CHAPTER 2: SYNTHESIS OF NANOSTRUCTURED TRANSITION METAL
A. Introduction
Non-oxide ceramics are ceramics consisting of a metal bonded with either metalloids or non-metals, other than oxygen, and have become an important class of materials. Non-oxides are rarely found in nature and are sometimes found in meteorites.
This rarity ensures that any commercial use of these materials will require synthetic processes.
Bursts of development have occurred depending on a particular need and some non-oxide materials have been developed more than others. For instance, Si3N4 was a highly pursued material beginning in the 1950’s for the use of lightweight ceramics for more efficient engines.[1-2] Although an all ceramic engine was never successfully implementeed, the intense research efforts to do so have resulted in the development of other important classes of materials (silicon aluminum oxynitrides, other oxynitrides, and oxynitride glasses for example). Overall, the possibility for replacing some engine components with Si3N4 and introduction of these materials in other suitable markets as processing was developed, tested, and improved. Si3N4-based materials are in some ways one of the most well understood advanced ceramic systems as a result of intense development.[3-5] Although silicon based non-oxide materials are a great example of successful and widespread commercialization of non-oxide materials, they only represent a small subset of ceramic non-oxide possibilities.
Ultra-high-temperature ceramics (UHTCs) are a particular subclass of non-oxides that had begun to be get some attention in the early 1960’s to mid 1970’s for materials that could be capable of withstanding the harsh environments of space applications.[6-7]
UHTC research has been substantially expanded in more recent years to meet new technological challenges, particularly as potential candidates for extreme environments associated with hypersonic flight, rocket propulsion, and space re-entry conditions.[8-11]
The definition of UHTCs varies, but in general corresponds to ceramics with melting temperatures above ~3000 K for use in high-temperature applications where temperatures may be near the range of 2000-3000 K.[12] Some of the transition metal borides, carbides, and nitrides meet this specification and some authors have limited their definition to these materials.[12-13] Table 2-I provides a summary of selected compounds demonstrating such high melting temperatures. The high melting temperatures and relatively good thermal-chemical-mechanical stability of these materials is attributed to covalent bonding. Adiabatic flame temperatures are also provided in the table and will be discussed later.
Table 2 – I. Melting Temperatures (Tm) and Adiabadic Flame Temperature (Tad) by Reaction from the Compound Constituents of Selected Ultra-High Temperature
Ceramics[14-15]
Carbides Tad (K)
Tm
(K) Borides Tad (K)
Tm
(K) Nitrides Tad (K)
Tm
(K) HfC 3900 4100 HfB2 3520 3640 HfN 5100 3600 TaC 2700 4100 TaB2 3370 3540 TaN 3360 3220 ZrC -- 3533 ZrB2 3310 3270 ZrN 4900 3230 TiC 3210 3210 TiB2 3190 3500 TiN 4900 3560
Although the melting temperatures of non-oxide UHTCs are high, special care must be considered for realizing the use of these materials at high temperatures without significant decomposition. Virtually all non-oxide ceramics in almost every application environment of interest are in a state of non-equilibrium with the atmosphere and the effective use of the material relies on the characteristics of the degradation processes involved. The most common non-equilibrium condition of interest is the result of being exposed to a chemically oxidizing atmosphere, which has been addressed by many authors not limited to the references provided for examples.[16-20] The oxidation resistance is a frequent deficiency in UHTCs, but can be improved with appropriate focus on compositional/composite design to engineer a protective oxidation scale.
The oxidation mechanism for a non-oxide can be classified into active (typically associated with weight and material loss that is continuous) and passive (typically
associated with weight gain with material conversion that slows with time). Active oxidation should be avoided completely for most applications. The typical weight gain associated with passive oxidation is the result of oxide scale formation and the oxidation slows with time because the scale acts as a barrier that limits further oxidation. The oxide scale can also be considered protective or non-protective. Non-protective oxide scales may result when the scale is porous, is not in equilibrium with the atmosphere, and/or cracking of the scale occurs (from physical damage, thermal shock, or stresses from interaction with the non-oxide bulk for example).
