ISO 7991-1987 Glass Determination of coefficient of mean linear thermal expansion《玻璃 热平均线膨胀系数的测定》.pdf

1、INTERNATIONAL STANDARD INTERNATIONAL ORGANIZATION FOR STANDARDIZATION ORGANISATION INTERNATIONALE DE NORMALISATION MEXjJYHAPOjJHAR OPrAHM3AMR IlO CTAH/JAPTkl3AMM 1 d 11 ,j b : .y 2. ,- : n*.P* B .i , _, -;_, i.i.ji,*:fs *:Li t) : The ratio of the change in length of a specimen within a temperature i

2、nterval to that temperature interval, related to the initial specimen length. It is given by the following equation: I - I, cd,; t) = f x - . . . 0 t - to to is the initial or reference temperature; t is the actual (constant or variable) specimen tem- perature; I, is the length at temperature to of

3、the specimen of glass under test (usually a rod made from the glass); I is the specimen length at temperature t. For the purposes of this International Standard, the nominal reference temperature, to, is 20 OC; therefore the coefficient of mean linear thermal expansion is denoted by a(20 YZ; t). 4 A

4、pparatus 4.1 Device for measuring the specimen length, to an ac- curacy of 0,l %. 4.2 Push-rod dilatometer, capable of determining changes in length of the specimen of 2 x 10e5 L, (i.e. 2 pm per 100 mm). The contact force of, the extensometer should not exceed 1 N. That force shall act through conta

5、cts of planes with spherical faces the radii of curvature of which shall be not less than the rod diameter of the specimen. In some special assemblies (see figure I) parallel planes are needed. The specimen-holding assembly shall ensure that the specimen is held firmly in position and shall prevent

6、even small changes in its alignment with respect to the push-rod axis throughout the test (see examples given in the annex). If the specimen-holding assembly is made of vitreous silica, see the precautions given in 7.2. From time to time, a performance test shall be carried out using a reference mat

7、erial (see clause 8). 4.3 Furnace, compatible with the dilatometer assembly, for temperatures up to 50 “C above the expected transformation temperature. The working position of the furnace relative to the dilatometer assembly shall be defined with a repeatability of 0,5 mm in both the axial and the

8、radial directions. Within the range of testing temperatures (i.e. up to tempera- tures about 150 OC below the highest expected transformation temperature, ty and at least up to 300 Cl, the furnace shall be capable of marntaining a constant temperature to + 2 OC over the whole specimen length. 4.4 Fu

9、rnace control device, suitable for the desired rate of increase in temperature up to (5 + I) OC/min within the test range (see 6.1) and for a cooling rate of (2 + 0,2) “Urnin for the annealing procedure according to 5.2. 1 IS0 7991 : 1967 (El 4.5 Temperature-measuring device (e.g. a thermocouple of

10、type E, J or K in accordance with IEC 684-11, capable of determining the temperature of the specimen to f 2 OC in the temperature range between to and t. 5 Test specimen 5.1 Shape and size The test specimen is usually in the form of a rod. Its shape depends on the type of dilatometer used. The lengt

11、h I, shall be at least 5 x lo4 times the resolution of the dilatometers measuring device for the change in length. NOTE - The specimen may be, for example, a rod either with a cir- cular cross-section having a diameter of 5 mm or with a square cross- section 5 mm x 5 mm, and between 25 and 100 mm in

12、 length. In cer- tain cases, a cross-section of at least 100 mm2 is more convenient (see the annex). 5.2 Preparation The test specimen shall be annealed before the test by heating it to about 30 YZ above the transformation temperature and then cooling it to about 150 OC below the transformation temp

13、erature at a rate of (2 + 0,2) Wmin, followed by further cooling to room temperature in draught-free air. 5.3 Number The test shall be carried out with two test specimens (see also 7.4). 6 Procedure 6.1 Choice of the test range In accordance with clause 3, the nominal reference temperature is 20 OC.

