广东以色列理工学院 Guangdong Technion-Israel Institute of Technology 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn Materials Science and Engineering Program 315040 - INTRODUCTION TO GLASS SCIENCE Final Exam B-Winter 2022 February 19th– 2023, 09:00-12:00 Your ID Number: First name and Surname: Guidelines: 1. Duration: 2 hours. Use of calculators and personal dictionaries is allowed. Electronic devices, reference materials, personal notes or any other extra material is not allowed. Please write your name and I.D number on the front of each page. 2. Explain your solutions, quote theorems you are using. No credit will be given for non-justified answers! 3. This exam is comprised of 4 questions, each of which is worth 25 points. The highest possible grade is 100. Good Luck! 1 广东以色列理工学院 Guangdong Technion-Israel Institute of Technology 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn Question 1 (25) a) (5) What is the distinction between an amorphous solid and a glass? In other words, what is the specific property of a glass as opposed to a general amorphous material. See exam A. b) (5) Based on the bellow classical diagram for a glass transition, briefly explain the differences between crystallization and glass transition in the context of thermodynamic transitions. Specifically explain (briefly) why the glass transition is not a conventional thermodynamic transition. See exam A c) (7) Briefly explain the main similarities and differences between the crystalline and the glassy counterparts, according to the Continues Random Network model (Zachariasen). Building blocks are about the same with crystalline: 1) Coordination number is the same as in crystalline solids (i.e. building blocks are the same). 2) Bond distances are almost the same with crystalline with low spread. Both are a direct consequence of the same basic chemistry rules when elements form bonds. But there is a big difference than crystalline: 1) A significant spread is permitted in bond angles. 2) There is no long-range order. Both are a direct consequence that the solid didn’t had time to reach the lower enthalpy structure. See relevant course slide 2 广东以色列理工学院 Guangdong Technion-Israel Institute of Technology 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn d) (8) The only stable crystalline composition for the Na-Cl system at ambient conditions is NaCl. Could you make a Na2Cl glass? Explain. (assume you can achieve the required critical cooling rate). In amorphous solids, composition (and thus properties) can be, almost, freely modified. Thus, almost any composition could form a glass. This exact example was discussed in course slides. Bonus!!! As we know the higher the cooling rate the higher the Tg. Is there any upper limit in the Tg we can achieve by increasing the cooling rate? Obviously, Tg cannot be higher than Tm, because in this case a liquid will be formed, instead of a glass. Question 2 (25) a) (8) A typical Angell diagram for three different families of glasses is provided in the figure. a) Indicate which family of glasses corresponds to which curve. This was included in summary slides. Instead of SiO2, a network glass is also correct. 3 广东以色列理工学院 Guangdong Technion-Israel Institute of Technology 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn b) What is the physical state of the materials at Tg/T=0.6? Either liquid or supercooled liquid (depends on Tm). b) (7) Levitation is a modern method aiming to facilitate formation of glasses that otherwise will require very high cooling rate (e.g. BMG). What is the main reason that Levitation allows the formation of such glasses? Aerodynamic levitation is the use of gas pressure to levitate materials so that they are no longer in physical contact with any container. In scientific experiments this removes contamination and nucleation issues associated with physical contact with a container. By eliminating completely any contact between sample and container, it is possible to study the sample with a very high degree of control and to access very high temperatures. c) (10) The Enthalpy as a function of the density for various B2O3 polymorphs are given in the below figure: a) at P= 0.0 GPa b) at P = 1.0 GPa, on the same scale as a). The enthalpy reference, H0, is for B2O3-I as shown by the horizontal dashed line. The vertical dashed line indicates the glass density. Based on this figure, explain why it is impossible to crystalize B2O3 at ambient conditions, while it is relatively easy at elevated pressures. At 0 GPa, there are a lot of structures with similar enthalpies, resulting to the formation of a glass even if cooling rate is low. However, at 1 GPa there is a structure with substantial lower enthalpy than all others, making formation of a crystalline solid straightforward. See similar question in exam A and the case of SiO2 discussed in course slides. 