Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si-based advanced ceramics, generally denoted as polymer-derived ceramics (PDCs). The polymer to ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep. In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior. From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso-range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components. Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities. Several key engineering fields suitable for application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food- and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics. The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro- and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon-based preceramic polymers.
(2) Special microstructural features of PDCs.
(3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines.
(4) Processing strategies to fabricate ceramic components from preceramic polymers.
(5) Discussion and presentation of several examples of possible real-life applications that take advantage of the special characteristics of preceramic polymers. Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN-based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.

Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics

The recent development in the field of superhard materials with Vickers hardness of ⩾40 GPa is reviewed. Two basic approaches are outlined including the intrinsic superhard materials, such as diamond, cubic boron nitride, C3N4, carbonitrides, etc. and extrinsic, nanostructured materials for which superhardness is achieved by an appropriate design of their microstructure. The theoretically predicted high hardness of C3N4 has not been experimentally documented so far. Ceramics made of cubic boron nitride prepared at high pressure and temperature find many applications whereas thin films prepared by activated deposition from the gas phase are still in the stage of fundamental development. The greatest progress has been achieved in the field of nanostructured materials including superlattices and nanocomposites where superhardness of ⩾50 GPa was reported for several systems. More recently, nc-TiN/SiNx nanocomposites with hardness of 105 GPa were prepared, reaching the hardness of diamond. The principles of de...

https://mediatum.ub.tum.de/doc/958917/file.pdf

The search for novel, superhard materials

Nanocomposites, a high performance material exhibit unusual property combinations and unique design possibilities. With an estimated annual growth rate of about 25% and fastest demand to be in engineering plastics and elastomers, their potential is so striking that they are useful in several areas ranging from packaging to biomedical applications. In this unified overview the three types of matrix nanocomposites are presented underlining the need for these materials, their processing methods and some recent results on structure, properties and potential applications, perspectives including need for such materials in future space mission and other interesting applications together with market and safety aspects. Possible uses of natural materials such as clay based minerals, chrysotile and lignocellulosic fibers are highlighted. Being environmentally friendly, applications of nanocomposites offer new technology and business opportunities for several sectors of the aerospace, automotive, electronics and biotechnology industries.

/pdf/nanocomposites-synthesis-structure-properties-and-new-4nejccptqe.pdf

Nanocomposites: synthesis, structure, properties and new application opportunities

A central challenge to modern materials science is the rational design and synthesis of new materials possessing exceptional properties. Recent advances in first-principles modeling methods and the availability of increasingly powerful computational resources make this goal increasingly achievable. The strength of these modeling methods lies in their predictive ability. They are able to reproduce the crystal structures and elastic properties of a large class of materials to within 2–3% of experimental values and have predicted a number of phase transitions that have been verified experimentally.Despite the power of these methods, the process of designing materials from first principles is not usually a straight-forward or simple one. It requires overcoming a number of obstacles, some of them quite formidable. First a calculable figure of merit that correlates well with the desired property must be identified. While this may be straightforward in some cases, in others—such as predicting the ability of a material to isolate radionuclides over million-year time scales—the process of reducing complex properties to a few calculable variables can be rather difficult. Next a promising chemical system and a realistic set of crystal structures must be selected. This is not trivial because predicting the structures that can crystallize in a given system can be exceedingly challenging. However a wide variety of methods are available to aid in the generation of promising structures — comparative crystallography, algorithms based upon the concepts of crystalline nets and close packing, modern alloy theory methods, and simulated annealing strategies being some examples.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400031420

Computational Alchemy: The Search for New Superhard Materials

Structural ceramic nanocomposites are reviewed with emphasis on the Al 2 O 3 SiC and Si 3 N 4 SiC systems. The incorporation of a nanosized second phase, such as SiC, into a ceramic matrix can lead to an improvement in mechanical properties. It is still unclear, however, whether those improvements can directly be related to an intrinsic ‘nanocompossite effect’ or to other factors. This review is divided into three parts. First, basic processing routes for nanocomposites, namely conventional powder processing, sol-gel processing and polymer pyrolysis are presented in detail. Second, the mechanical properties of different nanocomposites are compared. Finally, models which attempt to explain the improvements in these properties are explored. It will be shown that the strength increase can best be related to a reduction in processing defect size. For applications the most interesting properties of nanocomposites are their wear, creep and high temperature performance.

Structural ceramic nanocomposites

Three novel Si-C-B-N ceramic compositions, namely Si2.9B1.0C14N2.9, Si3.9B1.0C11N3.2 and Si5.3B1.0C19N3.4, were synthesized using the polymer-to-ceramic transformation of the polyorganoborosilazanes [B(C2H4Si(Ph)NH)3]n, [B(C2H4Si(CH3)NH)2–(C2H4Si(CH3)N(SiH2Ph))]n, and [B(C2H4Si(CH3)–N(SiH2Ph))3]n, where Ph is phenyl (C6H5), at 1050°C in argon. The Si-B-C-N ceramics exhibited significant stability with respect to composition and mass change in the temperature range between 1000° and 2200°C, including isothermal annealing of the samples at the final temperature for 30 min in argon. The mass loss rate at 2200°C was as low as 1.4 wt%·h−1 for Si5.3B1.0C19N3.4, 1.7 wt%·h−1 for Si2.9B1.0C14N2.9, and 2.4 wt%·h−1 for Si3.9B1.0C11N3.2. The measured amount of mass loss rate was comparable to that of pure SiC materials. As crystalline phases, β-Si3N4 and β-SiC were found exclusively in the samples annealed at 2200°C at 0.1 MPa in argon. For thermodynamic reasons, β-Si3N4 should have decomposed into the elements silicon and nitrogen at that particular temperature and gas pressure. However, the presence of β-Si3N4 in our materials indicated that carbon and boron kinetically stabilized the Si3N4-based composition.

