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Elemental Boron & Magnesium Boride synthesis - Review
Neale R. Neelameggham1*
1
Ind LLC, USA Elemental boron has a melting point of
[Invited Paper -Presented At the 17 th
International 2076 o C. It is considered a metalloid even though it
Symposium of Boron & Borides, Istanbul, Turkey, belongs to the metallic element group as aluminum
Sept.2011] in Group III of periodic table – where all other
elements are metals.The high melting point of the
Abstract elemental boron as well as its high affinity for
oxygen makes the preparation of pure elemental
The synthesis of pure elemental boron has boron more difficult than most other elements.
been elusive for over two centuries. The recent
understanding of magnesium boride as a The existence of pure and not so pure
superconductor at 39 K has brought about the need boron crystals was discussed in the article - Boron:
for understanding the causes of the difficulty in a Hunt for Superhard Polymorphs [A.R.Oganov et
preparing elemental boron and magnesium boride al. 2009]. It was highlighted that the search for
therefrom. This review article discusses some of making pure elemental boron continues to be going
the reasons for the difficulties in the chemical on since Gay Lussac and Humphrey Davy claimed
synthesis as well as the physics involved in these. the formation of elemental boron two hundred years
Some of the hypothesis outlined in this paper may ago. These initial discoveries are still valid as they
shed light into better synthetic methods. pointed to the direction for the existence of the
element even though they were shown later that the
Keywords: boron, elemental boron magnesium purity was less than 70%. Oganov discussed
boride, boron halides, Ellingham chart different polymorphs of crystal structures, noting
the revival of interest in making elemental boron to
help contribute towards the superconductor
1. Introduction
magnesium boride preparation since 2001.
Pure elemental boron is essential in nuclear In this overview, an attempt is made to give
industry as thermal neutron absorbers, in making possible reasons for the difficulties in making the
boron carbide control rods, and for making metal [or metalloid] while noting possible novel
magnesium boride superconductors. There are approaches based on thermochemical approach.
various other applications too. Most of the boron
obtained is amorphous; crystalline boron is The basic physical properties of elemental
available in much smaller quantities than boron which impact the production of the element.
amorphous boron. The total tonnage of boron is are shown below –
very low and production of boron element done by
a handful fo producers – H.C.Stark, Tronox ,
Solid density 2.36 g/cc
S.B.Boron, Pavezyum, several Chinese producers
and several Atomic Energy Agencies. It is difficult Liquid density at m.p. 2.08 g/cc
to obtain sintered elemental boron of high densities. Melting point 2076 °C
Any new economical approach in making elemental Boiling point 3927 °C
boron, crystalline in nature with ability to make Heat of fusion 50.2 kJ/mol, Heat of vaporization
components closer to theoretical density will bring 480 kJ/mol
in more high temperature and electrical Molar Heat Capacity 11.087 J/mol. K
applications.
2. Historic Review - Producing Elemental
*Corresponding Author: Neale R. Neelameggham, Boron
‘Guru’, IND LLC, 9859 Dream Circle, South Jordan, UT
84095. The chapter dedicated to Boron in “A
E mail: Neelameggham@gmail.com
Comprehensive Treatise on Inorganic and
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Theoretical Chemistry, Vol. V” – points to the
historic knowledge of borax by alchemists of the Besides using boric acid or its anhydride as
sixteenth century [J.W. Mellor, 1967]. Humphrey starting material, there have been several processes
Davy made crude elemental boron in 1807 followed to make elemental boron using boron halides as
by its preparation by Gay Lussac and Thenard in starting material in the nineteenth century which
1808. Both used potassium reduction of boric acid. were upgraded in the twentieth century using more
The announcement from both groups came in the electrical and electrochemical devices available.
