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Scalable fabrication protocols for the syntheses of structures with functional chemistries and complex morphologies that can be tailored over various length scales (even down to the nanoscale) may have a significant impact in a variety of current or potential applications. One paradigm for such fabrication involves separation of the processes for structure formation and for chemical tailoring; that is, a solid structure (a "preform") of a given chemistry may first be fabricated with desired macro-to-nanoscale morphological features, and then converted into a new chemistry via morphology-preserving gas/solid or liquid/solid reaction(s). The extensive literature available on the kinetics and phase evolution associated with such reactions in the field of high temperature oxidation/corrosion provides a rich source of mechanistic information that can be utilized for such materials "alchemy."

The purpose of this overview is to provide examples of the following three reaction-based approaches for the chemical conversion of shaped solid preforms: gas/solid displacement reactions, liquid/solid displacement reactions, and gas/solid oxidation reactions. Under appropriate conditions, each of these reaction-based approaches can be used to transform solid preforms into new nanostructured materials while preserving the preform morphology (note: a "nanostructured material" refers herein to a material with one or more physical features, such as a phasesize, crystal size, and/or pore size, with a dimension below one micrometer). The syntheses of two types of shaped, nanostructured materials will be discussed here: microscopic structures with nanoscale features (generated via the application of gas/solid displacement reactions to solid microscopic preforms), and macroscopic structures with nanoscale phase and/or crystal sizes (generated via the application of liquid/solid displacement reactions, or gas/solid oxidation reactions, to solid macroscopic preforms).

Biologically Replicable, 3-D, Microscale, Nanostructured Preforms: Diatom Frustules
Among the most complex-shaped, three-dimensional (3-D) nanostructured oxide templates available for the application and evaluation of shape-preserving gas/solid displacement reactions are the silica-based microshells (frustules) generated by diatoms. Diatoms are single-celled algae that populate a wide variety of aquatic environments.^1,2 Each diatom species generates an amorphous silica frustule with a particular 3-D shape and patterned nanoscale features (e.g., pores, channels, protuberances of 10¹–10² nm size), which suggests a strong degree of genetic control over the silica formation process.^1,3 Such species specificity (genetic control) of the diatom frustule morphology enables enormous numbers of similarly shaped frustules to be generated via sustained culturing (repeated doubling) of a single diatom species⁴ (e.g., 80 reproduction cycles from a single parent diatom cell can yield 2⁸⁰, or more than one trillion trillion, daughter cells with frustules of similar morphology). Furthermore, the 10⁴–10⁵ extant diatom species possess frustules with a spectacular variety of intricate 3-D morphologies.¹ Future genetic engineering of diatoms may allow for a wide range of tailored 3-D shapes.³ The precise (down to nanoscale), hierarchical (nano-to-microscale), complex (3-D), reproducible (genetically-controlled), and scalable (massively-parallel reproduction) nature of diatom microshell assembly under ambient conditions, coupled with the wide variety of selectable (and, in the future, perhaps tailorable) morphologies among extant diatom species, make diatom frustules unique and highly attractive as templates for synthetic chemical conversion into microscale nanostructures for potential devices applications (with a future potential for "Genetically Engineered Microdevices").^5–9

