Leonard V. Interrante

Coordination compounds of aluminum as precursors to aluminum nitride

Abstract Two different systems have been examined as potential sources of aluminum nitride, an important electronic and structural ceramic material. Cyclic organoaluminum amides obtained as intermediates in the thermolysis of trialkylaluminum : ammonia Lewis acid-base complexes have been used to obtain AlN powder and as precursors for the chemical vapor deposition of AlN films. The structures of two of these intermediates were determined by single-crystal XRD and the kinetics and thermodynamics of their formation and thermal decomposition reactions were also studied. The second system employs ethylenediamine as the Lewis base in combination with the R 3 Al (R = Me, Et) compounds. A 2:1 ratio of Et 3 Al with en yields a hydrocarbon-soluble, polymeric amide on thermolysis, which can be used to prepare AlN films by solution coating followed by pyrolysis in NH 3 . Lower proportions of R 3 Al to en, on thermolysis, lead to the formation of R m Al(en-2H) n cluster species that contain 5- and 6-coordinated Al atoms chelated and bridged by en-2H ligands. On further heating, these cluster species apparently go on to form cross-linked, insoluble, polymeric networks through condensation reactions involving the multiple N-H and R-Al groups on the periphery of the cluster molecules. The structures of two of these novel Al-en cluster compounds were determined by single-crystal XRD.

Studies of Organometallic Precursors to Aluminum Nitride

Abstract : The reaction of trialkylaluminum compounds with ammonia has been examined as a potential route to high purity AlN powder and to AlN thins films. This reaction proceeds in stages in which the initially formed Lewis acid/base adduct undergoes thermal decomposition to a series of intermediate alkylaluminum-amide and -imide species with increasing Al-N bonding, i.e., R3A1 + NH3 yields R3Al:NH3 yields AlN + 3RH (where R = CH3, C2H5, C4H9, etc.). The structure and properties of several of these species have been studied using various physical and chemical methods, leading to a better understanding of the chemistry of this novel A1N precursor system. The structure of the intermediate organoaluminum amide, (CH3)2AlNH2, has been determined by single crystal X-ray diffraction methods and found to contain molecular trimer units with a six- membered Al-N ring structure similar to those which make up the wurzite structure of AlN. This compound is readily volatile and has been used to deposit AlN thin films on Si surfaces by a low-pressure CVD process. This approach has also been used to prepare AlN as a high surface area, high purity powder. Keywords: Chemical vapor deposition; Organoaluminum amide.

High Yield Polycarbosilane Precursors to Stoichiometric SiC. Synthesis, Pyrolysis and Application

The synthesis and properties of two polycarbosilanes that have essentially a “SiH 2 CH 2 ” composition is described. One of these polymers is a highly branched hydridopolycarbosilane (HPCS) derived from Grignard coupling of CI 3 SiCH 2 CI followed by LiAIH 4 reduction. This synthesis is amenable to large scale production and we are exploring applications of HPCS as a source of SiC coatings and its allyl-derivative, AHPCS, as a matrix source for SiC- and C-fiber-reinforced composites. These polymers thermoset on heating at 200-400 °C (or at 100 °C with a catalyst) and give near stoichiometric SiC with low O content in ca. 80% yield on pyrolysis to 1000 °C. The second method involves ring-opening polymerization of 1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight, linear polymer that can be reduced to [SiH 2 CH 2 ] n (PSE), the monosilicon analog of polyethylene. In contrast to high density polyethylene which melts at 135 °C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On pyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC in virtually quantitative yield. The polymer-to-ceramic conversion was examined for PSE by using TGA, mass spec, solid state NMR, and IR methods yielding information regarding the cross-linking and structural evolution processes. The results of these studies of the polymer-to-ceramic conversion process and our efforts to employ the AHPCS polymer as a source of SiC matrices are described.

Fabrication of SiC matrix composites by liquid phase infiltration with a polymeric precursor

A process for the fabrication of SiC matrix composites has been developed which employs a liquid, highly branched, polycarbosilane (AHPCS). This polymer thermosets on heating at 200--400 C (or at 100 C with catalyst) and yields an amorphous SiC with low excess C and O content in 60--80% yield on pyrolysis to 1,000 C. Preforms consisting of C-coated Nicalon SiC fiber cloth, unidirectional Textron SiC SCS-6 fiber layups, or Mo boats packed with SiC whiskers, were infiltrated with the polymer, cured in an autoclave, and pyrolized to 1,000 C. Five to eight infiltration cycles gave net shape composites with final densities at 85--94% of theoretical. The results of 4-point flexure tests on the as-prepared Nicalon composites indicate flexure strengths (aver. 378 MPa) that are comparable to or better than similarly reinforced CVI-SiC matrix composites. The whisker and SCS-6 composites showed a small weight loss (10--20%) on heating to 1,500 C in Ar and little or no weight change or obvious embrittlement in air at 1,000 C.

