Heat flux type differential scanning calorimeter

Abstract: A heat conducting metal plate 3 soldered and held at an elliptical peripheral end portion to a thermally good conductor-make heat conducting plate support members 3, 4 forms a differential thermocouple between metal wires connected thereto, functioning as a differential heat flow detector excellent in response. This differential heat flow detector is not directly contacted with a heat oven 7, but fixed on a bottom plate of a heat oven 7 through a heat buffer plate 6 by screw fastening. Accordingly, thermal vibration detrimental to stability in heat flow signal is filtered and at the same time the detector can be mounted/dismounted and replaced.

Description: BACKGROUND OF THE INVENTION The present invention relates to a heat analyzing apparatus for measuring as to how a material property changes with temperature. More specifically, the invention relates to a differential scanning calorimeter of a heat flux type for measuring a differential heat flow excessively dissipated from and absorbed by a sample as compared to a reference substance based on a temperature difference (differential heat) between the sample and the reference substance. It is known for a heat flux type differential scanning calorimeter to have a detector structure including types as in Fig. 2 and Fig. 3. The type of Fig. 2 has a detector in a flat plate form soldered to side surfaces of a heating oven formed of a thermally good conductor having a sectional H-character form. The heat flow path within the detector is in a two dimensional form directed radially inwardly from the heat oven side surfaces to a sample section and a reference section The type of Fig. 3 is a mode that a detector directly is placed on a bottom plate of a heat oven formed of a thermally good conductor in a sectional H-character form, as seen in JP-A-60-64250U and in EP-A-645 619, the heat flow path within the detector is provided with a neck formed portion to suppress temperature distribution, and heat flows one dimensionally to the sample section and the reference section. In both the types of Fig. 2 and Fig. 3, the heat oven temperature is based on negative feedback control by a program temperature controller, and accurately controlled linearly with respect to time according to a ramp function. At this time, since the temperature difference between the sample section and the reference section is proportional to a difference in flow of the heat absorbed by or dissipated from them, when multiplied by a proper coefficient, it is regarded as an output of a differential heat flow. In this manner, one type of the differential scanning calorimeter that has a temperature difference output multiplied by a proper coefficient into rating to be dealt with as an output of a differential heat flow is called a heat flux type. In the differential scanning calorimeter of the Fig 2 type, there is a tendency to cause in the heat oven side surfaces a temperature distribution in a direction of A of Fig. 2. In such a case, in the detector directly fixed to the heat oven side surfaces, a temperature difference appear between the sample section and the reference section even in a state that no sample is placed. Due to this, a differential heat flow signal is varied to result in a defect that the accuracy in measurement lowers. Also, it is unavoidable for the detector to suffer exhaustion such as deterioration due to contamination caused by sample decomposition or exposure to high temperature. However, the detector is soldered to the heat oven side surfaces and difficult to be removed. There has been a necessity to replace the detector in accordance with its exhaustion together with a heat oven having its life still left. On the other hand, the differential scanning calorimeter of the Fig. 3 type has been proposed in order to solve the above-described problem. However, because the heat flow path was one dimensionally constituted, the efficiency of heat transfer is worse as compared to the two dimensional structure in the Fig. 2 type. As a result, there were a defect that the time constant in heat flow detection was increased, that is the detector response was lowered. Furthermore, both the Fig. 2 and Fig. 3 types, the heat oven of a good heat conductor in the sectional H-character form and the detector are directly soldered or contacted. It is difficult to avoid conducting fine temperature vibration in the heat oven due to temperature control from conducted to the detector. Due to this, the differential scanning signal is readily ridden on by vibratory noise which posed a factor of reducing signal sensitivity. French Patent Application FR 2428837, published on 11th January 1980, discloses a heat flux type differential scanning calorimeter, in which an electrically heated oven contains two sample tubes which are mounted in respective elastic supports The supports are in turn mounted on respective ceramic spigots on a metal base plate, which sits on the bottom floor of the oven. Heat flow is from the bottom of the oven through the base plate through the insulated spigots to the sample tubes. A differential thermocouple arrangement measures the temperature difference between the