Oxide, Oxide/Metal, and Metal Replicas via Oxidationreduction Displacement Reactions
The first demonstration of the use of a gas/solid displacement reaction to convert a nanostructured 3-D oxide microparticle (i.e., a silica diatom frustule) into an entirely new oxide composition, while retaining the microparticle morphology, was reported in 2002.⁷ Diatom frustules were exposed to magnesium vapor (generated by heating solid magnesium granules) at 900°C for 4 h to allow for a net magnesiothermic displacement reaction (Reaction 1), where {Si} refers to Si dissolved within a Mg-Si alloy liquid. (Note that all reactions are given in Table I.) Because the Mg:SiO₂ reactant ratio was maintained at a value well above the 2:1 molar ratio required by the stoichiometry of Reaction 1, the excess magnesium vapor continued to react with the silicon product to generate a magnesium-silicon liquid (note: the eutectic temperature for magnesium-rich compositions in the Mg-Si system is only 638°C¹⁰). This Mg-Si alloy liquid poured out of the reacted frustules to yield MgO-based structures that retained the 3-D frustule morphology and patterned pores (Figure 1).^5,7 This magnesiothermic process has also been used to convert "biosculpted" silica structures into magnesia replicas.¹¹ The exposure of a silica-precipitating peptide, derived from the silaffin-1A protein of the diatom Cylindrotheca fusiformis, to a tetramethylorthosilicate precursor solution under a linear shear flow condition resulted in the formation of interwoven microfilamentary silica fibrils. Upon reaction with magnesium gas at 900°C, the silica microfilaments were converted into nanocrystalline magnesium oxide replicas that retained the microfilamentary shape and fine (submicrometer) features.¹¹ Nanocrystalline magnesia in the form of such intricate porous 3-D microparticles (available in a wide variety of shapes as diatom frustules) or 2-D microfilamentary structures can be attractive for use in environmental (e.g., for heavy metal precipitation from, or neutralization of, acidic wastewater streams; for SO₂(g) removal from gaseous emissions), chemical/petrochemical (e.g., as an acid acceptor, filler, or thickening catalyst in the production of plastics), pharmaceutical (e.g., in cosmetics and ointments), or agricultural (e.g., in fertilizers, as a carrier for pesticides, in livestock feed) applications.^12–16

The gas/solid magnesiothermic reaction of an oxide template may also be used to generate a nanocrystalline elemental replica of the template. By lowering the magnesiothermic reaction temperature to 650°C, and by reducing the molar Mg:SiO₂ reactant ratio to a value just above that required by the stoichiometry of Reaction 1, the formation of an Mg₂Si compound and the Mg-Si alloy liquid were suppressed, so that nanocrystalline MgO/Si composite replicas of diatom frustules were formed, as indicated by Reaction 2.^17–21 The products of this reaction correspond to a co-continuous, nanocrystalline mixture of MgO (65.1 vol.%) and Si (34.9 vol.%). Owing to the continuity of the Si phase, porous Si replicas of the diatom frustules have been produced by selective acid dissolution of the interconnected MgO network.²¹ As revealed in Figure 2, the 3-D frustule morphology and features as small several tens of nanometers were well-preserved in the resulting nanocrystalline Si replicas. The Si frustule replicas possessed a much higher specific surface area (>500 m²/g) than the starting SiO₂ frustules,and contained a high population of nanoscale (<2 nm diameter) pores.²¹ Such a single porous silicon frustule replica was found to act as a rapid, sensitive, low-voltage, and minimally invasive gas sensor (Figures 2g and h).²¹ This now-patented^8,9,22,23 shape-preserving magnesiothermic reduction process has also recently been used to convert synthetic mesoporous silica films and colloidal silica assemblies into porous silicon replicas.^24–26 Such nanocrystalline porous silicon replica structures, generated from biosilica or synthetic silica templates, can be attractive for a number of chemical, biochemical, electrical, and optical applications (e.g., for sensing, biocatalysis, batteries, and photovoltaics).^21,24–29 Szczech and Jin³⁰ have subsequently reported that Mg₂Si-bearing diatom frustule replicas generated by this magnesiothermic conversion process may be useful for thermoelectric applications (note: the formation of such solid Mg₂Si, relative to solid Si or Mg-Si liquid products, may be controlled by adjustment of the Mg:SiO₂ reactant ratio at modest temperatures^20,21).