middle of 1808 within a few weeks of each other. Moisson boron powder was melted into bigger
Apparently both the products were later shown to lumps by using electric arc and hydrogen
contain only about 50% elemental boron, as the atmosphere [E.Weintraub, 1909]. Weintraub also
glassy boric oxide and the borides of container prepared boron from by passing boron trichloride
material are not attacked by the acid and stayed and hydrogen in an arc formed between water
mixed with the amorphous boron powder. cooled copper electrode. Apparently the beads of
melted boron which fell off from the electrodes
H Davy in 1809 , and others in the analyzed close to 100 per cent purity. This
following 40 years, electrolyzed fused borax technique is somewhat analogous to the manner in
[sodium borate] to boron [H. Davy, 1809], [R.D. which pure silicon is made from SiCl4 and
Thomson, 1831], [F. Wohler, 1856]. The hydrogen. A heated 30 mil resistor rod of
electrolysis surmised the formation of metallic tungsten over which the deposition of boron took
sodium which reduced boron oxide to boron place from the hydrogen reduction of boron
element. Again the product was a powdery trichloride [A. Warth, 1923].
material, containing less than 70% elemental boron
along with adhering salts and oxides and other Most of the boron made as powdery boron
metallic compounds [possibly borides] even after was all amorphous. Crystalline boron formation
being washed with hydrochloric acid followed by started being discussed when needle like rods were
rinsing with water. Some of these chemical prepared by using Warth technique in 1943. The
analyses were conducted by H. Moisson [H. icosohedral clusters of 12 boron atoms being
Moisson, 1892]. present in several of the crystalline boron in the
Several groups tried using rhombohedral form was pointed out in 1951 and
magnesium as the reducing agent, while trying to refinement to this was shown in years that
wash the magnesium oxide with hydrochloric acid followed. The refinement continues to the present
hoping to get a purer metal using boric acid or its time as shown in the paper by Oganov.
anhydride- boron oxide as starting material. [T. L.
Phippson, 1864] [F. Jones, 1879]. Moisson noted Newkirk divided the preparation of
that one of the two magnesium borides formed is as elemental boron into the following categories:
resistant to water and hydrochloric acid as
elemental boron causing difficulty in making pure • Reduction of Boron Compounds by Metals
boron. However, he was able to make 98.30% pure and Miscellaneous Agents
boron using several purification steps. The
preparation of crude boron with a 90 to 95% purity The metals used as reducing agent included
practised even in the twenty-first century is called alkali metals Li, Na, K, alkaline earth metals Be,
Moisson Process. Mg, Ca, other metals Al, Fe, Zn, Hg, and elements
C, Si and P. The boron raw material in most cases
were boric anhydride, and in some cases boron
The chapter on Elemental Boron, in the halides. Reactions using CaC2, CaH2 and WC as
book “Boron, Metallo-boron compounds and reducing agents were also reported. Many and
Boranes” in 1964 is one of the best compendiums most of these produced less than 98% pure boron,
on the subject [A.E.Newkirk, 1964]. This gives a and approaching the 98% purity took multiple steps
comprehensive discussion of the work done on of refining following initial reduction.
elemental boron in the twentieth century beyond the
earlier 19th century attempts mentioned earlier.
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• Reduction of Boron compounds by material. The deposits were crystalline when the
electrolysis temperature of reaction was higher than 1100 o C.
Fused salt electrolysis approach using • Thermal Decomposition of Boron
either melts containing borate or fluoborate as Compounds.
boron source with metal or carbon electrodes.
Since the electrolysis took place at temperatures [a] decomposition of boron hydrides: It was shown
much below the melting point of boron, the that boron hydrides can be thermally decomposed
deposits were powdery and had to be refined to elemental boron. [A.Stock, 1933]. Decomposing
further to get the purity. Kahlenberg claimed diborane to decaborane in the 300 – 900 o C
making 100% pure boron powder from a melt of produced amorphous boron. Decomposition of
B2O3-K2O-KF [H.H. Kahlenberg, 1925]. Twenty diborane on tantalum filaments around 950 o C and
five years later, Cooper showed that 99.7% pure low pressures produced crystalline boron, higher
boron could be obtained from B2O3-KBF4-KF [H.S. temperatures resulted in variation of crystallinity
Cooper, 1951]. Later studies showed the purity was [W.L. Robb et al. 1959] .
lower than claimed earlier. Again, the impurities in
the product were oxygen, and electrode material [b] decomposition of boron halides: The
difficult to separate from the boron product. Most decomposition of boron iodide can be carried out
of the boron by this technique was still amorphous on tungsten filaments around 1300 – 1400 oC
and non-crystalline. making 99.9% boron. [A.E. van Arkel, 1930].
