Reaction chemistry during self-propagating high-temperature synthesis (SHS) of H3BO3-ZrO2-Mg system

Article abstract of DOI:10.1016/j.materresbull.2007.01.016

In the present investigation, the ZrB₂ powder is produced by SHS of mixture containing H₃BO_3, ZrO₂ and Mg. The thermal analysis and XRD study reveal the reaction mechanism of ZrB₂ formation by SHS process. Synthesis of H₃BO₃-Mg system results in formation of Mg₃(BO₃)₂ and MgB₄ phases, whereas ZrO₂ is partially reduced to Zr₃O and Zr during synthesis of ZrO₂-Mg system. The reaction between elemental Zr and MgB₄ results in ZrB₂ formation. The particle size of ZrB₂ is found to decrease with the addition of SHS diluent.

Full text of DOI:10.1016/j.materresbull.2007.01.016

Materials Research Bulletin 42 (2007) 2224–2229
www.elsevier.com/locate/matresbu
Short communication
Reaction chemistry during self-propagating high-temperature
synthesis (SHS) of H BO –ZrO –Mg system
3
3
2
*
A.K. Khanra
Department of Metallurgical and Materials Engineering, IIT Kharagpur 721302, India
Received 12 August 2006; received in revised form 24 December 2006; accepted 24 January 2007
Available online 30 January 2007
Abstract
In the present investigation, the ZrB powder is produced by SHS of mixture containing H BO ZrO and Mg. The thermal
2
2
3
3,
analysis and XRD study reveal the reaction mechanism of ZrB
2
formation by SHS process. Synthesis of H
is partially reduced to Zr
system. The reaction between elemental Zr and MgB results in ZrB formation. The particle size of ZrB is found to decrease with
3
BO
3
–Mg system results
in formation of Mg
3
(BO
3
)
2
and MgB
4
phases, whereas ZrO
2
O and Zr during synthesis of ZrO –Mg
3 2
4
2
2
the addition of SHS diluent.
2007 Elsevier Ltd. All rights reserved.
#
Keywords: A. Carbides; Ceramics; C. Electron microscopy; X-ray diffraction; D. Defects
1
. Introduction
The ZrB is an excellent high-temperature ceramic material due to its high melting point, high wear and
2
oxidation resistance. It has several applications such as in hypersonic re-entry vehicles, nose caps, rocket nozzle
inserts, air-augmented propulsion system compounds, evaporation boats, cutting tools, cathode material for Al
extraction, etc. [1,2]. The ZrB can be prepared by borothermic reduction, carbothermal synthesis, SHS, gas-phase
2
combustion synthesis, etc. [3–6]. Last one decade the SHS becomes a suitable process for production of several
ceramic materials. The essential feature of SHS process is that the heat required to drive the chemical reaction is
supplied from the reaction itself. Once ignited, extremely high temperature (adiabatic temperature) can be achieved
due to the highly exothermic reaction proceeding in a short time. Preparation of boride powder from elemental
powder mixture results in high purity powder but makes process costly. Preparation of boride powder from the
mixture of boric acid, metal oxide and reducing agent is an alternative processing route and save the cost of
the starting materials. The SHS reactions are limited by kinetic considerations related to the rate of reactions and
rate of heat transfer ahead of the reaction front, which often results incomplete conversion. The details study of
SHS is useful to control the composition of product [7–9]. In the present investigation, the reaction mechanism
of ZrB formation by SHS process from mixture containing H BO ZrO and Mg is studied with the help of
2
3
3,
2
individual reactant mixture system. Present investigations also focus the effect of NaCl as a SHS diluent on ZrB2
particle size.
*
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0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2007.01.016