4 广东以色列理工学院 Guangdong Technion-Israel Institute of Technology Bonus!!! 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn If you increase the pressure on a B2O3 glass without changing temperature, do you expect the formation of a crystalline phase? Explain. Not necessary, there is an energy barrier for the transformation from glass to crystalline. Thus, most of the cases an increase of temperature is needed. Question 3 (25) a) (5) briefly explain the structure of a soda lime glass and the role of the different elements (Si, O, Na, Ca). See Exam A b) (12) One of the widely used procedures to increase the hardness of a glass is by ion-exchange: a) Briefly describe the experimental procedure during this process. 5 广东以色列理工学院 Guangdong Technion-Israel Institute of Technology 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn b) Could you use this procedure to increase the hardness of fussed silica (SiO2)? Of course not!!! SiO2 does not have any ions (e.g. Na) that are not part of the network. c) (8) Assuming that the introduction of a modifier results only to the creation of non-bridging oxygens (NBO). How many NBOs per Si atom are expected for the 0.333(Na2O)-0.666 (SiO2) glass? See exam A. There are equal amounts of Na and Si, each Na produces one NBO. Thus, the glass will have 1NBO/Si. Bonus!!! Assuming a) no Q0 and Q1 units and b) Q3 are 50% of the units in the above glass, what will be the ratio between Q4 and Q2 i.e. Q4/Q2? We need on average 1NBO/Si. Q3 has 1NBO/Si and the glass consists 50% of Q3. Thus, the remaining 50% of Q2 and Q4 should have on average 1NBO/Si. Q4 has 0 and Q2 has 2 NBOs, respectively. Thus, Q4 and Q2 should be equal and the ration Q4/Q2=1 6 广东以色列理工学院 Guangdong Technion-Israel Institute of Technology 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn Question 4 (25) a) (15) Consider the following TTT diagram for the three types of metallic glasses A, B, and C, and answer the questions given below: i) Estimate the critical cooling rate of glass ‘B’ when the melting point and temperature at the nose of TTT curve are 700 0C and 500 0C, respectively. Note: The critical cooling rate 𝑅𝑐 for the formation of a metallic glass is given as, ∆𝑇 𝑇𝑙 − 𝑇𝑛 𝑅𝑐 = = 𝑡𝑛 𝑡𝑛 Where, 𝑇𝑙 is the melting temperature, and 𝑇𝑛 and 𝑡𝑛 are the temperature and time at the nose of the TTT curve. This is just a calculation. You just need to roughly estimate tn from figure. ii) Discuss/compare the glass forming abilities of glasses A, B, and C. Glass forming abilities of the three glasses can be compared as follows: Glass Forming Ability of Glass C > Glass Forming Ability of Glass B > Glass Forming Ability of Glass A. This is a direct consequence of the cooling rate needed for each glass; lower the cooling rate higher the glass forming ability. 7 广东以色列理工学院 Guangdong Technion-Israel Institute of Technology 中国广东汕头市大学路241号 241 Da Xue Road, Shantou, Guangdong, China 邮编 Post Code: 515 063 电话 Tel: 86-754-8807 7073 传真 Fax: 86-754-8807 7034 www.gtiit.edu.cn iii) Comment on the statement: “Glass formation will be more favorable if the temperature is held constant above the melting line for an infinite period of time. Is this statement true?” Give a brief explanation. The given statement is incorrect. Explanation: A glass is only formed when the liquid is rapidly quenched below the glass transition temperature. Therefore, if the temperature is held constant above the melting line (i.e., melting temperature Tm) there will be no effect on the glass formation. In other words, no glass formation will take place and liquid will stay in molten state as long as it is kept at the temperature above the melting line. b) (5) Write down any three properties of metallic glasses that make them superior over their crystalline counter parts. The answer is correct if you write any THREE out of following: i) High Hardness ii) Excellent strength iii) Anti-corrosion iv) Biocompatibility c) (5) Enlist any of two methods for the synthesis of metallic glasses. The answer is correct if you write any TWO out of following: i) Melt Spinning ii) Splat Quenching iii)Water Quenching iv)Copper Mold Casting v) Levitation Bonus!!! Enlist one of the major drawbacks of the metallic glasses that hinder their applications as engineering materials. And also state a way/method to overcome this issue? The major defect in metallic glasses is that they are brittle. However, efforts are being made to overcome this issue by synthesizing the metallic glass composites. 8