Oxide Replicas via Metathetic (Halide) Displacement Reactions

Further Chemical Modification of Replicas via Coating and/or Additive Reactions

Nanostructured elemental replicas of shaped oxide templates, generated through a magnesiothermic reduction (displacement reaction) process, may also be used as templates for subsequent electroless deposition of other functional elements. The coating of relatively noble metals (Au, Ag, Pd) on porous nanostructured templates can be attractive for a number of catalytic (e.g., for CO(g) oxidation, fuel cell catalysts), electrochemical (e.g., sensors), electronic (e.g., electrodes), thermal (e.g., cryogenic heat exchangers), and other applications.^53–55 While the direct electroless deposition of noble metal coatings on silica templates is inhibited by the insulating nature of this oxide, porous silicon replicas possess appropriate electronic and chemical (reducing) characteristics for such deposition. Bao et al.⁵⁶ have recently synthesized nanocrystalline noble metal replicas of 3-D silica microparticle templates (diatom frustules) by first converting the silica into porous silicon (via magnesiothermic reduction), and then applying metal coatings onto/within the porous silicon via subsequent electroless deposition. Subsequent selective dissolution of the Si in an aqueous NaOH solution then yielded freestanding porous Ag, Au, or Pd structures that retained the 3-D morphology of the starting silicon frustule replicas (Figure 4).⁵⁶ The frustule-shaped silver, gold, and palladium microparticles possessed average crystal sizes of only 14, 50, and 43 nm, respectively.

Oxide replicas of patterned templates, generated via metathetic gas/solid displacement reactions, can also be used as reactive templates for the generation of functional multicomponent oxides. For example, nanocrystalline titania replicas produced by the reaction of silica templates with titanium tetrafluoride gas (using Reactions 3, 5a, and 5b) have been converted into barium titanate (BaTiO₃) replicas through hydrothermal reaction by Ernst, et al.^52,57 The titania replicas were sealed, along with barium hydroxide octahydrate and water, inside a Teflon chamber and then heated to 100°C for 48 h to allow for hydrothermal conversion into barium titanate. The resulting frustule-shaped particles were found to be comprised of nanocrystalline BaTiO₃ with an average crystal size of only 63 nm.^52,57

Several liquid/solid displacement reaction- based approaches have been developed to allow for the conversion of shaped, macroscopic, ceramic-bearing preforms into composites comprised of new ceramic and metallic or intermetallic products that retain the shapes and dimensions (to within 1%) of the starting preforms. Under appropriate conditions, these approaches have yielded composites with micrometer-to-submicrometer phase and/or crystal sizes. Two general categories of such liquid/ solid displacement reaction-based processes are reactive metal penetration and reactive casting. Reactive metal penetration refers to the conversion of a dense, shaped ceramic-bearing preform into a dense product, comprised of new ceramic and metallic or intermetallic phases, via the inward migration of a liquid/solid reaction front through the dense preform.^58–69 Reactive casting refers to the conversion of a porous, shaped ceramic-bearing preform into a dense ceramic/metallic or ceramic/ intermetallic composite via the infiltration (with or without the assistance of an applied pressure) of a metallic liquid through the porous preform and reaction of this liquid with one or more solid phases in the preform.^70–85