two sample tubes. Japanese Patent Application JP-A-05223764, published on 31st August 1993, relates likewise to an electrically heated oven containing two sample containers. The sample contamers in this case are both mounted on an angled metal member, which in turn is secured to the oven floor. A thermocouple arrangement again measures a temperature difference between the two sample containers. A heat flux type differential scanning calorimeter in accordance with the present invention comprises a heating oven with a bottom plate having a cylindrical internal space; a heat buffer means fixed on the bottom plate and formed of a material having a heat conductivity lower than a material of the heat oven; and a differential heat flow detector fixed on the heat buffer means, the differential heat flow detector being not in direct contact with the heat oven and heat flow between the heat oven and the differential heat flow detector being primarily through the heat buffer means, characterized in that the heat buffer means comprises a heat buffer plate. In a preferred embodiment of the invention the differential heat flow detector is constituted by a substantially flat heat-conducting metal plate. A differential thermocouple arrangement may be attached to the metal plate. The differential heat flow detector forming part of the calorimeter is fixed on the buffer plate by screw fastening, and can be mounted/dismounted and replaced if the screws are removed. Also, the differential heat flow detector is structured in a form that a heat-conducting metal plate in a flat plate form is held by a heat-conducting plate support member made of a thermally good conductor. A differential heat flow signal is outputted as a voltage between metal wires welded to the heat-conducting plate. Further, the heat-conducting metal plate is soldered at oval or elliptical formed peripheral end portions to the heat-conducting plate support member. A pair of convex portions are formed in a longer axis direction of the oval or elliptical of the heat conducting metal plate and in symmetrical position about a center thereof. When the temperature of the heating oven is controlled according to a ramp function, one part of the heat flows through the heat buffer plate to the differential heat flow detector in a state that fine thermal vibration is filtered. In the differential heat flow detector, the heat conducting plate support member made of a thermally good conductor functions as a heat sink. Because a difference in heat which flows from the heat sink to a sample section and a reference section composing the pair of convex portions in the heat conducting metal plate is measured by a differential thermocouple formed between the heat conducting metal plate and the metal wire, thus functioning as a differential scanning calorimeter. Embodiments of the present invention will now be described by way of further example only and with reference to the accompanying drawings, in which:- Fig. 1 is an explanatory view having a partly side sectional view showing one embodiment of the present invention. Fig, 2 is a side sectional view showing a conventional apparatus. Fig. 3 is a side sectional view showing a conventional apparatus. [Description of the Reference Numerals] 1 sample vessel 2 reference vessel 3 heat conducting plate 3a sample section 3b reference section 4 upper support plate 5 lower support plate 6 heat buffer plate 7 heating oven 8 set screw 9 lid 10 heater 11a sample side chromel plate 11b reference side chromel plate 12, 14, 15 chromel wires 13 alumel wire 16 double cored insulation tube Hereinafter, an example of a differential scanning calorimeter according to the present invention will be explained together with the drawing. Fig. 1 shows a preferred embodiment of a differential scanning calorimeter according to the present invention, wherein reference character 1 is a sample filled vessel and 2 is a reference substance filled vessel. The sample vessel 1 and the reference vessel 2 are respectively placed on convex portions symmetrically provided in the heat conduction plate 3 made of a constantan, i.e., a sample section 3a and a reference section 3b. The heat conduction plate 3 is soldered through a silver solder in a form clamped in a sandwich form between an upper support plate 4 made of silver in a ring form and a lower support plate 5 made of silver in a cup form. The upper support plate 4 and the lower support plate 5 both have their circular outer peripheries and inner peripheries ovally hollowed out. The lower support plate 5 has, at an underside, an inconel-made heat buffer plate 6 which has high heat proof quality and low heat conductivity as compared to silver. Incidentally, the lower support plate 5 and the heat buffer plate 6 have at nearly centered two positions signal line withdrawing small holes in two positions and fixing screw through-holes in outer four positions. Reference character 7 is a cylindrical heat oven made of silver in a sectional H-character form, and the heat oven 7 is wound at a side surface by a heater 10 with an insulation coating. The upper support plate 4, the heat conduction plate 3, the lower support plate 5 and the heat buffer plate 6 are fixed to