A.K. Khanra / Materials Research Bulletin 42 (2007) 2224–2229
2225
2
. Experimental procedure
In the present study, the ZrO powder (99.8% pure, and particle size <44 mm, S.D. Fine chemicals, Mumbai, India),
2
H BO (99.5% pure and particle size <44 mm, Loba-Chemie, Mumbai, India) and Mg (99.9% pure, and particle size
3
3
<
44 mm, Loba-Chemie, Mumbai, India) were used as raw materials. First the H BO and Mg (molar ratio of 2:3) were
3 3
dry mixed in a polythelene bottle for 3 h and small quantity of (ꢀ10 mg) mixture was used for differential thermal
analysis and thermogravimetry analysis (DTA/TG). The DTA/TG was carried at different rates of heating of 10 and
0 8C/min with continuous flowing of high pure (XL grade) argon gas. The H BO –Mg mixture was also heated in a
2
3
3
tubular furnace with resistance heating up to 1000 8C. After cooling the MgO was leached out by boiling in dilute HCl
solution. The phase analysis of product was characterized by X-ray diffraction (XRD) [Philips 1840, The
Netherlands]. Another mixture of ZrO and Mg (molar ratio of 1:2) was similarly tested by DTA/TG. The mixture was
2
heated in a furnace up to 1000 8C and MgO (reaction product) was removed. Then the product was analyzed by XRD
technique. The H BO , ZrO and Mg (molar ratio of 2:1:5) were dry mixed, which was used for DTA/TG study and
3
3
2
heated in a furnace up to 1000 8C. Similarly the synthesized product was characterized by XRD technique. The
experiment was also carried out by adding 20 wt% NaCl as a SHS diluent to the reactant mixture. These all
experiments were performed in high pure argon atmosphere and rate of heating was 20 8C/min. The morphology of
leached and dried product from H BO –ZrO –Mg system was studied by scanning electron microscope (SEM)
3
3
2
[
JEOL, JSM 840A, Japan] and transmission electron microscope (TEM) studies [CM 200, The Netherlands].
3
. Results and discussion
The typical DTA/TG plots of H BO –Mg system at different heating rates are shown in Fig. 1. The thermograms
3
3
show two endothermic peaks to appear at 139 and 169 8C, respectively. These two peaks are due to melting and
dissociation of boric acid. This is confirmed from individual DTA of boric acid and relevant thermodynamic data. The
endothermic peak due to melting of B O is not detected here. The thermograms reveal an endothermic peak at 650 8C
2
3
due to melting of Mg and then a sharp exothermic peak appears at 853 8C. The exothermic peak appearing at 853 8C
could be the reaction initiation temperature of H BO –Mg system. The rate of heating has insignificant effect on the
3
3
reaction initiation temperature. The TG plot shows the corresponding weight change taking place during the thermal
analysis. Initial sharp weight loss is due to removal of water from boric acid and then slight weight gain is found. This
small weight gain may be oxidation of unreacted Mg. The XRD pattern of H BO –Mg system shows presence of MgO
3
3
as a major phase with Mg–borate in the as synthesized powder (Fig. 2). The XRD pattern of leached powder shows
Mg–borate as major phase and MgB as a minor phase, respectively. The chemical reactions of H BO –Mg system
may be written as
4
3
3
2
H3BO3 ! 3H2O þ B2O3
(1)
(2)
4
B2O3 þ 7Mg ! MgB4 þ 2Mg3ðBO3Þ2
3 3 1 2 1 2
Fig. 1. Typical DTA/TG plot of H BO –Mg system: D , D —DTA at 10 and 20 8C rate of heating and T , T —TG at 10 and 20 8C rate of heating.

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A.K. Khanra / Materials Research Bulletin 42 (2007) 2224–2229
3 3
Fig. 2. XRD patterns for H BO –Mg system: (a) and (b) are unleached and leached powder.
During synthesis of H BO –Mg system the elemental boron may form, which is not detected by XRD analysis due
3
3
to amorphous in nature. The reaction between B O and Mg can be written as
2
3
B2O3 þ 3Mg ! 2B þ 3MgO
(3)
The DTA/TG plot of ZrO –Mg system shows only an exothermic peak at 645 8C (Fig. 3). The endothermic peak
2
due to melting of Mg is not detected, which indicates overlapping of the solid state reaction between Mg and ZrO on
2
melting reaction. The rate of heating has no effect on the exothermic peak position. The TG plot shows initial small
decrease in weight and then a rapid weight increase at a constant rate for both rates of heating. The weight gain during
thermal analysis however indicates MgO formation through reaction with adsorbed gases and residual oxygen in the
atmosphere. The XRD pattern of ZrO –Mg system is shown in Fig. 4. The unleached powder mainly contains MgO
2
and ZrO phase, whereas the leached powder shows ZrO as a major phase with Zr O and elemental Zr as minor
2
2
3
phases. Presence of ZrO as a major phase indicates it is partially reduced. The reduction sequence of ZrO can be
2
2
represented as ZrO ! Zr O ! Zr. It has been also seen that addition of extra Mg has no effect on XRD pattern of
2
3
synthesized product from ZrO –Mg mixture.
2
A typical DTA/TG plot for H BO –ZrO –Mg system is shown in Fig. 5. The DTA plot shows two endothermic
3
3
2
peaks at 139 and 169 8C and then an endothermic peak at 650 8C. Similar endothermic peaks are also found during
H BO –Mg system. After endothermic peak at 650 8C, a massive exothermic peak is found at 794 8C. The exothermic
3
3
peak at 794 8C for H BO –ZrO –Mg system may be the ignition temperature of exothermic reaction. The exothermic
3
3
2
peak at 650 8C, which is found during DTA of ZrO –Mg system, is not observed here. This is an unexpected matter and
2
further investigation is under progress. The rate of heating and NaCl addition seem to have no effect on ignition
2 1 2 1 2
Fig. 3. Typical DTA/TG plot of ZrO –Mg system: D , D —DTA at 10 and 20 8C rate of heating and T , T —TG at 10 and 20 8C rate of heating.