Linear and hyperbranched polycarbosilanes with Si-CH2-Si bridging groups: a synthetic platform for the construction of novel functional polymeric materials

Work carried in the authors’ laboratory on Si‐ CH2‐Si bridged polycarbosilanes is reviewed. In pursuit of high-yield polymeric precursors to silicon carbide, convenient synthetic routes to both linear and hyperbranched polycarbosilanes having a ‘[SiH2CH2]n’ compositional formula have been developed. The linear [SiH2CH2]n polymer was prepared by ring-opening polymerization of a substituted disilacyclobutane, and was studied both as an analogue of polyethylene and as a high-yield precursor to SiC. Elaboration of the methods employed to prepare this polymer has yielded a wide range of new poly(silylenemethylene)s (PSMs) of the type [SiRR’CH2]n, where R and R’ can be a wide range of different groups, including a series of symmetrically disubstituted polymers with R=R ’= F, alkyl and alkoxy which form crystalline solid phases and various amorphous, atactic polymers having different R and R’ groups. By using (Si)‐Cl replacement reactions analogous to those developed previously for polydichlorophosphazene, as well as hydrosilation reactions similar to those used for [Si(H)(Me)O]n, a series of side-chain polymers having various groups attached to Si through Si‐C or Si‐O bonded linkages were obtained. Similar polymer modification reactions have recently been developed for the branched oligomer/polymer analogue of these linear polycarbosilanes, leading to hyperbranched species with functional substituents, including a di(ethyleneoxy) methyl ether-terminated derivative which readily dissolves lithium salts. The results of studies of these novel ‘inorganic/organic’ hybrid polycarbosilanes are described and their properties are compared with those of related carbon-backbone and siloxane polymers. # 1998 John Wiley & Sons, Ltd.

Novel Solid State Materials Derived from Transition Metal Bis-dithiolene Complexes

Abstract The transition metal bis-dithiolene complexes (MS4C4X4) can be used as acceptor molecules in π-donor-acceptor-type compounds, or as radical anions in mixed valence compounds. These compounds are in the wide middle ground between the strictly organic or inorganic molecular solids on which extensive research has led to the discovery of molecular superconductors and to the development of new materials and concepts. Novel solid state materials derived from metal bis-dithiolene complexes also exhibit interesting original physical properties. In the (TTF)[MS4C4(CF3)4] series of compounds, depending on the nature of the metal ion, a wide range of novel magnetic phenomena ranging from bulk ferrimagnetism (M = Ni) to spin-Peierls transition (M = Cu and Au) have been observed. Metal bis-dithiolene complexes have also been used for preparing molecular superconductors such as (TTF)[M(dmit)2]2 and (Me4N)0.5[Ni(dmit)2] (dmit2- = 1,3-dithiol-2-thione-4,5-dithiolate) in which the conduction mechanisms are quite ...

A Novel Polycarbosilane-Based Low-k Dielectric Material

The electrical properties of a novel, cross-linked polycarbosilane, which is thermally stable up to 450°C, were investigated as a potential interlayer dielectric for advanced interconnect technology. This polycarbosilane is derived from a cyclolinear precursor polymer, which was spin-coated from xylene solution and cured at 300°C to induce cross-linking. The resultant films were found to have a dielectric constant of 2.32 (nonporous) and to exhibit excellent resistance to Cu diffusion under standard bias temperature stress (BTS) test conditions. This resistance to Cu diffusion is attributed to the fact that this polymer does not contain oxygen and that Cu ionization through metal oxidation at the metal-polymer interface can thus be prevented. It was further demonstrated that an ultrathin layer of polymer with a nominal thickness of 20 nm is sufficient to block the Cu ion penetration into an ultralow-k porous methylsilsesquioxane film when subjected to BTS. This polycarbosilane holds considerable promise for application as an interlayer dielectric in the next generations of high-performance integrated circuits.