The ideal UHTC from a design perspective will balance high melting temperature (to act as the bulk structural support) with a relatively stable oxide scale on the surfaces to protect the bulk from further oxidation at high temperatures. The general approach to this is by designing UHTC composites. The composite should form a dense scale on its surfaces that is as stable as possible with both the UHTC bulk and the atmospheric conditions. The stability of the scale with the UHTC bulk depends on chemical stability, good adhesion, similar thermal expansions to prevent stress development, and low thermal expansion to minimize the risk of thermal shocking the UHTC. The interactions between the oxide scale and the atmosphere should be minimal, requiring that the oxide scale that forms is also highly refractory. However, some lack of refractoriness may be beneficial for “healing” cracks that may form in the scale and will also require a balanced design approach.
One particular issue with exploring UHTC composites is that their high temperatures and stability make them expensive to produce and process, significantly hindering the ability to study them as extensively as would otherwise be possible. As an example, Levine et al. report on the study of ZrB2, but explicitly state that they did not study HfB2 because of cost limitations.[11] Although computer aided design and modeling will help select appropriate materials for the design of UHTC composite materials, experimental verification and testing is still a necessity. Utilizing nanostructured powders has the potential to reduce sintering temperatures, which could reduce processing costs.
There are several techniques currently utilized for the synthesis of nanostructured non-oxide ceramics.[21-22] For obtaining nanostructured non-oxides, the synthesis method
should result in nano-regime crystallite sizes and ideally have low levels of agglomeration, have high phase purity, have high chemical purity (including clean surfaces), and be a well understood and controllable process. An ideal synthesis method for ensuring a low cost would also be energy efficient, fast, scalable, and be able to produce a diverse group of materials (in this case a broad range of UHTCs that can be used for composite development). All of the typical synthesis methods satisfy some of these criteria, but often fall short for a few of the other desired characteristics. New techniques or modifications to currently used techniques that address these issues are desirable.
Table 2-I also demonstrates that these materials having high melting temperatures also have high adiabatic flame temperatures by the stoichiometric reaction between the metal and non-metal. The adiabatic flame temperature is the theoretical temperature that an exothermic reaction will achieve if all of the heat is transferred to the system (reaction products) with no heat loss to the environment and assuming full conversion of the reactants. Realistically heat is also lost to the environment. Highly exothermic reactions having high adiabatic flame temperatures (> 2073 K) are empirically suitable for self- propagating high temperature synthesis (SHS) of materials.[14] The high reaction temperature and short reaction time during SHS result in a reaction that approaches adiabatic behavior and therefore reaches a temperature that approaches the adiabatic flame temperature.
A promising and scalable solvothermal synthesis method was recently reported for synthesizing TaC and LaB6.[23-24] Scalability is important because large-volume production assists in driving costs downward. The solvothermal synthesis of these materials relies on SHS reactions within a melt. The SHS nature of the reaction provides energy efficiency, requiring only a low amount of external energy to ignite the reactants, which also contributes to low synthesis costs. Similar to traditional SHS, the reaction is fast, assisting scalability by allowing for large quantities of powders to be produced rapidly. Similar techniques have been demonstrated in kg-quantities.[25-27] The melt reduces grain size, overcoming the difficulty to produce fine powders from traditional SHS. The empirical relationship for the successful synthesis of a material by solvothermal synthesis is expected to be similar to that for the parent SHS technique.
Thus, reactions having adiabatic flame temperatures >2073 K, like many listed in Table 2-I, are good candidates for the solvothermal synthesis method.
By eliminating the use of pressure for solvothermal synthesis, the necessity of an autoclave was eliminated and the reaction could be performed in simpler apparatus. The simpler apparatus is not as costly as an autoclave and the thin walls of the reaction vessel allow for much faster heating and cooling rates when compared to using an autoclave, which helps make the process faster from start to finish and minimizes agglomeration caused by reprecipitation.
The solvothermal process has been used to obtain TaC powders that were nanostructured, phase pure, and had a low level of agglomeration.[23-24] Some flexibility in various responses versus processing variables was also demonstrated.[24] In fact, the only appealing aspects of an ideal process that were described earlier that have not been studied for this solvothermal synthesis method include chemical purity and demonstrating that a broad range of materials can be synthesized using this method. The primary chemical impurities of non-oxides include carbon or carbon containing impurities, oxygen, metals and halogens. Removal of these impurities from non-oxides is already well studied.[28] Therefore, the purpose of this work will be to demonstrate the potential of the solvothermal synthesis method for making a diverse set of UHTC powders and begin to define the criteria that defines the conditions for successful reactions.
It is important to note that the purpose is to demonstrate the synthesis of these materials, not to optimize the synthesis. Some aspects of process optimization have been provided previously.[24]