1、IS0 7884-l INTERNATIONAL STANDARD INTERNATIONAL ORGANIZATION FOR STANDARDIZATION ORGANISATION INTERNATIONALE DE NORMALISATION MEXAYHAPOAHAR OPTAHl43AIJWl IlO CTAHAPTMSAklM .;p&;j ,: : : . 9 Y, ,; _, S-II .,“yf t ;.: Y: ;1;4 ,!.-, 5.f” o is the surface tension in millinewtons per metre. For Q = 2,5 g
2、/cm3 and o = 300 mN/m the viscosity is da = 107.6 dPas In most cases Of2 lies close to that value; it shall be used if the values of Q and cr are unknown. In extreme cases qf2 ranges from 107,5 to 108 dPas. The softening point can also be determined by the method described in IS0 7884-2. 3.3.3 annea
3、ling point rYf3: The temperature determined in accordance with the method described in IS0 7884-7. NOTE - From the beam bending method a non-equilibrium viscosity of 1O1a,2 dPas is assigned to the annealing point. From various measurement techniques, a non-equilibrium viscosity of 101s to lots.2 dPa
4、s has been found to approximate to the viscosity at the annealing point. 3.3.4 strain point r9f4: The temperature determined in accor- dance with the method described in IS0 7884-7. NOTE - From the beam bending method a non-equilibrium viscosity of 1014.7 dPas is assigned to the strain point. From v
5、arious measurement techniques, a non-equilibrium viscosity of 1014,5 to 1014J dPas has been found to approximate to the viscosity at the strain point. NOTE - The order of magnitude of the constants for technical glasses is as follows : A = - 100 (decadic logarithm of viscosity in decipascal seconds)
6、 B = 103T c = 102oc 4.2 Determination of the constants Three pairs of measured temperatures and equilibrium viscosities i = 1, 2, 3, covering the range of the measurement and separated sufficiently from each other, are chosen to calculate the constants of the VFT-equation by means of equa- tions (3)
7、 to (5) : c=v, + (192 - l9,) . it93 - L91) * (lg 43 - lg 472) (lY2 - V,) . (lg 1/3 - Ig tll 1 - h93 - I. ( therefore, any change in composition of the glass should be avoided as far as possible, in particular by taking the following precautions : - do not grind to powder (to avoid water adsorption);
8、 - do not heat above the measuring temperatures if possi- ble, or heat only for as short a time as possible (to avoid loss of volatile components); - do not heat for a long time within the temperature region of maximum rate of crystallization; - select pieces without inhomogeneities, inclusions or b
9、ub- bles. 1) Pythagoras IS an example of a suitable product supplied by W. Haldenwanger Technische Keramik GmbH I: 0,015 0 700 7,80 4:6 0,020 0 600 10.74 883 0,036 0 950 I,86 23 0,011 2 850 3,38 4,l 0,017 6 103 750 5,55 7,4 0,032 0 “short” i: 7,78 12,6 0,054 8 IO,66 13,4 0,058 0 600 1354 13,4 0,058
10、0 5 / IS0 7884-l : 1987 (E) As an example for a typical technical glass (reference glass DGG 1, see table 31, table 2 gives a survey on the influence of the error in viscosity introduced by rounding the constants of the VFT-equation. Table 2 - Influence on error introduced by rounding constants A, B
11、 and C Rounding of constants A, B, C Temperature Relative error (Arlr) x 100 % in the case of constant to . significant to the nearest . . . ff A B C figures unit(s) in the last digit OC maximum maximum maximum 1400 0,oz 0,Ol 5 2 800 0.02 O.J 0,03 560 0.08 0,ll 1400 0,05 0.02 5 5 800 0,06 0,lO 0,08 550 0,19 0.28 1400 or1 0,04 4 1 800 O,l 02 02 550 0,4 33 1400 02 O,l 4 2 800 02 0,4 0,3 550 0.8 1,1 1400 0,5 4 5 800 0.6 I,0 % 550 1.9 2:8 1400 Or4 3 1 800 12 ii? 1,6 550 3:9 5,5 6
