J. Am. Ceram. Soc., 92 [1] 26–31 (2009)
DOI: 10.1111/j.1551-2916.2008.02835.x
r 2008 The American Ceramic Society
ournal
J
Acid Leaching of SHS Produced Magnesium Oxide/Titanium Diboride
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Jonathan Y. Lok, Kathryn V. Logan,* ** and Jairaj J. Payyapilly*
Department of Materials Science and Engineering, Virginia Polytechnic and State University, Blacksburg, Virginia 24060
The stoichiometric self-propagating high-temperature synthesis
SHS) thermite reaction involving magnesium (Mg), titanium
dioxide (TiO ), and boron oxide (B O ) forms MgO and tita-
(
5Mg þ TiO2 þ B2O3 ! 5MgO þ TiB2
(1)
2
2
3
nium diboride (TiB ) as final products. Selective acid leaching
2
is used to remove the MgO leaving TiB powder. This study
2
Stoichiometric amounts of the reactants are intimately mixed
and reacted in a silica crucible. A resistance heated NiCr wire
is used to initiate the reaction with a variable transformer.
The reaction requires a relatively low-energy input and
takes o1 min to complete. The products formed by Eq. (1)
investigates the acid leaching of SHS-produced MgO/TiB2
powders and a stoichiometric mixture of commercially obtained
MgO and TiB powders. Leaching was conducted at pH levels
2
of 4.0, 2.5, and 1.0 by the introduction of concentrated aliquots
of HNO . This method maintains a minimum pH target
3
2
consist of an agglomerated MgO encasing TiB platelets that
throughout the leaching process, thereby sustaining a dynamic
concentration to remove the oxide. The optimal leaching condi-
tions were determined to be at 901C at a minimum pH target of
10
must be broken up to facilitate the removal of MgO.
Classical handbook literature by Samsonov et al. and more
recent work by Sangwal et al. have compared the kinetics of
2.5 for the SHS-produced product. At these conditions, conver-
2
MgO dissolution and TiB yield amongst various acids. Strong
acids are thermodynamically more favorable than weak acids or
electrolyte solutions. Of the strong acids, nitric acid had the
sion percentages of 83%–84% of MgO were measured with
only trace amounts of TiB measured in the solution (o100
2
lg/L). Conversion percentages for each leaching condition
highest TiB
atures. Others have leached the MgO/TiB
acids, particularly H SO , HCl, and HNO
2
yield over a range of concentrations and temper-
and dissolution mass of solid MgO and TiB at each pH are
also reported. Results from powder X-ray diffraction confirm
the removal of MgO and minimal dissolution of TiB , and in-
2
2
products with strong
Leaching with
11–16
3.
2
4
2
HCl in particular resulted in a pyrophoric product, possibly due
to the higher surface area that was produced from dissolution of
particles at the nanoscale level. Others have reported the use of
dicate the formation of unidentified compounds. Inductively cou-
pled plasma mass spectrometry (ICP) was used to analyze the
ionic composition and extent of leaching. Scanning electron mi-
croscopy was used to observe the particle morphology of the
leached powders.
10
HCl with varying results, showing both excessive and acceptable
Quantified results of the dissolution of TiB
2
have not been found in the literature.
17,18
.
losses of TiB
2
I. Introduction
II. Experimental Procedure
ITANIUM diboride is a refractory ceramic known for its high
hardness, strength to density ratio, and wear resistance.
(1) Synthesis
1
T
Stoichiometric amounts of reactant powders were mixed ac-
cording to Eq. (1). Three, 130.0 g batches (96.7 g MgO and 33.3
g TiB theoretical yield) were mixed to produce all powders used
Potential applications include wear-resistant coatings and bal-
listic armor. Industries are interested in TiB for its chemical/
environmental passivity, tribiological properties, electrical con-
2
2
in this study. Pigment grade TiO anatase (38.3 g, 1.0 mm mean
particle size) and B O (33.4 g, 189.5 mm mean particle size),
2
2
ductivity, and thermal shock resistance. Of the various methods
to synthesize TiB , carbothermic/borothermic reduction and
2
3
2
both from Fisher Scientific (Fair Lawn, NJ), were weighed and
mixed in an alumina mortar and pestle with a light shearing
motion. Once homogenized, 58.3 g (643.7 mm mean particle size)
of Mg flakes from Reade Manufacturing (Manchester, NJ) were
added to the mortar. The powders were completely homoge-
nized when a light-gray appearance of the mixture was achieved.
The mixture was transferred to a slip cast fused silica crucible,
and placed in an enclosed, air-tight, refractory-lined reaction
chamber. A second crucible was placed on top of the first cru-
cible with a 1 kg weight to retain the powders within the closed
system. To initiate the reaction, a NiCr wire was connected to a
variable transformer (12 A, 120 V). The wire was placed in the
crucible, just touching the top of the reactant powders. The
voltage was increased at a rate of about 2.5 V/5 s.
self-propagating high-temperature synthesis (SHS) are the
have investigated
3
–7
most prevalent. Recent works by others
cold milling, rapid carbothermal reduction, and single-step bo-
ronation as low-energy alternatives. Commercially, carbother-
mic and borothermic reduction is used to reduce rutile TiO
2
to produce TiB . While exhibiting excellent control of reaction
2
dynamics, the carbothermic and borothermic reduction path-
ways are energy intensive. They are kinetically favorable above
8
,9
2
to synthesize TiB is a fast, low-energy alternative. The overall
0001 and 12001C, respectively. The magnesium SHS method
2
reaction is shown in Eq. (1).
T. Besmann—contributing editor
(
2) Processing
Manuscript No. 24270. Received January 27, 2008; approved October 15, 2008.
Presented at The 31st International Cocoa Beach Conference and Exposition
on Advanced Ceramics & Composites (ICACC), Daytona Beach, FL.
Work was funded by a grant for the National Institute of Aerospace (Grant Award
No.: VT-03-01).
The cemented MgO/TiB
2
agglomerates were ball milled to break
apart the reacted mass in preparation for acid leaching. A 4.9 L
Type 304 stainless steel mill jar and 1.27 cm Type 302/304 stain-
less steel balls were used for milling. The particle size effect of
acid leaching is not considered a part of the scope of this work,
and is a fixed parameter. Previous work showed no further
*
Member, The American Ceramic Society.
*Fellow, The American Ceramic Society, 1992.
*
z
Present address: Exxon Mobil Research and Engineering, Materials Engineering,
Fairfax, VA 22037.
wAuthor to whom correspondence should be addressed. e-mail: jairajp@vt.edu
reduction in SHS MgO/TiB agglomerate size after 18–20 h of
2
2
6