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Laboratory Report

Structure and Properties of Non-metals Experiment 4 Viscosity and Softening Point of Soda-lime and Borosilicate Glasses Yufei Chang • Group X5 • 19/10/07


The aim of this experiment is to ditermin the viscosity and softening point of soda-lime and borosilicate glasses by fibre elongation method. Measuring the viscosity of glasses over an appropriate temperature. From the experiment results we got viscous is flow an activated process. And at any temperature the viscosity of a borosilicate glass is greater than that of a soda-lime glass. These will be discussed at the end of this report.

The method we used is suspending the glass fibre(must have a circular cross section) vertically in a small tube furnace . Measuring the temperature and elongation in every minute. In order to carry out this experiment we need use micrometer to get the the diameter(between 0.55 and 0.75 mm which is uniform over the entire length). The glass should be 23.5 cm long and not including the top head. Measurement of elongation:Preheated the furnace to 500째C. (take care when lower the glass fiber into the furnace). After about 5 minutes, the furnace temperature should be rising steadily at 5째C per minute. We have to use telescope(with a travel on it and used like a vernier scale) to focus the bottom of the fiber. Reading the number of elongation and furnace temp every minute as the fiber flowing downwards. These reading should be continued until the travel on the telescope can on longer follow the extension. Record the measurement in table like

Time(min)

Furnace temp (째C)

Elongation reading (cm)

Elongation rate(mm/min)

The experiment data-block is attached at the end of this report.


Plot the data on a semi-log graph paper ,x-axis is the temp and y is elongation rate. The y-axis is log axis and x is linear axis.


The softening point temperature is corresponding to the elongation rate 1.00 per min. And from the graph above we could easily find the softening point of Soda-lime glass is 718°C,and Borosilicate is 838°C approximately. To calculate the viscosity η. Use

L=initial length of the fiber (in m) L’= initial length of the fiber below the centre of the furnace ρ= density of the glass (kg/ m³) E= elongation rate of the fiber (m/s) g=9.81 and give the viscosity in Pa s. To determine the activation energy for viscous flow in the glass use equation η = Aexp(Q/RT) where A is contant Q is activation energy in KJ/mol R= Gas constant (8.314 J/mol k) the density of Soda-lime glass is 2.49×10³ kg/m³ , Borosilicate is 2.23×10³ kg/m³ Transfer this equation into linear correlation ln(η) = ln(A)+Q/RT since Q/R is a gradient of this line ,we could get it by plotting a graph

Temp

1/T

Elongation rate

Elongation rate

°C

Kˉ¹

(mm/min)

(m/s)

Viscosity (Pa s)

ln(η)


From the graph , we get the gradient of best fit line for Soda-lime is 45472 for Borosilicate is 35741 Hence we get the Q of Soda-lime glass is 3.78×10⁶ KJ/mol and Borosilicate glass is 2.97×10⁶KJ/mol

Conclusion:After this experiment, we can find that the viscosity of “Boro” at any temperature is higher.The softening point and activation energy of Soda-lime glass is higher.

Discussion: 1

For most materials, it must overcome an internal resistance of that material to de-

form like fluid . This is the prerequisite for viscous flow.The measure of viscous flow is viscosity and it is commonly perceived as resistance to flow or fluid friction. The resistance is the internal force provided by bonds. If a certain material made a viscous flow ,it must provide enough energy to overcome the force,to break bonds. And the threshold energy is know as activation energy. Since this energy needed this is an activated process. (There are also ideal fluid or inviscid fluid which has no resistance to couse viscous flow)


2

No negative sign in the exponent in the above equation is due to viscosity inverse

proportional to the elongation rate.

3

Soda-lime glass can be create by melting a mixture of silicon dioxide,sodium carbon-

ate,and either calcium oxide. Sodium and calcium ions are inserted into the silicate ion structure such that tetrahedrons of silicon ans oxygen atom are stretched. Borosilicate glass is created by adding boron to the ordinary glass made by SiO₂.And it has a higher viscosity than Soda-lime glass. This is due to the properties of their bonds. There are many B-O bonds in Borosilicate glass, the bond energy is 536 KJ/mol , it’s higher than Si-O ( 452 KJ/mol). Each boron ion could bond with three oxygen ions, Si can only bond with one.So the energy needed to make Borosilicate viscous flow is higher , when at the same temperature,the viscosity of “Boro” is greater.


Time(min)

Furnace temperature(℃)

Microscope reading(cm)

Elongation rate(mm/min)

0

650

18.616

0

1

651

18.59

0.26

2

652

18.6

-0.1

3

653

18.606

-0.06

4

655

18.61

-0.04

5

658

18.604

0.06

6

662

18.56

0.44

7

666

18.59

-0.3

8

671

18.585

0.05

9

676

18.55

0.35

10

681

18.53

0.2

11

687

18.524

0.06

12

692

18.508

0.16

13

698

18.484

0.24

14

705

18.446

0.38

15

711

18.418

0.28

16

717

18.352

0.66

17

723

18.296

0.56

18

730

18.18

1.16

19

736

18.056

1.24

20

742

17.836

2.2

21

747

17.616

2.2

22

753

17.322

2.94

23

758

16.842

4.8

24

764

16.18

6.62

25

770

15.226

9.54


Time(min) Furnace temperature(℃)

Microscope reading(cm)

Elongation rate(mm/min)

0

750

18.61

0

1

752

18.614

0.04

2

755

18.62

0.06

3

759

18.628

0.08

4

764

18.61

0.18

5

770

18.674

0.64

6

775

18.678

0.04

7

782

18.684

0.06

8

788

18.67

0.14

9

794

18.642

0.28

10

800

18.52

1.22

11

807

18.488

0.32

12

814

18.47

0.18

13

819

18.422

0.48

14

826

18.386

0.36

15

832

18.326

0.6

16

838

18.266

0.6

17

844

18.18

0.86

18

849

18.08

1

19

855

17.966

1.14

20

860

17.808

1.58

21

866

17.622

1.86

22

871

17.374

2.48

23

877

17.008

3.66

24

882

16.746

2.62

25

887

16.366

3.8


Lab report Viscosity and Softening Point of Soda-lime and Borosilicate Glasses