Unformatted text preview:

Addenda to Girolami et. al. Experiment #1, Preparation of YBa2Cu3O7-xIntroductionWhile the introduction in your textbook is good, it fails to discuss a keyrelationship between the oxygen content of your superconductor and itsstructure. Therefore, you will need to read at least one of the first 3 referenceslisted below before you attempt to draw conclusions from your data (and wellbefore the night your report is due):C & E News, December 21, 1992, pages 24-41 (very readable).Cava, Scientific American, Aug 1990, pages 42-49. Very readable withnice color graphics. Available in Young library, oversize section, callnumber T1.S5.Cheetham and Dey Solid State Chemistry: Compounds, 1992 (onreserve). Read relevant parts of the chapter on superconductivity.Sleight, Science, 1988, 242, (Dec 16), p 1517-1527. (more complex)In addition, we will be performing powder X-ray diffraction on your product, soyou will need to read the following items:West, Anthony Solid State Chemistry and its Applications. Chapter 2.1(Solid State Reactions)ibid, Chapter 5 (powder X-ray diffraction...this is a very long chapter; justtry to get the basic idea for now)Synthesis (Day 1)Do not follow the procedure in your text book. Your starting materials forthis experiment will be Y2O3, BaCO3 and CuO. It is up to you to determinethe proper amounts of reagents to use for this reaction, but you should tryto make 3 -5 grams of product (your yield will be nearly quantitative).Make sure you come up with a balanced equation for the synthesis of yourproduct and record your calculations in your notebook! Follow this genericprocedure:Place your raw materials in a mortar and pestle. Question: Why are wedoing this?Add enough acetone to form a thick slurry (makes it easier to grind).Grind your slurry until you have a homogeneous gray powder. Continuegrinding until all of the acetone has evaporated (10 to 15 minutes total).Place your powder into an alumina boat and place it into a tube furnace.The one we're using is shown below. Fire your material overnight at 940degrees. It is not necessary to flow oxygen in this first step, although youcan try it if you wish. Arrange with your instructor to come in the followingday for approximately 30 minutes to perform the following two steps.Your instructor will turn the furnace off the following morning. Remove yourmaterial from the furnace using a push rod. Let your product cool to roomtemperature. Place it in a mortar and pestle and regrind the material (don'tbother using acetone). Question: Why are we doing this?Press your product into pellets using the pneumatic pellet press. Placeyour pellets and any leftovers in the tube furnace and heat it under a slowflow of pure oxygen at 940 degrees Celsius overnight. Question: why are2we doing this? Your instructors will turn off the furnace the followingmorning.Characterization (Day 2)Characterize your alleged superconductor by the following three methods.For the first two methods, you will need to regrind a portion of yourmaterial (without acetone). Be sure to save one of your pellets for the thirdmethod!Iodometric Titration. Follow the procedure given in your textbook. Iodometric titrations are verysensitive to oxygen (but not water). You can attempt to do the titration onthe Schlenk line if you wish.Powder X-ray Diffraction. Powder XRD is often used by chemists, physicists, geologists andmaterial scientists to characterize unknown microcrystalline materials.Please read the appropriate background readings before attempting thisexperiment.To use X-ray equipment you have to be a certified operator and have aradiation badge through the University. Therefore, your TA will run theXRD of your sample for you, but you will accompany him to the XRD lab.He will explain the workings of the unit.Your instructor will demonstratehow to prepare a sample for the XRD during the first laboratory period.Interpret your XRD spectrum, taking particular note to account for anylikely impurity phases, such as the "green phase". You will have to go tothe library and look up XRD spectra of authentic samples of thesematerials (the reference in Angelici is not acceptable for comparison). Youmust consult the primary literature to write your lab report!Magnetic Levitation.The TC of your material is above that of liquid nitrogen. Therefore we willtest your sample to see if it exhibits the Meissner Effect. Question: Whatthe heck is the Meissner effect?Place your oxygen-treated pellet into a shallow dish (a cut-off styrofoamcoffee cup works well). Carefully fill the dish with liquid nitrogen. Caution:Liquid nitrogen is -196 degrees C (77 K) and can cause instantaneousfrostbite! The nitrogen will boil off very rapidly at first -- add more liquidnitrogen as necessary.3Your TA will give you a very powerful (but small) neodynium magnet. Usea pair of non-magnetic tweezers to place the magnet above your cooledsample. If your sample is superconducting, the magnet will levitate abovethe sample. If so, measure the distance that the magnet is elevated. Besure the magnet is dry before returning it to the instructor.If we had time, we would perform resistivity or SQUID measurements todetermine the TC of your material. In addition, we could perform aRietveld analysis of your material to try and "solve" the crystal structure.Finally, we could determine the oxygen content using thermogravimetricanalysis (TGA) as a comparison to our titration. These are powerful toolsthat you may want to consider in your independent or


View Full Document

UK CHE 450 - LECTURE NOTES

Download LECTURE NOTES
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view LECTURE NOTES and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view LECTURE NOTES 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?