Materials and Methods for Synthesis of SmCo nanoparticles

Darren Steele

Mentor: Jim Spencer

Synthesis of Samarium Cobalt Nanoblades

Introduction:

As particle accelerator technology takes on smaller dimensions such as those found in photonic crystal fibers (PCF’s) it is imperative that all of the major supporting components of a traditional accelerator be brought down to the same scale. One of the key components to any accelerator is the magnets used for altering the charged particles’ trajectories. For conventional accelerators, electromagnets are employed, but at the micron scale a new technology must be implemented. In 2008 researchers at Northeastern University and Sandia National Laboratory published a paper that announced a new method to synthesize samarium cobalt nanoblades with dimensions on the order of 100 nm which is composed of recyclable and environmentally friendly materials.¹ SmCo alloy is a rare-earth–transition-metal (RE-TM) permanent magnetic material, which is characterized by having a sufficiently high Curie temperature, a high magnetization, and a high magnetocrystalline anisotropy. The first two of these properties are provided predominantly by the sublattice of the 3d element Cobalt, while the third is due mainly to the rare-earth samarium, sublattice.² If such material could be produced to be air-stable, the material would presumably possess the bulk material’s ability to withstand high temperatures, radiation and magnetic fields, hence would be capable of being implemented into new accelerator concepts such as the PCF’s for testing. See Figure 1 for the conventional unit cells of SmCo₅ and Sm₂Co₁₇.² Note that the SmCo₅ phase takes on a hexagonal close-packed structure.

Figure 1: Samarium cobalt alloy phases. SmCo₅ is on the right.
The previously introduced method proposed one year ago is a method classic to nanoparticle synthesis and commonly referred to as the polyol process. Salts, in this case Sm(NO₃)₃·6H₂O and Co(NO₃)₂·6H₂O, are placed in a polyol medium, specifically tetraethylene glycol, which serves three functions. It dissolves the salts as a solvent, releasing metal ions into solution, acts as a stabilizing ligand, and acts as a reducing agent, which refers to its ability to reduce metal ions into atoms by providing electrons through its own oxidation to lower the ions’ positive charge to neutral.³ The metal atoms group together forming nuclei composed of the most stable lattice structures, which by mixing the proper stoichiometric amounts, could be SmCo₅, Sm₂Co₁₇ or others. The nuclei germinate into final nanoparticles whose shapes and size depend on the many parameters of the process of their creation. PVP is added to prevent aggregation of nanoparticles and to provide a means of protection from oxidation. It will be attempted to synthesize SmCo₅ as it is the simpler of the two SmCo high performance permanent magnetic phases found in Figure 1. The chemical synthesis will involve small yields on the order of tenths of a gram and take place at the Geballe Laboratory of Advanced Materials (GLAM). The product of each synthesis will be characterized individually via standard XRD and SEM techniques at the Stanford Nanocharacterization Laboratory (SNL) in order to analyze the effects of varying parameters on crystal phase purity, nanoparticle shapes and size distributions. It would be necessary to measure the magnetic properties of the particles to assess their value in future particle accelerator applications.
Chemicals:
-99.9% purity Sm(NO₃)₃·6H₂O

-98% purity Co(NO₃)₂·6H₂O

-Tetraethylene glycol

-Polyvinylpyrrolidone (PVP), Average molecular weight: 10,000 g/mol

-200 proof HPLC grade ethanol

-Hexane
Materials:

-500 mL three neck flask

-100 mL graduated cylinder

-100 mL beaker

-Condenser

-Rubber septum

-Thermocouple glass adaptor

-Magnetic stir bar

-Heating mantle with thermostat control and thermocouple

-Glass vials

-50 mL centrifuge vial

-Micro spatula

-Micro pipette

-Glass joint grease

-Liquid nitrogen

Safety Considerations:
When working in a wet chemical lab it is imperative to wear proper personal protection equipment (PPE). For the purposes of this experiment, nitrile gloves, complete coverage safety goggles and a lab coat are to be worn and maintained at the experimental site. MSDS information reports none of the listed chemicals have serious acute toxicity concerns. Review Table 1 for experimental hazard descriptions and hazard controls.

Table 1: Experimental Hazards and Controls

Hazard	Hazard Description	Probability/Severity	Control	Hazard Risk with control implemented
Chemical exposure to skin	Irritation or chemical absorption through skin. Acute and chronic risks found in MSDS.	High probability of skin contact/ Low severity of direct external exposure.	Utilization of proper PPE. Nitrile gloves and lab coat. Store unused reagents in respective chemical cabinets and chemical waste in designated waste containers.	Very low.
Chemical exposure to eyes	Irritation or blindness. Specific descriptions found in MSDS.	Moderate probability of eye contact/ Moderate to high severity from eye contact	Utilization of proper PPE. Complete closure safety goggles.	Very low.
Chemical exposure by inhalation of ingestion	Specific effects depend on dose and given in MSDS. Chronic toxicity effects are noted but not fully evaluated for both metal nitrates.	Moderate probability/ Low to moderate severity	Dry chemicals and liquids should be handled in fume hood as much as possible, storage containers should be closed after use, and all spills should be cleaned up immediately.	Very low.
Direct exposure to liquid nitrogen	Contact of liquid nitrogen by skin or clothes may result in severe burns and permanent tissue damage.	Low probability/ Low to high severity depending on circumstance.	Store liquid nitrogen out of immediate work area in properly labeled dewar and utilize standard liquid nitrogen PPE including cryogenic gloves and safety goggles during transfer.	Moderately low.
Contact of heating mantle or hot glass	Moderate burns may result from contacting heated objects.	Low probability/ Low severity.	Maintain knowledge of where heated objects are located and allow hot glass to return to room temperature touching glass with back of hand prior to handling.	Very low.
Ignition of samarium cobalt alloy or tetraethylene glycol during reaction phase.	Heating up to 300°C provides conditions for combustion of SmCo metal and glycol vapor, if oxygen gas and an ignition source are present in reaction vessel.	Low probability/ Moderate severity	Allow for several vacuum-nitrogen cycles prior to heating. Close fume hood during reaction in the unlikely event that a systematic failure results in combustion. Ensure no flames or source of sparks are present in fume hood during reaction.	Moderately low.
Condensation of oxygen in Schenk line trap	Liquid oxygen is extremely dangerous and reacts violently with organic compounds. If it collects in trap and then evaporates, it may result in Schenk line manifold explosion.	Moderate probability/ Moderate to high severity depending on concentrations of organic compounds in cold trap.	When shutting down Schlenk line manifold, remove liquid nitrogen from trap and allow glass to return to room temperature prior to shutting off vacuum pump.	Very low.