Novel Silicon‐Boron‐Carbon‐Nitrogen Materials Thermally Stable up to 2200°C

In the present work a Si 3 N 4 composite containing 24 wt% SiC particulates was prepared by sintering a polysilazane-derived amorphous SiNC powder. Al 2 O 3 and Y 2 O 3 were added as sintering additives. The amorphous powder was pressureless sintered to 97% relative density at 1750°C in nitrogen atmosphere. α-Si 3 N 4 and β-SiC were generated in situ due to the crystallization of the amorphous powder during the heating in the densification process. Subsequently, α-Si 3 N 4 is transformed to the β-form by the liquid phase sintering mechanism. A fine-grained microstructure (d 50 = 0·2 μ m) with the SiC particulates homogeneously distributed in the Si 3 N 4 -matrix was found in the densified samples. Room temperature fracture strength ( σ B ) and fracture toughness (K Ic ) were similar to those measured in conventionally processed composites. However, the fracture strength was characterized by a relatively low standard deviation indicative of the better reproducibility of the properties due to improved homogeneity in the developed microstructure.

Sintering of amorphous polymer-derived Si, N and C containing composite powders

Thermodynamic calculations in the systems Al-Cl, Al-Cl-N, and H-Al-Cl-N were used to assess the capabilities of AlCl3 or mixtures of AlCl3 with Al to produce AlN by chemical vapor deposition (CVD) techniques. Direct nitridation (N2 as reaction agent) is possible only at high temperatures (≥1500 K), using AlCl3–Al mixtures. Reaction with NH3 at equilibrium gives low yields but the suppression of NH3 dissociation yields near 100%, which makes the method suitable for powder production, coating, and single-crystal growth. AlN with less than 1 wt% oxygen was obtained from technical grade AlCl3 by this process. The formation of both amorphous AlN powder and crystalline AlN coatings was observed. It is assumed that the formation of AlCl3·xNH3 adducts by mixing of Al-Cl vapor and NH3 at temperatures ≤1273 K prevents NH3 dissociation and favors the production of amorphous AlN.

Thermodynamic and Experimental Study of High-Purity Aluminum Nitride Formation from Aluminum Chloride by Chemical Vapor Deposition

The crystallization of amorphous polymer-derived Si 3 N 4 was investigated. The transformation of the amorphous into the crystalline α-phase was determined to be diffusion-controlled. In this report, the rate constants and the activation energy as well as the temperature-dependent induction period τ of the crystallization reaction were calculated by using a modified Avrami-Erofe'ev equation.

Crystallization behaviour of amorphous silicon nitride

Die thermische Zersetzung von Amin-Boranen im Temperaturintervall von Raumtemperatur bis 1050°C sowie die Phasenumwandlung des aus BH3 · C5H5N erhaltenen Ruckstands bei Temperaturen bis 2200°C wird untersucht. Der Pyrolyseverlauf wird thermogravimetrisch und massenspektroskopisch verfolgt. Nach elementaranalytischen und photoelektronenspektroskopischen (ESCA) Ergebnissen handelt es sich bei den Zersetzungsprodukten um einphasiges Borcarbidnitrid (BxCyNz), einem Bor und Stickstoff enthaltenden pyrolytischen Kohlenstoff. Dies wird auch durch die analytische Elektronenspektroskopie bestatigt. Durch nachfolgende „Auslagerung” des aus Pyridin-Boran erhaltenen amorphen BxCyNz in einer Heispresse bei 1800°C und 190 MPa Druck werden whiskerformige Kristalle, deren d-Werte auf die Bildung von BN und/oder Graphit hinweisen, erhalten.



Amine-Boranes as Precursors for Boron Carbonitride



Amine-Boranes have been investigated with respect to their thermal decomposition in the temperature range from rom temperature to 1050°C. The phase transformation of the pyrolytic residue derived from BH3 · C5H5N subsequently heat-treated at temperatures up to 2200°C was studied. The progress of the pyrolysis has been detected by thermal analysis (TGA) and mass spectroscopy. Chemical analysis and ESCA measurements show that the pyrolytic products are single-phase boron carbonitride (BxCyNz) which can be described as a boron and nitrogen containing pyrolytic carbon. This is also supported by means of analytical electron microscopy. Hot pressing of the pyridine-borane-derived BxCyNz at 1800°C and a pressure of 190 MPa yields whiskerlike crystals whose d-values are in accordance with that of BN and/or graphite.

Ralf Riedel

Papers

Novel Silicon‐Boron‐Carbon‐Nitrogen Materials Thermally Stable up to 2200°C

Sintering of amorphous polymer-derived Si, N and C containing composite powders

Thermodynamic and Experimental Study of High-Purity Aluminum Nitride Formation from Aluminum Chloride by Chemical Vapor Deposition

Crystallization behaviour of amorphous silicon nitride

Amin‐Borane als Precursoren für Borcarbidnitrid