A.K. Khanra / Materials Research Bulletin 42 (2007) 2224–2229
2227
2
Fig. 4. XRD patterns of ZrO –Mg system: (a) and (b) are unleached and leached powder.
temperature. It has been also seen that addition of NaCl as SHS diluent to reactant mixture has insignificant effect on
ignition temperature, which indicates that it does not affect the mechanism of the present SHS process and the NaCl is
remaining neutral. TG plot shows initial rapid decrease of weight due to loss of H O and then there is no weight change
2
observed till exothermic reaction. The weight decrease during exothermic reaction may be due to flying away of some
particles from the crucible during DTA/TG analysis and the vigorous exothermic reaction could be the cause for this.
The XRD pattern of H BO –ZrO –Mg system is shown in Fig. 6. The XRD pattern of the powder (unleached) shows
3
3
2
presence of MgO as a major phase along with ZrB and ZrO phases. The leached powder shows presence of ZrB as a
2
2
2
major phase along with ZrO and B Zr phases (Fig. 6B). The sequence of chemical reactions of H BO –ZrO –Mg
2
51
3
3
2
system can be written as
MgB þ Zr ! MgB þ ZrB
MgB2 þ Zr ! ZrB2 þ Mg
Zr þ 2B ! ZrB2
(4)
(5)
(6)
4
2
2
However, presence of Mg (BO ) is not detected during XRD of H BO –ZrO –Mg system. The presence of
3 2
3
3
3
2
Mg (BO ) phase in H BO –Mg system and absence of Mg (BO ) phase in ZrO –H BO –Mg system is also
3
3
3 2
3
3
3 2
2
3
3
observed by Weimin et al. during combustion synthesis of TiB [9]. This indicates it may not form or even if it forms, it
2
may subsequently dissociate to form other species. The detail of investigation is under progress.
Fig. 5. Typical DTA/TG plot of ZrO
heating.
2 3 3 1 2 1 2
–H BO –Mg system: D , D —DTA at 10 and 20 8C rate of heating and T , T —TG at 10 and 20 8C rate of

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A.K. Khanra / Materials Research Bulletin 42 (2007) 2224–2229
Fig. 6. XRD patterns of different powders of H
leached powder (20 wt% NaCl addition).
3 3 2
BO –ZrO –Mg system: (a) unleach powder, (b) leached powder (without NaCl addition) and (c)
In the present investigation presence of unreacted ZrO in the product indicates an incomplete conversion. Addition
2
of extra Mg (molar ration of H BO :ZrO and Mg 2:1:8) also has no effect of XRD result. A similar phenomenon is
3
3
2
also found during synthesis of ZrO –Mg system. In case of production of TiB production from mixture of H BO ,
2
2
3
3
TiO and Mg, the unreacted TiO is not found, which may indicates complete conversion [10]. The ZrO may be much
2
2
2
more stable oxide than TiO which always leads to incomplete conversion. The XRD pattern in case of NaCl addition
2
,
shows presence of extra ZrO peak. The appearance of extra peak may indicate decrease of conversion, which results
2
from loss of heat due to NaCl addition. The XRD patterns indicate line broadening of highest intensity peak (1 0 1) for
NaCl added sample as compared to without NaCl added sample, which is due to diluent addition. Applying Scherrer
formulae, the crystallite size of ZrB are found to be ꢀ75 and 43 nm for without NaCl and 20 wt% NaCl added
2
samples, respectively. The TEM images of SHS powders are shown in Fig. 7. It also shows presence of agglomeration
of spherical particles and the particle size decreases with the NaCl addition. During synthesis NaCl melts and
vaporizes and deposit on ZrB particle, which reduces the grain growth of particle. The corresponding selected area
2
diffraction pattern (SAD) shows presence of mirror images of dots (inset of Fig. 7a and b), which indicates twin
formation in the particle.
2
Fig. 7. TEM images of leached ZrB powder: (a) without NaCl addition and (b) 20 wt% NaCl addition.

A.K. Khanra / Materials Research Bulletin 42 (2007) 2224–2229
2229
4
. Conclusions
An attempt has been made here to study the reaction mechanism during production of ZrB by SHS technique.
2
During SHS, the H BO is reduced to Mg (BO ) and MgB whereas ZrO is partially reduced to Zr O, Zr. Then
3
3
3
3 2
4,
2
3
elemental Zr reacts with MgB for ZrB formation. Incomplete reduction of ZrO results in presence of unreacted
2
4
2
ZrO in the product. Addition of diluent to the reactant mixture helps in decrease of ZrB particle size.
2
2
Acknowledgements
Author is grateful to Prof. M.M. Godkhindi of IIT Kharagpur and Dr. L.C. Pathak of NML, Jamshedpur for their
valuable suggestion during experiments.
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