Methods:

Weigh 0.15 g of samarium nitrate hexahydrate, 0.48 g of cobalt nitrate hexahydrate, and 0.83 g of PVP in a 200 mL three-neck-round-bottom flask. (Note: If only smaller flasks are available, downsize reagent quantities while maintaining 1:5 molar ratio of samarium to cobalt precursor). Add 100 mL of tetraethylene glycol. Total metal ion concentration should be approximately 0.02 M. Place rubber septum on one of the two side necks. Add magnetic stir bar into flask and place thermocouple along with greased glass plug adaptor into the remaining side neck. (Note: The glass adaptor may need to be wrapped in teflon tape to ensure no leakage) After ensuring heating mantle and thermostat control are plugged in to wall outlet, place flask in mantle. The proposed synthesis will deviate from the original by being conducted using a Schlenk line technique as opposed to the more laborious glove box method. A typical double manifold Schlenk line may be seen in Figure 2.

Figure 2: Schlenk line double manifold apparatus.⁴

Provide adequate fresh grease to designated Schlenk line joint and attach condenser. Grease condenser joint and attach flask. Ensure that the Schlenk line, condenser, flask and mantle assembly is secure, open vacuum line stopcock, turn on vacuum pump and after two minutes submerge the Schlenk line’s low-temperature trap in liquid nitrogen. (Important: Transfer liquid nitrogen from storage dewar into a transfer container, then pour into an adequate thermos which will submerge the trap to at least two-thirds of its length). Set thermostat control to 100°C but do not begin heating. Open dry nitrogen gas valve and adjust flow rate until a steady flow is observed at the bubbler. Close stopclock for fifteen seconds and reopen to allow vacuum. Repeat vacuum-refill cycle two more times and close stopcock such that the reaction vessel is exposed to positive pressure nitrogen gas. Note that the fraction of air remaining in a flask after n vacuum-refill cycles is given by the expression (f)ⁿ where f is the fraction of original atmosphere in the flask after one cycle.⁵ Begin heating. Heat solution for one hour and observe for signs of black residue. If no product appears, allow another half an hour of heating. If still no solids, raise temperature.

Upon observation of reasonable product, turn thermostat control to room temperature, open stopcock to restore vacuum, remove liquid nitrogen containing thermos and vent vacuum line to allow liquid caught in trap to thaw to be properly disposed in designated waste containers.⁶ Once the system has returned to room temperature, turn off vacuum pump and close nitrogen gas valve. Feel that the reaction vessel has indeed cooled by pressing the back of the hand against the glass and remove the vessel from the Schlenk line. If the solids settle to the bottom, remove sufficient liquid such that solids may be transferred along with a maximum of approximately 50 mL of liquid to a centrifuge vial; otherwise transfer entire contents to multiple centrifuge vials to prevent loss of product. Place vial(s) in centrifuge and after solids are packed well, remove remaining liquid into beaker for proper disposal. Rinse solids with ethanol and repeat centrifuge process. Transfer solids to storage vial (25 ml) and disperse in hexane. Label vial with contents and a number to be recorded along with reaction conditions for distinction purposes. Repeat procedure for higher temperatures up to 300°C, longer times, different order of addition of reagents, and varying PVP to nitrate ratio to determine optimal parameters for desired nanoparticle shapes and sizes.⁷ Each reaction product should be placed in separate hexane filled vials and stored together on designated storage rack. Employ powder x-ray diffraction (XRD) if available and scanning electron microscope (SEM) techniques to verify structural composition and nanoparticle geometry and distributions. Specific procedure for such techniques will depend on specific instruments located at the SNL. Obtain size and shape distribution and measure magnetic properties of particles.
References:
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^2. Pfeiler, W. Alloy Physics: A Comprehensive Reference, 1^st ed., Wiley: New York, 2007; pp 878-879.
^3. R.S. Ningthoujam, N.S. Gajbhiye, and Sachil Sharma, J. Phys. 72, 3 (2009); p 577.
^4. Shriver, D.F.; Drezdzon, M.A. The Manipulation of Air-Sensitive Compounds, 2^nd ed., Wiley: New York, 1986.
^5. Girolami, G. S.; Rauchfuss, T.B.; Angelici, R.J. Synthesis and Technique in Inorganic Chemistry, 3^rd ed., University Science Books: Sausalito, CA, 1999; p 173.
^6. Kubiak, C.P. Kubiak Lab Manual. http://kubiak.ucsd.edu/manual/schlenktech.php (accessed July 21, 2009).
^7. A. Slistan-Grijalva et al., Materials Research Bulletin, 43, (2008); p 91.