The Design, Syntheses, and Application of Group 13 Molecular and

Chapter 12

The Design, Syntheses, and Application of Group 13 Molecular and Polymeric Precursors to Advanced Ceramics

Downloaded by UNIV OF LEEDS on October 4, 2015 | http://pubs.acs.org Publication Date: June 3, 2002 | doi: 10.1021/bk-2002-0822.ch012

M a r k J. Pender, Kersten M. Forsthoefel, and Larry G . Sneddon* Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323

New decaborane-based single-source molecular and polymeric precursors to boron carbide have been developed that enable the formation of boron carbide and boron-carbide/silicon -carbide composites in processed forms, including nanostructured materials.

The development of efficient methods for the production of complex structural and electronic materials in usable forms with controlled structures, orders and porosities is one of the most important problems of modern solid state chemistry and materials science. The chemical precursor approach, in which a polymeric or molecular precursor is first formed into the desired shape and then decomposed to the final solid-state material with retention of this shape, has been shown to be an important new route for producing many ceramics in a wide range of processed forms and sizes (/). In our work, we have successfully employed this approach to design new processable precursors to technologically important boron- and silicon-nitride ceramic materials, including: (1) the polyborazylene precursor to boron nitride (2); (2) borazinemodified hydridopolysilazane (3) and borazine-silazane copolymer (4)

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© 2002 American Chemical Society

In Group 13 Chemistry; Shapiro, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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precursors to S i N C B ceramics; (3) second-generation dipentylaminepolyborazylene (5) and pinacolborane-hydridopolysilazane (6) polymeric precursors to B N and S i N C B ceramic fibers (7); and (4) precursors to both metal-boronitrides (8,9) and metal-borides (70). As described in the following sections, our most recent work has focused on the design of new single-source precursors to boron-carbide and boroncarbide/silicon-carbide composites that allow the formation of these ceramics in technologically important processed forms, including films, fibers and nanostructured materials.

Single-Source Boron Carbide and Boron-Carbide/SiliconCarbide Precursors Boron carbide is a highly refractory material that is of great interest for both its structural and electronic properties (77). O f particular importance are its hightemperature stability, high hardness, high cross-section for neutron capture, and excellent high-temperature thermoelectric properties (12). This combination of properties gives rise to numerous applications, including uses as an abrasive wear-resistant material, ceramic armor, a neutron moderator in nuclear reactors, and, potentially, for power generation in deep space flight applications. Boron carbide is normally represented by a B4C (B12C3) composition with a structure based on B u C icosahedra and C-B-C intericosahedral chains. However, singlephase boron carbides are also known with carbon concentrations ranging from 8.8 to 20 at%. This range of concentrations is made possible by the substitution of boron and carbon atoms for one another within both the icosahedra and the three-atom chains. While boron carbide powders are easily made by the direct reaction of the elements at high temperatures, new synthetic methods that allow the formation of pure boron carbide in processed forms still need to be developed. Several investigations have found that boron carbide is indeed formed upon the pyrolysis of boron-based polymers suggesting that an efficient polymer-based route is possible. For example, Seyferth showed that pyrolysis of decaboranediamine and -diphosphine polymers gave B N and BP, respectively, mixed with B C and carbon (75). Likewise, we showed (14) that pyrolysis of decaboranedicyanopentane polymers yields predominately B C with a minor B N component (77). We also found that, upon pyrolysis, polyvinylpentaborane yields pure B C with high ceramic yields; however, due to the thermolytic and hydrolytic instability of this polymer, it has no significant technological potential. Thus, new efficient polymeric precursors to B C are needed that both give pure materials and have sufficient hydrolytic and thermal stabilities to enable processing. 4

4

4

4


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We have now achieved the synthesis of the first decaborane-based polyolefin polymer, polyhexenyldecaborane, and demonstrated that it is, indeed, an excellent precursor to boron carbide in processed forms (15). The synthesis of the monomeric building block, 6-hexenyl-decaborane, that was needed for the construction of the polymer was made possible by our discovery of a new metalcatalyzed method (Eq. 1), involving the Cp2Ti(CO)2 catalyzed reaction of hexadiene with decaborane (16).

