CHAPTER 2: SYNTHESIS OF NANOSTRUCTURED TRANSITION METAL
B. Experimental Procedures
Thus, reactions having adiabatic flame temperatures >2073 K, like many listed in Table 2-I, are good candidates for the solvothermal synthesis method.
By eliminating the use of pressure for solvothermal synthesis, the necessity of an autoclave was eliminated and the reaction could be performed in simpler apparatus. The simpler apparatus is not as costly as an autoclave and the thin walls of the reaction vessel allow for much faster heating and cooling rates when compared to using an autoclave, which helps make the process faster from start to finish and minimizes agglomeration caused by reprecipitation.
The solvothermal process has been used to obtain TaC powders that were nanostructured, phase pure, and had a low level of agglomeration.[23-24] Some flexibility in various responses versus processing variables was also demonstrated.[24] In fact, the only appealing aspects of an ideal process that were described earlier that have not been studied for this solvothermal synthesis method include chemical purity and demonstrating that a broad range of materials can be synthesized using this method. The primary chemical impurities of non-oxides include carbon or carbon containing impurities, oxygen, metals and halogens. Removal of these impurities from non-oxides is already well studied.[28] Therefore, the purpose of this work will be to demonstrate the potential of the solvothermal synthesis method for making a diverse set of UHTC powders and begin to define the criteria that defines the conditions for successful reactions.
It is important to note that the purpose is to demonstrate the synthesis of these materials, not to optimize the synthesis. Some aspects of process optimization have been provided previously.[24]
which correspond to the reaction equations given in Table 2-II, and hand mixed using a mortar and pestle. This mixture was added to a 95 ml fused-silica test tube. The reductant metal, either lithium or calcium, was weighed out in the amount given in Table 2-IV and added on top of the reactant mixture, but not mixed because the reductant is added in granular form to reduce its reactivity. The test tube was then lightly sealed with a rubber stopper and removed from the argon atmosphere to be ignited underneath a chemical fume hood.
Table 2 – II. Balanced Solvothermal Synthesis Reactions of Various Ultra-High Temperature Ceramics
Compound Reaction
TaB2 2TaCl5 + 8NaBH4 + 20Ca → 2TaB2 + 4B+5CaCl2 +15Ca + 8Na +16H2
HfB2 HfCl4 + 6B + 8Ca → HfB2 + 4B + 2CaCl2 + 6Ca ZrB2 ZrCl4 + 6B + 8Ca → ZrB2 + 4B + 2CaCl2 + 6Ca HfC HfCl4 + 3C + 12Li → HfC + 2C + 4LiCl + 8Li ZrC ZrCl4 + 3C + 12Li → ZrC + 2C + 4LiCl + 8Li TaN TaCl5 + 3NH4Cl + 15Li → TaN + 2NH3 +8LiCl + 7Li + 3H2
HfN HfCl4 + 3NH4Cl + 24Li → HfN + 2NH3 +7LiCl + 17Li + 3H2
ZrN ZrCl4 + 3NH4Cl + 12Li → ZrN + 2NH3 +7LiCl + 5Li + 3H2
BN NaBH4 + 3NH4Cl + 18Li → BN + 2NH3 + 3LiCl + 15 Li + 3H2
Table 2 – III. Supplier Information for Raw Materials Reactant Reactant Information
TaCl5 99.8%, Alfa Aesar, Ward Hill, MA NaB4H 98%, Alfa Aesar, Ward Hill, MA
Ca Granular, 99%, Fisher Scientific, Pittsburgh, PA HfCl4 99.9%, Alfa Aesar, Ward Hill, MA
B amorphous (submicron), Sigma-Aldrich, St. Louis, MO ZrCl4 98%, Alfa Aesar, Ward Hill, MA
C Lampblack 101, Degussa, Parsippany, NJ Li granular, 99%, Sigma-Aldrich, St. Louis, MO NH4Cl Fisher Scientific, Pittsburgh, PA
Table 2 – IV. Reactant Amounts used to Produce Boride, Carbide, and Nitride Powders MXy Compound M Precursor (g) X Precursor (g) Reductant (g)
TaB2 5.30 2.24 5.94 (Ca)
HfB2 4.80 0.97 4.81 (Ca)
ZrB2 6.20 1.72 8.52 (Ca)
HfC 5.04 0.57 1.31 (Li)
ZrC 6.77 1.05 2.42 (Li)
TaN 5.51 2.47 1.60 (Li)
HfN 4.99 2.50 2.60 (Li)
ZrN 6.64 4.57 2.37 (Li)
BN 1.52 6.47 5.03 (Li)
Prior to igniting, the test tube was rotated at an unspecified angle to mix the granular reductant with the other powder reactants. Once mixed the test-tube stopper was loosened so that it rested on the top and would allow for outgassing. The test tube was lowered into the center of a tube furnace that was held at either 548 K if lithium is used as the reductant or 873 K if calcium was used as the reductant. Once the samples were thoroughly ignited, they were immediately removed from the furnace and allowed to air quench.
