MIXKD FUSION ANALYSIS 



ployed: the components may be thoroughly 

 mixed physically or a contact preparation 

 may be made. Melting and solidification 

 phenomena are usually observed on prepara- 

 tions contained between a microscope slide 

 and cover-glass; however, it is sometimes 

 desirable to contain the mixture in a sealed 

 micro-capillary tube. 



Equipment. The equipment necessary 

 for microscopic mixed fusion analysis is usu- 

 ally quite simple. Any microscope with 

 magnification up to about 200 X and capable 

 of accommodating a hotstage is satisfactory. 

 Some method for achieving polarized light 

 is desirable; although this may be as simple 

 as two pieces of "Polaroid" mounted to serve 

 as a polarizer and an analyzer. For observa- 

 tions at various temperatures, a good hot- 

 stage is mandatory. Hot stages covering the 

 range 30-350°C are commercially available 

 as is a coldstage capable of functioning above 

 — 100°C. In addition, there are published in 

 the literature designs for both hot and cold 

 stages applicable at more extremes of tem- 

 perature. Temperatures are measured either 

 with a thermometer or a thermocouple. The 

 commercial stages employ thermometers. 

 Temperatures and heating rates are usually 

 controlled with a variable resistor or a 

 variable transformer. It is imperative that 

 the temperature measuring devices be 

 accurately calibrated with known melting 

 point standards at a heating rate nearly 

 identical with that at which an unknown is 

 to be measured. Usually, a standard heating 

 rate (such as 3°C/min) is selected and all 

 calibration and melting point determinations 

 are made at this heating rate. It is very 

 helpful to have a good voltmeter in parallel 

 with the hotstage heating element so that 

 heating rates may be approximately ad- 

 justed to the desired value at any given 

 temperature from predetermined stage char- 

 acteristics. 



A hot bar is a very useful adjunct to the 

 hotstage microscope, especially for contact 

 preparations. These devices are commer- 



cially available or may be constructed in the 

 laboratory. However, it is possible to employ 

 an alcohol lamp, a micro burner, or even a 

 soldering iron to achieve the same ends. A 

 micro sul)limator for purification of com- 

 pounds is also a useful item for microscopic 

 mixed fusion analysis. 



Contact Preparation Methods. Con- 

 tact preparation methods rapidh^ yield in- 

 formation concerning the nature of the phase 

 diagram of a two-component system. A more 

 restricted definition of microscopic mixed 

 fusion analysis would include only contact 

 preparation methods. A contact preparation 

 is made in the following fashion. A small 

 amount of the higher melting component (A) 

 is melted between a microscope slide and 

 cover-glass so that, on solidification, approxi- 

 mately one half of the area of the cover-glass 

 contains crystalline material. A small 

 amount of the lower melting component (B) 

 is then placed adjacent to the crystals of 

 component (A) and in contact with the 

 cover-glass. When the slide is warmed so 

 that component (B) melts, the melt flows 

 under the cover-glass and dissolves a portion 

 of component (A). The entire preparation is 

 then allowed to solidify. It is freciuently de- 

 sirable, in order to insure adequate mixing, 

 to melt back the preparation so that most 

 of the higher melting component is melted 

 and then to allow the entire preparation to 

 resolidify. 



A concentration gradient, ranging from 

 pure A on one side to pure B on the other 

 side, exists in such a preparation. All inter- 

 mediate compositions exist in some area of 

 the zone of mixing. ^Microscopical observa- 

 tion of the mixing zone dm'ing the cooling 

 process yields information concerning the 

 nature of the phase diagram between A and 

 B. If the system is simple eutectic, both 

 components will crystallize rather rapidly 

 vmtil the crystal fronts reach the mixing 

 zone. Crystal growth will then proceed at a 

 greatly reduced rate. When the eutectic tem- 

 perature is reached, fine grained crystals of 



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