the heating oven 7 through set screws 8 at 4 positions (two positions only illustrated). A silver lid 9 is placed on an upper portion of the heat oven 7 to form a close space including the sample section 3a and the reference section 3b, thus achieving thermal evenness by eliminating temperature fluctuation due to convection. Also, the upper support plate 4, the heat conduction plate 3 and the lower support plate 5 have a side surface outer diameter smaller than an inner diameter of the heat oven 7, resulting in free of contact. Accordingly, even where there is a temperature distribution in a direction A of an inner diameter of the heat oven 7, the temperature distribution does not be conveyed to the heat conduction plate 3. The most part of the heat flow directed from the heat oven 7 to the heat conduction plate 3 is conducted to the heat conduction plate 3 by way of the bottom plate of the heating oven 7 and the lower support plate 5. Accordingly, the temperature distribution in a planar direction within the heat conduction plate 3 is greatly reduced as compared to a method that heat directly comes from the side surfaces of the heat oven 7. On an underside of the sample portion 3a and the reference portion 3b of the heat conduction plate 3, small-sized chromel-made disk 11a and 11b are respectively spot-welded at multiplicity of points. Further, In a bottom surface center of the sample-side chromel plate 11a, chromel wires 12, 14 and an alumel wire 13 are welded. On the other hand, at a bottom surface center of the reference-side chromel plate 11b, a chromel wire 15 is welded. The chromel wire 12 and the alumel wire 13 is passed to the underneath of the sample section 3a by way of a sample-side alumina-made double-cored insulation tube 16 penetrated through the lower support plate 5, the heat buffer plate 6 and the heat oven 7. Also, the chromel wires 14 and 15 are passed to the underneath of the reference section 3b by way of a reference-side alumina-made double-cored insulation tube 16. Next, the operation of the apparatus shown in Fig. 1 is explained. First, a measurer opens the lid 9, places on the sample section 3a and the reference section 3b, respectively, a sample vessel 1 packed with a sample to be measured and a reference vessel 2 packed with a reference substance having been confirmed of thermal stability in the temperature range that measurement is to be made, and then closes the lid 9. Next, the measurer inputs a temperature program to a heating oven temperature controller (not shown), to give an instruction of starting measurement. The temperature controller supplies a power to the heater 10 such that an actually measured temperature at a particular point of the heating oven 7 is brought into agreement with the programmed temperature by a well-known negative feedback control, thus exactly controlling the temperature of the heating oven 7. If a temperature difference is caused between respective parts such as the heating oven 7, the heat buffer plate 6, the lower support plate 5 and the heat conduction plate 3, then a heat flow occurs in proportion to the temperature difference and thermal conductance. As a result, the temperature of the sample vessel 1 and reference vessel 2 varies following a temperature of the heat oven 7. At this time, the temperature of the sample within the sample vessel 1 is accurately measured by a chromel-alumel thermocouple with the chromel wire 12 and alumel wire 13 soldered to the underside of the sample-side chromel plate 11a. On the other hand, a temperature difference between the sample vessel 1 and the reference vessel 2, which is an origin of a differential heat flow signal between the sample and the reference substance is determined, as follows. In a circuit from the chromel wire 14 via the sample-side chromel plate 11a, sample section 3a, heat conduction plate 3, reference section 3b and reference-side chromel plate 11b to the chromel wire 15, the chromel wire 14 and the sample-side chromel plate 11a as well as the reference-side chromel plate 11b and the chromel wire 15 are formed of a same material chromel. Accordingly, if the temperature of each section changes, no electromotive force is generated. Also, because the interior of the heat conduction plate 3 is formed of a same material constantan, a temperature change of each section if caused generates no electromotive force. On the other hand, chromel-constantan multiple points of junction points are formed between the sample-side chromel plate 11a and the sample section 3a. The potential of chromel side 11a, 14 increases with increase in average temperature on the junction points. Similarly, in the junction point of the reference-side chromel plate 11b and the reference section 3b, the potential on the chromel side 11b, 15 increases with increase in average temperature on the junction point portion. In this manner, it is understood that the potential difference between the chromel wires 14 and 15 represents an electromotive force of a differential thermocouple representing a difference between a junction average temperature between the sample section 3a and the sample-side chromel plate 11a and a junction average temperature between the sample section 