As recent as in 2010, a presentation [c] decomposition of Borides and borohydrides:
described a molten salt electrolysis of boron. The Several processes were attempted in decomposing
electrolyte system evaluated was MgF2-NaF-LiF borides such as magnesium borides or sodium boro-
with B2O3 as the source of boron. It was noted that hydrides and were not effective like the other
the product was agglomerated boron spheres. Part decompositions.
of the elemental boron produced diffused inside the
cathode from the boronization of the iron. Even this There are various techniques used in the
process produced boron of purity less than about reduction of boron oxides; one of the newer
90% [P. Taylor et al. 2010]. techniques studied in the laboratory scale is called
mechano-chemical approach using high energy ball
• Reduction of Boron Compounds by milling. [R. Riccerri, 2003], [H.D. Almadari et al.
Hydrogen 2003]. This technique is still prone to producing
only impure boron due to the nature of the
Reduction of non-volatile compounds with production technique of solid state processing.
hydrogen did not seem to result in boron free of
boric oxide and or water. Among the boron halides There have been multiple studies done in
tested by this method only boron trichloride refining the impure boron. Refining may be of the
resulted in good products, while use of boron tri- powdery material or of a partly sintered boron
fluoride required temperatures beyond practical product. A technique of making 99.99+ pure boron
limits. The hydrogen reduction of BCl3 required using an iodide method has been described, along
special reactors such as [a] Reactors using an arc or with indicating that there is a boron bromide –
spark, [b] Hot tube reactors such as heated quartz hydrogen reduction which can also provide similar
tube, [c] Hot filament reactors. Of these, hot purity [A.F.Armington et al. 1964]. High purity
filament reactors seemed to produce higher purity boron is used as thermal neutron absorber in
material most of them above 99% purity. Initial nuclear reactor making it one of the few important
filament which proved worthwhile was tungsten, applications [U.S.Atomic Energy Commission,
other filaments that were applicable included 1972]. Electrolytic boron from fused salt
tantalum or molybdenum. Reductions were also electrolysis is used in making Boron carbide for
done with boron or carbon or titanium as filament control rods for nuclear reactors. A 95 to 99% pure
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boron is known to be made by electrowinning report on the formation of Mg3B2 by magnesium
from potassium fluoroborate dissolved in a metal reaction with either boric anhydride or boron
mixture of molten potassium fluoride and trichloride [F. Jones et al. 1881]. Forty years later
potassium chloride for this purpose [A. Jain et it was shown that other magnesium borides such as
al. 2008]. MgB2 and Mg2B4 also exist [M.W. Travers et al.
1912]. Another forty years later, the existence of
Mg3B2 was refuted – noting that it may be a
It is noted that there have been two historic
mixture of magnesium metal and magnesium
waves of discoveries in developing and
Boride MgB2 [V.R. Russell, 1953]. The thermal
understanding crystalline polymorphs of boron.
relationship among magnesium borides were
The first historic wave in the 1950s by GE and
developed for further understanding [A. Chretian,
other researchers who showed the presence of
1962]. Magnesium borides with more boron are
minor impurity atoms present in the crystalline
also known to be formed at higher temperatures by
lattice such as B50C2 or B50N2. This was followed
volatilization of magnesium.
by pure boron phase crystal structure identification
in years following 1957 – 58. [A. Oganov et al.
Many of the MB2 type borides are known
2009]. The effect of compressing the pure boron at
to have high electrical conductivity for over 50
10 Gpa pressure and 1800 to 2300 K has been
years. In 2001 the discovery of superconductivity
described [R. Wentorff, 1965]. The advent of
at 39 K in magnesium diboride was announced [J.
magnesium boride’s superconductivity at 39 K in
Nagamatsu et al. 2001]. This announcement
2001 revived the interest in studying the behavior
enthused researchers worldwide rushing to make
of pure boron for the second wave including
magnesium diboride and superconducting
finding superconductivity in elemental boron by
components thereof since that time.
compressing pure crystalline boron to 250 GPa at
room temperature providing a Tc at 11.2 K
[M.I.Eremets et al. 2001].
