Preparation of titanium diboride nanopowder

Article abstract of DOI:10.1134/S0020168510060105

We have studied the reaction between NaBH₄ and TiCl₄ at elevated temperatures in the range 570-1020 K and pressures of up to 10 MPa, with no solvent. The results indicate that nanoparticulate tita- nium diboride forms at temperatures

Full text of DOI:10.1134/S0020168510060105

ISSN 0020ꢀ1685, Inorganic Materials, 2010, Vol. 46, No. 6, pp. 614–616. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © S.E. Kravchenko, V.I. Torbov, S.P. Shilkin, 2010, published in Neorganicheskie Materialy, 2010, Vol. 46, No. 6, pp. 691–693.
Preparation of Titanium Diboride Nanopowder
S. E. Kravchenko, V. I. Torbov, and S. P. Shilkin
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia
eꢀmail: kgv@icp.ac.ru
Received May 5, 2009
Abstract—We have studied the reaction between NaBH4 and TiCl4 at elevated temperatures in the range
570–1020 K and pressures of up to 10 MPa, with no solvent. The results indicate that nanoparticulate titaꢀ
nium diboride forms at temperatures above 820 K. According to electron microscopy data, the titanium
diboride powder obtained at 1020 K consists of spherical particles 35–50 nm in diameter, in reasonable agreeꢀ
ment with the equivalent particle diameter of
Ӎ45 nm evaluated from the specific surface area of the TiB2 and
with the crystallite size Dhkl 30 nm evaluated from Xꢀray diffraction data.
Ӎ
DOI: 10.1134/S0020168510060105
INTRODUCTION
In this paper, we describe a different process for the
synthesis of nanoparticulate TiB2, which utilizes the reacꢀ
tion between TiCl4 and NaBH4 at elevated temperatures
and pressures, with no solvent. As distinct from the proceꢀ
dure reported by Chen et al. [11], the reaction mixture was
heated not in an argon atmosphere but in vacuum (initial
Modern materials research is paying a great deal of
attention to the synthesis and characterization of refracꢀ
tory compounds in an amorphous nanoparticulate or
nanocrystalline state. The unique functional properties of
such materials are due to the specific features of their
nanostructure: the small size of crystallites and pores, the
structure and composition of interfaces, the presence of
residual stress and defects, and others [1–4].
pressure of 1.33
hydrogen released in the reactor was within 10 MPa.
×
10–1 Pa), so that the pressure of the
EXPERIMENTAL
Nanoparticulate titanium diboride is of particular
interest among refractory compounds. Only a few proꢀ
cesses for the synthesis of nanoparticulate ТiB2 have been
proposed to date. Most of them involve the thermolysis of
titanium borohydride or its derivatives [5–9] according to
the scheme
Starting chemicals. Sodium borohydride of 99.5%
purity was prepared by crystallizing a commercial reagent
from a 1 N NaOH solution, followed by drying in a vacꢀ
uum of 1.33
10–1 Pa at 370 K. Offꢀthe shelf titanium tetꢀ
×
rachloride was vacuumꢀdistilled over copper turnings just
before the synthesis.
Characterization techniques. Xꢀray diffraction
measurements were made on an ADPꢀ1 automatic
t
⎯⎯→
Ti(BH4)3 ·
nSolv
TiB2 +0.5B2H6 +4.5H2 +
nSolv, (1)
where Solv stands for dimethoxyethane, tetrahydrofuꢀ
ran, diglyme, triglyme, or another solvent. The therꢀ
molysis process is run at a constant temperature in
vacuum using conventional techniques or laser irradiꢀ
ation [9]. The TiB2 prepared according to scheme (1)
is Xꢀray amorphous and can be crystallized by vacuum
annealing between 1120 and 1270 K.
diffractometer (Cu
K radiation). The lattice parameꢀ
α
ters of TiB2 were determined with an accuracy of
0.0003 nm or better. From the width of powder diffracꢀ
tion peaks, we evaluated the crystallite size using the
Scherrer formula: Dhkl
mal to the hkl plane). Here,
unity), = 1.54178
=
k
λ
/
k
βhklcosθhkl (along the norꢀ
is a constant (taken to be
and is the half width at
Å
,
β
λCuK
α
Scheme (1) represents an overall reaction, which
involves many steps, and the resultant titanium diboride is
contaminated with oxygen, carbon, and boron or its
hydrides, which has an adverse effect on the nanostrucꢀ
ture of the TiB2. Nanoparticulate TiB2 can also be preꢀ
pared by reacting sodium vapor with a mixture of TiCl4
and BСl3 or by the reaction [10]
half height for the diffraction peak in radians. Elecꢀ
tronꢀmicroscopic examination was performed on an
EMVꢀ100 BR transmission electron microscope.
Xꢀray photoelectron spectra were obtained on a Varian
IEEꢀ15 spectrometer (Mgꢀanode tube,
As the bindingꢀenergyꢀscale reference, we used the
C1 level (285.0 eV).
Thermogravimetric scans were combined with mass
spectrometric analyses of the decomposition products on
and also by reacting Ti with BBr3 in the presence of an STA 409PC Luxx simultaneous thermal analyzer in an
sodium metal at 670 K [11]. argon atmosphere at a heating rate of 10 K/min. The speꢀ
hν
= 1253 eV).
s
t
⎯⎯→
TiCl3 + 2LiBH4 +LiH
TiB2 + 3LiCl + 4.5H2, (2)
614

