DO ONLY QUESTION (7.1) UPRATE!
DO ONLY QUESTION (7.1) UPRATE!
DO ONLY QUESTION (7.1) UPRATE!
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DO ONLY QUESTION (7.1) UPRATE!
DO ONLY QUESTION (7.1) UPRATE!
Experiment 4 Phase diagram of a binary solid-liquid system 1. Objective 1.1. To master the principle of drawing phase diagrams by thermal analysis method. 1.2. To construct the phase diagram by measuring the cooling curves. 2. Introduction Phase diagram are often used to represent the relationship between the state, composition, temperature and pressure of systems. The T-x graph with composition as an independent variable and temperature as a dependent variable is the most common type of phase diagram. Solid-liquid phase diagrams are of great value in the technical study of alloys, ceramics and in the recovery of a salt by crystallization from a mixture of salts. Fig. 1 shows the solid-liquid phase diagram of Bi-Cd system. In this diagram we are plotting temperature versus the mass fraction of Cd. At the left, the curve intersects the ordinate at the melting point of pure Bi, and at the right, it intersects the ordinate at the melting point of pure Cd. Note that the freezing point of the solution decreased as we move away from either pure Bi or pure Cd. This is a phenomenon of freezing point depression. The minimum in the freezing-point curve is called the eutectic, and a horizontal line has been drawn along the eutectic temperature. 1 596 E E 4 546 A D DN 249 B C D C B2 D t/min - 0.0 0.2 0,8 1.0 Cd В. l+Cd(s) 416 1+Bi(3) B Bi(s)+Cd(s) 0.4 0.6 Ві w(Cd) Fig. 1 Step cooling curve and phase diagram of Bi-Cd system. To construct such a phase diagram for a binary mixture, thermal analysis is commonly used experimental method in phase diagram studies. This can be achieved by recording cooling curves for sample with known composition as a function of time. The break in the cooling curve gives information of the phase transition temperature. The data point in the phase diagram is identified by the composition of the sample and its phase transition temperature. The reasonable linking of all the data points sketches out the line and regions of the phase diagram. Samples are needed to be heated until complete dissolution occurs, then allow it to cool slowly and measure the temperature at regular time intervals. As shown in Fig. 1, cooling curve A is for pure Cd. The sample cools at an approximately constant rate. Once the temperature is reached melting point of Cd, a “halt” is observed in the cooling curve. The temperature of the substance remains constant until all of the sample freezes. Then the temperature drops rapidly again. The same thing happens at the composition of a eutectic (cooling curve C) and pure Bi (cooling curve E). Cooling curve B is for a mixture with composition between
pure Cd and the eutectic. On this cooling curve we have a break (change in slope) at a point where solid Cd is crystallizing out. Because the heat evolved by solidification partly offsets the heat lost by radiation and conduction to the cold surroundings, a slow rate of cooling is observed. The melt becomes richer in component Bi as component Cd is separating out, and the freezing point of the melt decreases along the curve. The sample then has reached the eutectic composition and a “halt” is observed in the cooling curve. At this temperature, both Bi and Cd will crystallize together. The cooling curve D is similar with curve B. 3. Apparatus and Reagents Burner; Bi; Cd. 4. Procedure 4.1. Sample preparation The samples were placed in the following mass fractions, loaded into six hard tubes. NO.1 NO.2 NO.3 NO.4 NO.5 NO.6 100% Cd 20% Bi 40% Bi 58% Bi 80% Bi 100% Bi 4.2. Draw the cooling curves of the six samples by burner. 5. Data Analysis 5.1. Present the results in the Table 1 Table 1 Experimental data obtained from the step cooling curve Bi content 0% 20% 40% 58% 80% 100% Break point temperature Platform temperature 5.2. Construct a solid-liquid phase diagram for the binary system. 5.3. Find the eutectic temperature and assign the phases of regions in the diagram. 6. Notes 6.1. Make sure that at the beginning of each cooling curve the sample is at equilibrium and in the molten state. 6.2. The thermocouple should be in the sample. 7. Exercises 7.1. What are the factors influencing the slopes of each segment of the cooling curve and the length of the horizontal segment? 7.2. Why should the cooling rate not be too fast during the experiment?