25 Jun 2013

The Synthesis and Characterization of Cobalt Spinels

Lab Experiment

Submitted by Rebecca Ricciardo, The Ohio State University
Categories
Description: 

In this lab, students will use solid-state methods to synthesize cobalt and chromium spinels, ZnCr2O4, ZnCo2O4, CoAl2O4, and CoCr2O4. They will (1) characterize their structure with X-ray powder diffraction (XRD) and (2) characterize the color using UV-Vis diffuse reflectance spectroscopy.

Learning Goals: 

Upon completing this lab, students will:

  • Know how to synthesize polycrystalline materials using solid state synthesis techniques.
  • Gain experience preparing solid samples for diffraction analysis and collecting XRD data.
  • Be able to index XRD patterns from cubic materials to determine unit cell parameters.
  • Relate reflectance spectra to the visible colors of materials.

 

Equipment needs: 

Analytical balances

Mortars and pestles (agate or porcelain)

Crucibles (alumina or porcelain)

High temperature furnace (1000˚C)

Powder X-ray diffractometer

UV-Vis spectrometer equipped to do diffuse reflectance

Implementation Notes: 

This lab is the first lab in an upper level inorganic laboratory course. It is completed first, because the students must weigh out reagents and grind them in a mortar and pestle and then heat them for 8 hours at 1000˚C. The synthesis does not take a full lab period, which allows time for the first-day housekeeping items to be distributed and discussed. During the second lab period, student samples are returned and the students collect their XRD data and UV-Vis data. There is enough time for students to begin working through the analysis in class.

Note: This lab lends itself to many adaptations.

  • If there is no XRD access readily available for students, patterns may be provided so that students can index these, even if they have not collected the data.
  • The electronic transitions of these compounds may be analyzed with more detail (ie. assigning transitions).
  • Magnetic measurements may be made and the number of unpaired electrons deduced (this is Exp 3 that is referred to in the attached lab document).

 

Time Required: 
Sample must be heated for 8-10 hours, so the synthesis and characterization must be completed at two separate times: (1) 1-hour period for synthesis of compounds and (2) a 3-hour lab period for analysis.
Evaluation
Evaluation Methods: 

Students complete pre- and post-lab questions; these are within the document. Upon completion of the lab, the student is to write a lab report. A lab report is written in a format similar to that of a paper submitted for publication. Sections include the title & authors, abstract, introduction, experimental, results, discussion, conclusion and references.

Evaluation Results: 

The average score for the prelab assignements was 65% with a high of 100% and a low of 35%. The average score for lab reports was 92% with a high of 100% and a low of 71%. 

Students were able to index the patterns using the supplement (attached) as a guide. 

Creative Commons License: 
Creative Commons Licence

Comments

I have a question about the implementation of this lab.  We do have a programmable tube furnace but I was thinking it would be more efficient to do multiple samples in our non-programmable muffle furnace.  Any thoughts about whether just putting it in a cold muffle furnace and turning it on would be a slow enough ramp to do this, or if there is a way to ramp slowly but manually?    

Hi Hilary,

Do you have an estimate of your natural ramp rate if you just turn the furnace on and set to a high temperature? The rate we use, 10 C/min, is fairly quick. I suspect that the ramp up rate is not that important, but the ramp down rate may have more of an impact. 

If you are able, it will work just fine to manually control the temperature: increase/decrease by 10 degrees every minute. However if your furnace is well insulated, it may naturally ramp down slowly. In my experience, I suspect that you would be fine only watching it on the way down from 1000 to 600. Past 600 it is likely that the natural furnace cooling will be sufficiently slow.

All the best,
Rebecca

So just to report back.  I finally did this lab with the muffle furnace (and  scaled down to 1 g in mini crucibles).  The Co versions worked well, but the Zn versions seem like they haven't reacted entirely, particularly the ZnCr2O4 version.  The latter is a kind of light green with brown specs so I'd imagine I need to go longer or hotter.   Any suggestions?   It is supposed to be black-brown right?      

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