After cooling to room temperature, the reaction products were removed from the test tube by adding water, thus forming either LiOH or Ca(OH)2, depending on the reductant that was used, to free the reaction products. The contents were emptied into a 250 ml beaker. Additional water was added to the beaker to create a 100 ml suspension, which was magnetically stirred for 1800 s, followed by ultrasonication for 1800 s. All ultrasonication for this work was performed using an ultrasonic cleaner with the perforated tray accessory (Model FS30, Fisher Scientific, Pittsburgh, PA). The ultrasonic cleaner was filled with water until the water level was even with the level of suspension in the 250 ml beaker. All further washing utilizes a similar ultrasonication step. An additional 900 sec of magnetic stirring was performed after ultrasonication. The suspension was centrifuged at 15,500 × g for 300 sec and decanted to prepare for the powder washing procedures so that the lithium or calcium hydroxide can be removed.
Lithium hydroxide is very soluble in water. Three consecutive water wash cycles were performed for all experiments that used lithium. A water-wash cycle is defined as creating a 100 ml suspension of the powder using deionized water, magnetically stirring for 1800 s, ultrasonicating for 1800 s, magnetically stirring for an additional 900 s, centrifuging the powders at 15,500 × g for 300 s, and decanting the fluid above the powders. After the three cycles, the powders were left under a fume hood to dry.
Powders which contained calcium hydroxide were washed and rinsed according to a procedure that was previously defined.[24]
Powder X-ray diffraction (XRD) was performed using a diffractometer (Siemens D5000, Siemens, New York, NY) by scanning from 15 to 85° 2θ using CuKα radiation, a step size of 0.04° 2θ and a dwell time of two sec after dispersing the powders on a zero- background holder. Software (Jade 8, Materials Data, Inc., Livermore, CA) was used to qualitatively evaluate the phases present in the powders. Additional analyses were performed on the powders that were close to being optimized. The analysis software was also used to analyze the average crystallite sizes for these samples. The crystallite sizes were determined using the Williamson-Hall technique.[29] Previous work, which uses cross-characterization, suggests that this method is accurate within approximately 10% error.[23] Since the crystallite size analysis depends on accurate full width at half maximum (FWHM) measurements for the peaks, whole profile fitting was used to help reduce error in the FWHM values.
Specific surface area (SSA) measurements were determined using the Bruauer- Emmett-Teller (BET) method (Tristar 3000, Micromeritics, Norcross, GA) after degassing in argon for 1 day at 423 K. Density measurements were performed by helium pycnometry (AccuPyc II 1340, Micromeritics, Norcross, GA) after drying the powders in an oven at approximately 253 K for 1 hour. The crystallite size was computed from the SSA and density measurements to compare to the crystallite size calculated from the X- ray diffraction data.
Thermogravimetric analysis (TGA) was performed in air using a thermogravimetric analyzer (Q50, TA Instruments, New Castle, DE) by heating from 323 to 1273 K using a heating rate of 5 K/min. This method was used to evaluate weight losses caused by adsorbed molecules, weight gains due to oxidation, and weight losses
associated with elimination of free carbon for example. Physisorbed molecules are typically observed as low temperature weight loss. Oxidation is observed as a weight gain. Free carbon reacts with oxygen at elevated temperatures and manifests as a weight loss as gas is evolved.
Dynamic light scattering (DLS) was performed (NanotracTM ULTRA, Microtrac, Montgomeryville, PA) after dispersing 0.02 grams of powder in 30 ml of deionized water and allowing the suspension to magnetically stir for one hour, followed by 300 sec of ultrasonication prior to measurement. The DLS measurements were repeated four additional times with 30 sec of ultrasonication between each measurement. Each run consisted of an average of five 30-sec measurements as is recommended by the manufacturer of the instrument and ASTM standard E2490-09.
Scanning electron microscopy (SEM) was performed (FEITM Quanta 200F, FEI Company, Hillsboro, OR) to observe particle characteristics and supplement particle and crystallite size analyses. SEM samples were prepared by dispersing 10 mg of powder into 25 ml of water, magnetically stirring for 1 hr, ultrasonicating for 10 min, drop coating onto a silicon wafer, and drying in an oven set at 363 K.
C. Results and Discussion