4. Recent Development on Magnesium
While crystallinity of high purity elemental Boride
boron may be of importance on its own electrical
properties, amorphous boron of high purity may There have been two aspects to the
suffice in making the compounds. One of the preparation of magnesium boride. One is the
recent patent applications notes that high purity formation of the compound and the other the
amorphous boron of very fine size is required for forming of the superconducting component for
making superconducting magnesium boride using applications such as in NMR and superconducting
hydrogen reduction of BCl3 with a catalyst such as magnets of high current densities.
tantalum or tungsten from which the amorphous
boron is continuously removed. by inventive Magnesium diboride is usually synthesized
mechanical means [A. Aubele, 2010]. A novel by several methods. One of which is the high
approach of electrolyzing molten sodium temperature reaction between boron and
tetraborate at 1000 oC is patented recently to make magnesium powders. Formation begins at 650 °C;
boron in an economical fashion[H. Ylidran et al. however, since magnesium metal melts at 652 °C,
2006]. the reaction mechanism is considered to be
moderated by magnesium vapor diffusion.
Superconducting magnesium diboride wire can be
3. Magnesium Boride
produced through the Powder In Tube (PIT)
process. In the in situ variant, a mixture of boron
Formations of borides were known since
and magnesium is poured into a metal tube, which
the time of H Davy in 1808. He noted that
is further processed by wire drawing. Then the wire
potassium and boron formed potassium boride,
is heated to the reaction temperature to form MgB2
decomposed into KOH and BH3 by action of water
inside. It is noted that MgB2 tends to decompose
[H. Davy, , 1808]. References cited indicate that
into other compounds around its melting point
F. Jones and R L Taylor were one of the earliest to
around 850 o C. The powder in tube method of
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making magnesium boride can be used further And at high temperature, there are these
making encapsulated wires [R, Fulkiger et al. phenomena that MgB2 can also exist or MgB2
2003]. directly decomposes into Mg and amorphous B.
Without higher pressures increase of temperature
This can be divided into formation of MgB2 by decomposes MgB2 into Mg and MgB4. Preparation
of nanoparticles of magnesium boride by reaction
[a] mechano chemical formation of powder between lithium borohydride and magnesium
[b] chemical synthesis hydride at 450oC with demonstrated 38.7 K Tc was
[c] vapor deposition of powder reported by Jun Lu, and others [J. Lu et al. 2011].
[d] plasma synthesis of powder
[e] self propagating high temperature An in situ synthesis of superconducting
synthesis magnesium boride fibers in magnesium matrix
using a unique approach of infiltrating aligned
Formation of the superconducting boron fibers with liquid magnesium allowing high
components by packing densities of magnesium boride in
magnesium matrix – exhibiting the 39 K Tc of
[a] powder in tube synthesis of wires MgB2 [J.D. deFaouw et al. 2003].
[b] continuous film vapor deposition onto
parallel electrical conducting substrates A pulse plasma synthesis describes
chemical bonding in magnesium in the making
The magazine Superconducting Science microcrystalline powder of magnesium boride and
and Technology has scores of articles published notes a Tc determination of 37.3 K of a conductor
during the past ten years on magnesium boride made from the powder [J. Schmidt et al. 2003]. It
made by different techniques along with end has been described that traditional preparation of
product superconductor component testing. There nanocrystalline boron-containing material is rather
are several companies engaged in producing the complicated but can be simplified by
magnesium diboride powders and superconductor mechanochemical route [F. KH. Urakev et al.
evaluation mostly funded by research grants; 2004].
patents in this field are numerous to note in the
small overview paper. Here we show some 5. Fundamentals on pure boron formation.
highlights of studies being done.