3b and the sample-side chromel plate 11b. Accordingly, it can be seen that the potential difference between the chromel wires 14 and 15 represents a temperature difference between the sample vessel 1 and the reference vessel 2 as measured by a chromel-constantan thermocouple. In order to calibrate the heat flow sensitivity of the present apparatus, the temperature difference may be divided by a thermal resistance of heat flow detection according to the principle of heat flux type differential scanning calorimeter. Also, the output can be calibrated by experimentally determining the heat resistance by measuring enthalpy-known latent heat such as melting heat quantity of indium so that the peak area of a differential heat flow signal areas with the known latent heat. As above, according to the present invention, because there in no contact between the heat oven side surface and the differential heat flow detector side surface, even if a temperature distribution which is liable to occur in the heat oven side surface occurs, a temperature difference hardly appears between the sample section 3a and the reference section 3b thus providing an effect of readily stabilizing the differential heat flow signal. Also, the detector formed by the upper support plate 4, the heat conduction plate 3 and the lower support plate 5 is merely fastened by set screws 8 to the heat oven 7 through the heat buffer plate 6. Because no seizure occurs between the silver and inconel at a temperature of less than 750 degrees, an effect was also obtained that, if the wires 12 - 15 are removed, only the detector can be replaced as a single unit. Further, because the heat flow buffer plate is interposed between the heat oven and the detector, it is possible to prevent fine temperature vibration caused by heat oven temperature negative feedback from being conveyed to the detector. Thus, an effect was obtained that signal accuracy is improved. Moreover, because the heat flow path within the detector is two dimensionally configured, the time constant for heat flow detection can be shortened with a result that a differential scanning calorimeter having an acute peak and high in response was provided. In particular, in the present invention the use of a structure of soldering the heat conduction plate peripheral end in an oval form as a compact shape including the sample section and the reference section in symmetrical positions can reduce the thermal capacity and heat resistance within the heat conduction plate and greatly improve the response, as compared to the conventional example having a circular peripheral end.

Claims: A heat flux type differential scanning calorimeter comprising a heating oven with a bottom plate (7) having a cylindrical internal space; a heat buffer means (6) fixed on the bottom plate and formed of a material having a heat conductivity lower than a matenal of the heating oven; and a differential heat flow detector (3-5, 11-15) fixed on the heat buffer means (6), the differential heat flow detector (3-5, 11-15) being not in direct contact with the heating oven and heat flow between the heating oven and the differential heat flow detector (3-5, 11-15) being primarily through the heat buffer means (6), characterized in that : the heat buffer means comprises a heat buffer plate (6). A calorimeter as claimed in claim 1, wherein the differential heat flow detector (3-5, 11-15) comprises a substantially flat, heat-conducting metal plate (3). A calorimeter as claimed in claim 2, wherein the differential heat flow detector comprises a differential thermocouple arrangement (12-15) attached to the heat-conducting metal plate (3). A calorimeter as claimed in any one of the preceding claims, wherein the fixing of the differential heat flow detector (3-5, 11-15) on the heat buffer plate (6) is made by a screw fastening method (8), so that the differential heat flow detector (3-5, 11-15) can be mounted or dismounted and replaced by screw operation. A calorimeter as claimed in claim 4 as appendant to claim 3, wherein the differential heat flow detector (3-5, 11-15) further comprises a heat-conducting plate support member (4, 5) formed of a material having a higher heat conductivity than the heat buffer plate (6) and to which the heat-conducting metal plate (3) is secured, and a plurality of metal wires (12-15) welded to the heat-conducting metal plate (3) to form the differential thermocouple arrangement, measurement being made on a difference in heat flow absorbed or dissipated by a sample and a reference substance from a voltage between the metal wires. A calorimeter as claimed in claim 5, wherein the heat-conducting metal plate (3) is welded at an oval or elliptical peripheral end-portion to the heat-conducting plate support member (4, 5), a pair of convex portions being formed in a longer axis direction of the oval or elliptic and in symmetrical positions about a center thereof, respective of which correspond to positions of placing a sample vessel (1) and a reference vessel (2).

Inventors: KINOSHITA RYOICHI, NAKAMURA NOBUTAKA, KINOSHITA, RYOICHI, NAKAMURA, NOBUTAKA
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1999
Main classification: G01N25
IPC codes: G01N25, G01K17