Future directions in solid state chemistry: report of the NSF-sponsored workshop

Abstract A long-established area of scientific excellence in Europe, solid state chemistry has emerged in the US in the past two decades as a field experiencing rapid growth and development. At its core, it is an interdisciplinary melding of chemistry, physics, engineering, and materials science, as it focuses on the design, synthesis and structural characterization of new chemical compounds and characterization of their physical properties. As a consequence of this inherently interdisciplinary character, the solid state chemistry community is highly open to the influx of new ideas and directions. The inclusionary character of the field’s culture has been a significant factor in its continuing growth and vitality. This report presents an elaboration of discussions held during an NSF-sponsored workshop on Future Directions in Solid State Chemistry , held on the UC Davis Campus in October 2001. That workshop was the second of a series of workshops planned in this topical area. The first, held at NSF headquarters in Arlington, Virginia, in January of 1998, was designed to address the core of the field, describing how it has developed in the US and worldwide in the past decade, and how the members of the community saw the central thrusts of research and education in solid state chemistry proceeding in the next several years. A report was published on that workshop (J.M. Honig, chair, “Proceedings of the Workshop on the Present Status and Future Developments of Solid State Chemistry and Materials”, Arlington, VA, January 15–16, 1998) describing the state of the field and recommendations for future development of the core discipline. In the spirit of continuing to expand the scope of the solid state chemistry community into new areas of scientific inquiry, the workshop elaborated in this document was designed to address the interfaces between our field and fields where we thought there would be significant opportunity for the development of new scientific advancements through increased interaction. The 7 topic areas, described in detail in this report, ranged from those with established ties to solid state chemistry such as Earth and planetary sciences, and energy storage and conversion, to those such as condensed matter physics, where the connections are in their infancy, to biology, where the opportunities for connections are largely unexplored. Exciting ties to materials chemistry were explored in discussions on molecular materials and nanoscale science, and a session on the importance of improving the ties between solid state chemists and experts in characterization at national experimental facilities was included. The full report elaborates these ideas extensively.

Preparation and thermal properties of asymmetrically substituted poly(silylenemethylene)s

Poly(silylenemethylene)s of the types [SiMeRCH 2 ] n and [SiHRCH 2 ] n were prepared by the ring-opening polymerization (ROP) of 1,3-disilacyclobutanes (DSCBs) containing n-alkyl substituents, such as C 2 H 5 , n-C 3 H 7 , n-C 4 H 9 , n-C 5 H 11 , and n-C 6 H 13 , or a phenyl group on the Si. These new polymers include a monosilicon analog of poly(styrene), [SiHPhCH 2 ] n . Improved synthesis routes to the DSCB monomers were developed which proceed through Grignard ring closure reactions on alkoxy-substituted chlorocarbosilanes. All of these asymmetrically substituted polymers were obtained in high molecular weight form, except for [SiHPhCH 2 ] n . The configurations of all of the polymers were found to be atactic. The aryl-substituted polymers have higher glass transition temperatures (T g s) and thermal stability than those of the alkyl-substituted poly(silylenemethylene)s. Unlike the polyolefins of the type [C(H)(R)CH 2 ] n , where T g drops continuously from R = Me to n-Hex, the T g s of the n-C n H 2n+1 (n = 2-6)-substituted [SiMeRCH 2 ] n PSMs appear to reach a maximum (at -61°C) for the R = n -Pr-substituted polymer. Moreover, where it was possible to make direct comparisons among similarly substituted atactic polymers, all of the poly(silylenemethylene)s were found to have lower T g s than their all-carbon analogs.

Identification of the gas-phase products which occur during the deposition of AIN using the organometallic percursor: [(CH3)2AINH2]3

Abstract Molecular beam-mass spectrometry is used to analyze the gas-phase products occuring during chemical vapor deposition (CVD) of AIN using tris-dimethylaluminum amide, [(CH3)2AINH2]3 (I), as a precursor. By correlating the ion signals in the mass spectra for the case of I to the ion signals in the mass spectra of the deuterated analogue, [(CH3)2AIND2]3, it is possible to assign tentative structures to the ion signals in the mass spectra of the gas flow emerging from the reactor. It is shown that time-of-flight velocity spectra can be used to determine the molecular weight of the species which generate the ion signals in the mass spectra. For the case of high pumping speeds or short reactor residence times, the time-of-flight velocity spectra indicate that the trimeric form of the precursor is the only species present in the gas phase at a reactor temperature of 100°C. However, at low pumping speeds the time-of-flight velocity spectra support the existence of dimeric as well as trimeric forms of the precursor at a reactor temperature of 100°C.