14、 For practical reasons, however, the measurement may be started between 18 and 28 OC. The preferred final actual temperature is 290 OC t 310 OC. If this is not practical, then the alternative values 190 OC t 210 OC, or, in special cases, !XC t 105OC or 390C t 410C may be chosen. The corresponding no

15、minal values of t are 300 OC, 200 OC, 100 OC, and 400 OC, respectively. All readings of temperatures and temperature differences shall be taken with an accuracy to 2 OC. Though these actual values are used in the calculations in accordance with clause 7, the test range shall be expressed in terms of

16、 the nominal tem- peratures (see 7.4). For a given coefficient a(20 OC; t) ex- pressed in terms of the nominal temperature, no influence on the value of the coefficient can be detected within the limits specified for the preferred actual temperatures. 6.2 Determination of the reference length Determ

17、ine the reference length 1, of the annealed specimen (see 5.2) to an accuracy of 0,l % at the reference temperature t,. Subsequently insert the specimen into the dilatometer and wait for about 5 min before beginning the test as described in 6.3 or 6.4. 6.3 Test at increasing temperature Determine th

18、e position of ;he dilatometer at the initial temperature to and take this reading as zero for the uncorrected change in length, AI, which will be measured. Subse- quently set the furnace control device (4.4) to the desired heating programme and start the programme. Record the temperature t and the r

19、elated change in length Al, until the desired final temperature has been reached. NOTE - The rate of temperature increase should not exceed 5 Wmin. As the dilatometer readings of Al, are recorded during the increase in the temperature between to and t (values chosen in accordance with 6.11, it shoul

20、d be borne in mind that a temperature difference will exist between the hot junction of the thermocouple and the test specimen; therefore a correction shall be applied to the apparent temperature of the test specimen. NOTE - The magnitude of this correction depends on the rate of temperature change

21、and the rate of heat exchange between the fur- nace and the test specimen. It is essential that the correction is deter- mined experimentally by comparison with measurements at constant temperatures. 6.4 Test at constant temperature Determine the position of the dilatometer at the initial temperatur

22、e to and take this reading as zero for the uncorrected change in length, AI, which will be measured. Subse- quently heat the furnace to the selected final temperature t and hold it constant to f 2 OC for 20 min. Then take from the dilatometer reading the value of Al,. NOTE - Although the test at inc

23、reasing temperature (6.3) enables a set of coefficients cd,; t) with various values of t to be determined in one test run, the test at constant temperature (6.4) should be preferred if only one final value of t is required since this test affords the better precision. 7 Expression of results 7.1 Cal

24、culation of the final length From the measured change in length, Al,as, the corrected length I at temperature t is calculated using the following equa- tion : I = I, + AI, -I- Ala - AIB . . . (2) where the correction terms AIo and AIB are explained in 7.2 and 7.3 respectively. 7.2 Calculation of the

25、 expansion of the specimen- holding assembly In the case of a simple push-rod dilatometer, the correction term Alo in equation (2) is the thermal expansion of that part of 2 IS0 7991 : 1987 (E) the specimen-holding assembly alongside the specimen, hav- ing the length I, at temperature t,. In the cas

26、e of a differential push-rod dilatometer, the correction term Alo is the expansion of a reference rod with the specimen length 1, at temperature to. In either case, the correction term Alo is calculated using the following equation : Ala = I, . aa . (t - t,) . . . (3) where oo is (in the case of a s

27、imple push-rod dilatometer) the coefficient of mean linear thermal expansion of the material from which the specimen-holding asembly is made or (in the case of a differential push-rod dilatometer) the coefficient of mean linear thermal expansion of the material of the reference rod. If specimen-hold

28、ing assemblies, push-rods or reference rods are made from vitreous silica which is essentially hydroxyl-free, the values of oo given in the table may be used. Before these parts of the dilatometer are used for the first time, they shall be annealed for 7 h at 1 100 OC, and then cooled from 1 100 to

29、%I0 OC at a constant rate of 0,2 Y/min. In order to avoid devitrification of vitreous silica the surfaces shall be kept clean. It is recommended that they are cleaned twice with analytical-grade alcohol, after which contact with bare fingers shall be avoided. Table - Coefficient of mean linear therm

30、al expansion aa for vitreous silica 21 NOTE - The values of oa given in the table are altered if the system is heated to above 700 OC. 7.3 Determination of the dilatometer correction The dilatometer correction term A/a is needed mainly because of irregularities in temperature distribution within the