PREPARATION OF TITANIUM DIBORIDE NANOPOWDER
615
NaBH4 + TiCl4 reaction conditions and products (5 g (0.13 mol) of NaBH4 and 3.45 g (0.019 mol) of TiCl4)
T
, K
τ
, h
Composition
a
, nm
c
, nm
570
670
20
20
15
10
10
No TiB
–
–
–
–
2
2
No TiB
820
TiB
TiB
TiB
0.3024
0.3025
0.3027
0.3214
0.3212
0.3213
2.03
2.01
2.0
870
1020
cific surface area S of our samples was determined by BET microscopy results, the titanium diboride powder
analysis of lowꢀtemperature krypton adsorption isoꢀ obtained at 1020 K consists of spherical particles 35–
therms obtained after removing volatile impurities from 50 nm in diameter (figure), in reasonable agreement with
the solid phase in a vacuum of 1.33
The molecular area of adsorbed krypton was taken to be corresponds to an equivalent particle diameter of
19
10–20 m2. The error of determination was within at a TiB2 theoretical density of 4.5 g/cm3 [12]. The crystalꢀ
×
10–3 Pa at 570 K. its measured specific surface area,
S
= 29.6 m2/g, which
45 nm
Ӎ
×
10%. Hydrogen was determined on an LKhMꢀ8MD lite size evaluated from Xꢀray diffraction data is Dhkl
chromatograph equipped with a katharometer detector, 30 nm.
Ӎ
using a 1ꢀmꢀlong, 3ꢀmmꢀdiameter column packed with
tricresyl phosphate (30 wt %) on zeolite 545. The flow rate
of argon carrier gas was 30 ml/min.
To more accurately determine the surface composiꢀ
tion of the titanium diboride powders, we measured their
Xꢀray photoelectron spectra. The results showed that the
major component of the powders was TiB2, with Ti2p3/2
Boron was determined by potentiometric titration of
the mannitol–boric acid complex with an alkali after titaꢀ
nium precipitation from the solution, and Ti was deterꢀ
mined by chelatometric titration in the presence of Xyleꢀ
nol Orange, using standard procedures. The pressure in
the system was measured with certified reference manomꢀ
eters of accuracy class 0.4.
All manipulations with the titanium diboride, includꢀ
ing sampling, were carried out under an argon atmoꢀ
sphere or vacuum.
and B1s binding energies of 454.4 and 187.5 eV, respecꢀ
tively, which is consistent with typical binding energies in
metal borides [5, 14]. In addition to the lines due to titaꢀ
nium diboride, the spectra showed weak features arising
from boron and titanium oxides and elemental boron,
suggesting that the surface of the TiB2.0–2.03 particles was
covered with slight amounts of boron and titanium
oxides, with elemental boron inclusions. According to
Xꢀray diffraction results, the titanium diboride had a hexꢀ
agonal structure. Its lattice parameters (table) agree well
Experimental procedure. TiCl4 and NaBH4 were
reacted as follows: A silica ampule containing titanium
tetrachloride and an excess of sodium borohydride was
placed in a stainlessꢀsteel autoclave under an extrapure
argon atmosphere. The autoclave was then cooled to
200 K, evacuated for 1 min, and heated at constant temꢀ
perature for 10–20 h. Next, the reactor was cooled to
room temperature, and the reaction products were evacuꢀ
ated for 0.5 h. After opening the reactor, the products were
sequentially washed with distilled water cooled to 270 K,
acetone, and ethanol and then evacuated at 310 K for 5–
with those reported in the literature for TiB2 [12]:
a =
0.3026 nm and = 0.3213 nm. When heated from 290 to
c
6 h to a residual pressure of 1.33
10 –1 Pa. According to
×
chemical analysis data, the resultant black powder had
the composition TiB2.0– 2.03 and a hexagonal structure. Its
lattice parameters (table) agree well with those reported
in the literature for TiB2:
0.3213 nm [12].
a = 0.3026 nm and c =
RESULTS AND DISCUSSION
The table presents our results on the reaction between
NaBH4 (5 g, 0.13 mol) and TiCl4 (3.45 g, 0.019 mol) at
temperatures from 570 to 1020 K. These data demonꢀ
strate that the formation of nanoparticulate titanium
200 nm
diboride occurs for
T
≥
820 K, that is, at temperatures
Electron micrograph of the titanium diboride.
where NaBH4 decomposes [13]. According to electron
INORGANIC MATERIALS Vol. 46
No. 6 2010

616
KRAVCHENKO et al.
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1270 K in an argon atmosphere, the material showed no
exothermic or endothermic transformations and no
weight loss.
Thus, nanoparticulate TiB2 can be prepared by reactꢀ
ing NaBH4 and TiCl4 in a steel reactor at
T ≥ 820 K.
ACKNOWLEDGMENTS
We are grateful to R.A. Andrievski for his interest in
our work and for fruitful discussions.
This work was supported by the Presidium of the Rusꢀ
sian Academy of Sciences through the basic research proꢀ
gram no. Pꢀ9.
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thesis and Characterization of TiB2 and VB2 Nanopowꢀ
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2010