Boron is a solid with a very high melting
An informative study in Turkey, noted that point, and usually forms as a powdery material.
magnesium diboride powder can be synthesized Since there is no medium for its growth in the
from commercial purity [95%] boron and rapidly solidified form it is amorphous in nature. In
magnesium; and that powder in iron tube drawn to cases when solid reactants are chosen the product is
wires showed higher Tc about 20 K compared to mixed with other solid reactants or solid state
powder in copper tube. It notes that the latter products. In cases, where the feed material are
problem was probably caused by the formation of gases [boron chloride + H2, or borane] and boron is
magnesium copper intermetallic. This study also the only solid product, crystallinity is found only
has a comprehensive literature search on where ultra-high temperatures are available as in
magnesium boride synthesis including in situ as plasma or arc processes. It seems that boron
well as ex situ methods [N. Hozrum, 2008]. formation is similar to silicon formation; but Boron
has a melting point of 2076 oC - higher than silicon
The synthesis and properties of Magnesium whose melting point is 1414 oC - making it more
Boride by the Self propagating high temperature difficult to melt form solid pieces of boron to
synthesis [SHS] has been reported [Huimin Lu et facilitate exploiting other electrical properties of
al. 2006]. They note that at normal pressure, the boron.
decomposition temperature of MgB2 is less than A special Ellingham Chart of metal-oxygen
1320K; but higher gas pressure is beneficial to reaction with boron-oxygen is prepared and is
lifting the decomposition temperature of MgB2. shown in Figure 1, to note what may be the readily
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available reductants. But this simple approach does inside a heated quartz tube where a shell formed of
not give the side reactions involved in getting pure boron kept the contamination of the rest of the
boron element, as the information on other boron formed from the walls of the reactor or by
contaminant borides and borates are not shown. partly co-reduced silicon in the shell. Use of
However, this Ellingham chart shows that the thermochemical studies being important in finding
greatest separation of the curves is between Boron- better processes is also noted in this paper as well
oxygen and silicon-oxygen, and that silicon metal [D. Agaogullari et al. 2011].
can reduce boron oxides easily. This fact is partly
utilized in making high purity [99.99% pure] and
crystalline boron by hydrogen reduction of BCl3
Figure 1. Ellingham chart –with Boron Oxygen reaction
The understanding of the fundamentals and refractory and reactive metals. Note that, BCl3
use of thermochemical equilibria calculations done or other boron halides is similar to SiCl4,
by Ind LLC, indicate the possibility of forming TiCl4, ZrCl4 that are reduced by metals or
pure boron element by other metallothermic electrochemically. Metallothermic processes of
reactions of boron tri-halides, with special boron halides can take place at a lower
controls. Kroll’s research on boron involved
temperature than hydrogen reduction.
purification of boron made by magnesium
reduction [W.J. Kroll et al. 1959]. Such metal Processes are amenable for large scale
reduction processes are likely to be more production compared to present Moisson
economical than processes–such as shown in the process. Patent applications on making high purity
making of 99.999% pure boron made for semi- boron by metallic reductants using thermochemical
conductor investigations, from high purity boron principles are underway, which could lead to more
trichloride and hydrogen reduction of boron uses of elemental boron and less expensive methods
halides on hot filaments at above 1300 o C [T. of preparing superconducting MgB2, as well as
Niemyski et al. 1962]. possibly finding more applications for boron.
It is suggested to use metallothermic
methods with proper appreciation of techniques
developed for large scale production of
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Acknowledgment Magnesium Technology 2006, Ed. A.A.
Luo, N.R. Neelameggham, and R.S. Beals ,
The author wishes to thank Prof. Onuralp TMS (The Minerals, Metals & Materials
Yucel for inviting me to give this overview and Society), (2006) 355.
fundamental approach to pure elemental boron and [14] A. Jain, S. Anthonysamy, K.
magnesium boride, and to acknowledge facilitation
by Ind LLC of the project in making boron by more
Ananthasivan, R. Ranganathan, V. Mittal,
economical techniques. S.V. Narasimhan and P.R. Vasudeva Rao:
Materials Characterization, (7) 59 (2008)
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