning, when the entire apparatus is at a room temperature, the temperature of the furnace  21  is equal to a temperature of the sample  1  or reference substance  2 . Accordingly, using the temperature of the furnace  21 , it is possible to calibrate a temperature origin (room temperature) of the sample  1  and reference substance  2 . However, if the temperature of the furnace  21  is scanned, generally a temperature difference of several degrees to several tens of degrees occurs between the furnace  21  and the sample  1  and the reference substance  2 . Therefore, it is impossible to use the temperature of the furnace  21  as substitution for the temperature of the sample  1  or reference substance  2 . 
     Although the sample  1  and reference substance  2  expand as the temperature rises, the amount of expansion at that time appears as relative displacement of core  7   a ,  8   a  to the differential transformer  7   b ,  8   b  provided at a branch portion of the sample-side probe  5  or reference-side probe  6 , and each is detected by the sample expansion measuring circuit  27  or the reference substance expansion measuring circuit  28 . Incidentally, strictly speaking, although the expansion amounts to be measured by the sample expansion measuring circuit  27  and reference expansion measuring circuit  28  are differences between the sample  1  and the sample holder  3 , and between reference substance  2  and the sample holder  3 , they can be neglected when the expansion amount of the sample holder is sufficiently small as compared with those of the sample  1  and reference substance  2 . 
     Because the reference substance  2  has a known coefficient of thermal expansion, what the expansion amount is at a certain temperature is already known. In other words, it is possible to know the mean temperature of the reference substance  2  from an expansion amount of the reference substance  2 . The reference expansion/temperature converter  29  acts to convert the expansion amount of the reference substance  2  into a temperature of the reference substance  2 . Because the sample  1  and the reference substance  2  are in the same shape and placed symmetrical within the furnace  21 , the temperature difference between the both is extremely small. The temperature of the reference substance  2  can be used as substitution for a temperature of the sample  1 . Accordingly, the output of the reference expansion/temperature converter  29  represents a temperature signal for continuously measuring the sample  1  temperature. 
     In the case that the expansion of the sample holder  3  cannot be neglected, an accurate temperature can be determined if sending to the reference substance/temperature converter  29  the data extruding an affection of an expansion amount of the sample holder  3 , i.e. the data of an expansion amount of the sample holder  3  added to an expansion amount measured by the reference expansion measuring circuit. In this case, the sample holder has a known coefficient of thermal expansion. 
     On the other hand, in the subtractor  30  is determined an output difference between the sample expansion measuring circuit  27  and the reference expansion measuring circuit  28 , representing a differential expansion (difference in expansion amount) between the sample  1  and the reference substance  2 . The effect of expansion of the sample holder  3  is cancelled and hence not contained in the output of the subtractor  30 . Accordingly, the expansion amount of the sample  1  can be accurately determined by simply adding the known expansion amount of the reference substance  2  to the output of the subtractor  30 . The resultant data are continuously outputted as TMA signal. 
     From a temperature signal and TMA signal of the sample  1  thus obtained, the analysis is further proceeded hereinunder similarly to the usual thermal analysis case. 
     Incidentally, the present embodiment was explained on the case that the physical property to be measured is a material expansion based on the structure of the differential type TMA apparatus. It is however natural that the application of the present invention is not limited to the differential type TMA apparatus. For example, if the reference substance and the sample are sequentially measured under a same condition, the present invention can be applied to a non-differential type TMA apparatus. 
     If the elastic modulus, dielectric constant or heat capacity of material is selected as a physical property to be measured, it is also possible to construct a DMA, DETA or DSC apparatus with high temperature measuring accuracy with the invention applied. 
     As discussed above, according to the present invention, because a temperature of a sample can be determined through observing an expansion amount of a reference substance, there is no necessity of arranging a temperature sensor such as a thermocouple in the vicinity of the sample. Accordingly, troublesome temperature calibration is unnecessary. 
     Further, because the reference substance entirety plays a role as a temperature sensor, there is no tendency of appearing a difference in heat capacity between the sample and the temperature sensor. Accordingly, temperature detection error will not appear that is due to a difference in thermal response between the sample and the sensor. For example, even where measurement efficiency improvement is emphasized and temperature scanning rate is raised, it is possible to suppress the reduction in temperature measurement accuracy to minimum. 
     Furthermore, because in principle a mean temperature including a temperature distribution inside the substance is outputted as a temperature signal, even where the sample is increased in diameter and a temperature distribution exists inside the sample, it is possible to accurately determine a sample mean temperature. That is, released from problems of temperature measurement resulting from relative position, contactability, contact point or the like of the large diameter sample and the temperature sensor, which occur in measurement by a conventional type temperature sensor. 
     As a result of this, for example, in a precise measurement of expansion coefficient wherein temperature measurement accuracy is important, measurement accuracy can be greatly improved. If in a measurement with same accuracy, it is possible to shorten a measurement time by increasing a temperature scanning rate as compared to the conventional method. Thus, measurement efficiency can be greatly improved.