The 6-hexenyl-decaborane monomer is isolated in high yield (>90%) as an air stable liquid. Polymerization of the compound was then achieved (Eq. 2) by employing the Cp ZrMe2/B(C F5)3 catalyst system (17) to yield white polymeric materials which are both air stable for extended periods, and highly soluble in organic solvents. 2

6

The spectroscopic data for the polyhexenyldecaborane polymers are in excellent agreement with the inorganic/organic hybrid structure proposed in Eq. 2, consisting of a polyolefin-backbone with pendant decaboranes. The polymer Tgs are in the 50-60°C range and the T G A study in Figure 1 showed that polymer decomposition does not begin until ~225°C. Thus, the polymers are stable as melts. According to the T G A , the ceramic conversion reaction is essentially complete by 600°C. The observed T G A (65%) and bulk (60%) ceramic yields are close to the theoretical ceramic yield of 68% (Eq. 3). A s presented in Figure 2, X R D studies of the black, glassy ceramics obtained upon bulk pyrolyses of the polymer showed that samples heated to only 1000°C were amorphous, but those heated at 1250°C (lh) exhibited the onset of boron carbide crystallization.



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Figure 1. Thermogravimetric (TGA) study ofpolyhexenyldecaborane

1400°C

5B C +C H 4

X

7

1 6

+ 16H

2

(3)

Theoretical Ceramic Yield: 68% Observed Ceramic Yields: TGA: 65% Bulk: 60%

Highly crystalline materials were obtained by heating the ceramics to 1850°C. The diffuse reflectance infrared spectrum, Figure 2, of a sample heated at 1250°C (lh) showed the characteristic boron carbide bands at 1605 and 1144 cm" (18). 1

021 1250X

10" "

2b" ' ' W ' ' '40

50 ' W

TwoTheta (Degrees)

70 '

8Ό " ' SfO

DRIFT

XRD

Figure 2. X-ray Diffraction (XRD) and diffuse reflectance infrared (DRIFT) studies of ceramics derivedfrom polyhexenyldecaborane



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Elemental analyses of the resulting boron carbide ceramics derived from polyhexenyldecaborane indicate compositions in the range of B 4 C . A s discussed earlier, boron carbide can have a range of compositions ranging from 8.8 to 20 at.% carbon and it has been shown that solid-state properties, such as hardness and thermoelectric efficiency, vary with the boronxarbon ratio. The ceramics derived from the polyhexenyldecaborane are thus on the carbon-rich side of the range of boron carbide compositions. However, by altering the boron to carbon ratio of the starting precursor, we have now also developed a new decaborane-based molecular precursor to boron-rich compositions. The new precursor was synthesized by again employing the Cp2Ti(CO)2 catalyzed reaction of B10H14 with hexadiene, but the reactant ratios were altered to ensure hydroboration of both olefinic units. A s shown in Eq. 4, the resulting 6,6'-(Œfe)6-(Bi Hi3)2product is obtained as an off-white, air stable solid in over 90% isolated yields. 0

The T G A study in Figure 3 of the 6,6'-(CH2)6-(BioHi3)2 ceramic-conversion reaction showed that decomposition begins near 220°C and is essentially complete by 400°C.

Figure 3. TGA study of 6,6 '(CH2)6-(BioH13)2 i


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The X R D spectra of bulk powder samples of 6,6'-(CH)6-(BioHi3)2 pyrolyzed at 1000°C (3h) indicated that they were amorphous, but powders heated at 1025°C (3h) exhibited the characteristic boron carbide diffraction pattern. Likewise, DRIFT spectra of powders heated at 1025°C (3h) showed the major boron carbide bands. Since the boron to carbon ratio (20:6) in 6,6'-(CH )6-(BioHi3)2 is much higher than that in the polyhexenyldecaborane (10:6), it was expected to provide access to boron-rich boron carbide compositions. Elemental analyses of the 1000°C (B, 88.39; C, 11.45%) and 1025°C (B, 89.04; C, 10.76%) pyrolyzed powders showed no measurable hydrogen and established B7.7C and B .3C compositions, respectively. Thus, these studies (19) demonstrated that 6,6'(CH )6-(BioHi )2 is, in fact, an excellent precursor to boron-rich boron carbide. Both the polyhexenyldecaborane polymer and the 6,6'-(CH2)6-(BioHi ) compound appear to be ideal precursors for the synthesis of boron carbide materials in processed forms: (1) they are readily synthesized in large amounts using metal catalyzed reactions; (2) they contain no other ceramic forming elements and have complementary boron to carbon ratios, thus yielding carbonrich or boron-rich boron carbide compositions, respectively, upon pyrolysis; (3) they are stable as melts, thus allowing the use of melt-processing methods; and (4) upon pyrolysis, both precursors undergo a crosslinking reaction at relatively low temperatures (~220°C) that retards loss of material by volatilization, thereby generating high ceramic and chemical yields. One of the limitations on the use of boron carbide is its relatively low oxidation temperature (600°C

The Design, Syntheses, and Application of Group 13 Molecular and

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