31、 transient range between the specimen at temperature t and the exten- someter at ambient temperatures. The dilatometer correction term should be determined by means of a blank test. In the case of a simple push-rod dilatometer, the specimen for the blank test is made of the same material as the dila

32、tometer. If that material is vitreous silica, the specimen for the blank test shall be annealed in accordance with 7.2. In the case of a differential push-rod dilatometer, two identical specimens of any suitable material can be used. The measurements on glass and the blank test shall be carried out

33、under identical conditions. The blank test shall be repeated at least whenever a performance test in accordance with clause 8 is carried out. 7.4 Calculation of the coefficient of mean linear thermal expansion In order to calculate the coefficient of mean linear thermal ex- pansion, a(t,; t), insert

34、 the measured values of I, and AI, the corrections established in accordance with 7.2 and 7.3, and the actual values of to and t (with t corrected, if it is determined using the test at increasing temperature) into the following equation : a(t,; t) = +x f%,S -I- Ala - Als . . . 0 t - to Calculate a(

35、20 T; 300 T), a(20 T; 200 T), a(20 T; 100 T) or a(20 OC; 400 W for the two test specimens (5.3) to two significant figures if a(20 T; t) 10 x 10-a K-l. If the results for the two test specimens differ by not more than 0,2 x low6 K-l, take the arithmetic mean. If the difference is larger, repeat the

36、test with two other test specimens. 8 Performance test In order to check that the whole test device is functioning cor- rectly, the test procedure and calculation laid down in clauses 6 and 7 shall be carried out on a specimen of a reference material, the value of the coefficient of mean linear ther

37、mal expansion of which is certified. I) Recommended reference materials are as follows : - vitreous silica annealed according to 7.2; - sapphire single crystal; - chemically pure platinum. NOTE - Sintered alumina fAl203) as a reference material is very insen- sitive to the thermal treatment applied

38、in the test procedure laid down in this International Standard. However, the values of the mean linear thermal expansion coefficient differ from one rod to another. The shape and dimensions of the reference specimen shall be similar to those of the specimens usually tested in the test device. Care s

39、hall be taken to ensure that the thermal expansion behaviour of the reference material is not altered by the test. If the reference material is a glass, it shall be annealed (or re- annealed) in accordance with 5.2, unless other procedures are specified by the certifier. 1) Enquiries about sources o

40、f certified reference materials (Cl3M.s) may be addressed to the Secretariat of REMCO, International Organization for Standardization (ISO), 1, rue de Varembe, Case postale 56, CH-1211 Geneva 20, Switzerland. 3 IS0 7991 : 1997 (El 9 Test report The test report shall include the following information

41、 : a) reference to this International Standard; b) specification, type and state of delivery of the glass tested : c) shape, size and number of test specimens; d) type of push-rod dilatometer used; e) type of test run (constant or increasing temperature, rate of increase); f) coefficient of mean lin

42、ear thermal expansion a(20 OC; t) expressed in 10-s K -1 - to two significant figures, if a(20 “C; t) 10 x 10-s K-1. For the temperatures t, and t, use the nominal values (see 7.4). 4 IS0 7991 : 1987 (El Annex Devices for self-adjusting alignment of specimen and push-rod axis (This annex forms an in

43、tegral part of the standard. 1 Ideally the axes of the test specimen and push-rod coincide, and the length I, should lie in the same axis. In practice, small deviations between the axes of the test specimen and push-rod may occur. Such deviations are negligible only when that misalignment remains co

44、nstant throughout the test. Similar considerations hold true for the push-rod direction and the working direction of the extensometer. Changes in alignment (e.g. caused by vibration of the apparatus) shall be avoided by appropriate devices as shown in the examples (figures 1 and 21. An example for m

45、inimizing changes in alignment in a dilatometer assembly working almost vertically is illustrated in figure 1. The guiding devices made from platinum wire prevent further lateral changes in the position of specimen and push- rod once the stable position is achieved by slight shaking. The axial movements caused by thermal expansion, however, are not hindered. Exactly vertically mounted dilatometer assemblies have been found to be the most sensitive with respect to changes in alignment during the test. 5