Reactive Metal Penetration

The average size of the oxide and metal colonies within co-continuous alumina/aluminum-bearing composites formed by reactive metal penetration of silica-bearing or mullite-bearing preforms at 1,000–1,200°C is typically several micrometers, although a significant population of submicrometer sized ligaments has also been observed.^59,64,67 The oxide and metal colony size can be appreciably reduced, however, through alloying additions to the molten aluminum. For example, Strange and Breslin,⁶⁸ and Evarts⁶⁹ have reported that the introduction of copper into the aluminum melt has a dramatic impact on the scale of the microstructure of composites formed by reactive metal penetration. These authors immersed dense amorphous SiO₂ preforms into Cu-Al melts comprised of 30–63 at.% (50–80 wt.%) Cu at 1,100–1,150°C. The rates of linear penetration for these copper-aluminum alloy melts into the silica preforms were lower than for pure molten aluminum. Dramatic reductions in the colony sizes of the co-continuous oxide and intermetallic (AlCu, Al₂Cu) phases were observed for melts with ≥50 at.% Cu (Figures 6a–d). The nanostructured Al₂O₃/AlCu/ Al₂Cu composites were also found to exhibit substantially higher values of hardness than microstructured Al₂O₃/ AlCu/Al₂Cu composites.⁶⁹ Yoshikawa et al.⁶⁶ have reported that the reactive metal penetration of molten Fe-Al alloys into dense amorphous SiO₂ rods at 1,200–1,300°C yielded co-continuous, nanocrystalline Al₂O₃/Fe_xAl_y composites. ⁶⁶ With increasing iron content in the melt (from 5–30 at.% Fe), the rate of reactive metal penetration into the silica preform decreased and composites with iron-aluminum intermetallic phases of higher iron content were generated (i.e., predominantly Al₃Fe for a 20 at.% Fe melt vs. predominantly Al₅Fe₂ for a 30 at.% Fe melt).⁶⁶

Reactive Casting

The sizes of ceramic and metal or intermetallic phases generated within reactively cast composites can be appreciably refined through the use of fine-scale reactant phases in the starting porous preforms, rapid molten metal infiltration at a modest temperature, and modest post-infiltration heat treatment.^71,72,82 For example, Al₂O₃/ Ti(Al,Si)₃-based composites with phase sizes of a few micrometers to submicrometers have been produced by: i) attrition milling of a mixture of fine Al₂O₃ (1.2 μm ave. size) and TiO₂ (0.5 μm ave. size) powders, ii) compacting and sintering the powder mixture for 0.5 h at 1,150°C (to generate a rigid preform with a porosity of 49 vol.%), iii) rapid squeeze casting (within a few seconds) of a molten Al-Si alloy at 700°C into the porous preform, and then iv) annealing of the resulting infiltrated preform for 3 h at 800°C.^72,82 While the as-cast specimen contained unreacted TiO₂, Al, and Si phases and no apparent Ti(Al,Si)₃ product (as determined from x-ray diffraction analysis), the subsequent 800°C/3 h thermal treatment resulted in complete conversion of the as-cast specimen into an intimate mixture of Al₂O₃ and Ti(Al,Si)₃. Such fine-scale, reactively cast, near net-shape Al₂O₃/Ti(Al,Si)₃ composites possessed fracture strengths (4 point bending) of 490–540 MPa, fracture toughness (by indentation) values of 5.0–8.6 MPa·m^1/2, and hardness values of 5.7–7.4 GPa.^72,82 A similar approach (milling of fine oxide/metal powders, partial sintering, rapid squeeze casting, and modest annealing after casting) has been used to fabricate near net-shaped metal matrix composites (Fe-Ni-Cr-based or Ni-Co-Cr-Al-based) reinforced with nanoscale Al₂O₃ filaments (via the reactive casting approach known as the infiltration Metal Matrix Composite process^81–85). The fracture strengths of reactively cast Fe-Ni-Crbased alloy composites reinforced with nanoscale Al₂O₃ at 550°C and 900°C were found to be almost twice the values of a similar reference metal alloy lacking the alumina phase.⁸³ A derivative of such reactive casting ("in-situ" or "short-distance" infiltration^82,85,87,88), involving local aluminum melting and brief reaction within attrition-milled oxide/metal mixtures under an applied pressure, has also been used to synthesize dense metal alloy matrix composites reinforced with interconnected Al₂O₃ filaments of a few hundred nanometers thickness. Such Fe-Cr-Ni alloy/ nanofi lamentary Al₂O₃ composites have exhibited fracture strength and fracture toughness values of 1,100 MPa and 18 MPa·m^1/2, respectively, in threepoint bending.⁸⁸

Two general oxidation-based approaches that are capable of converting shaped, macroscopic, freestanding, solid metal-bearing preforms into oxide products that retain the preform shape with relatively little or no change in dimensions (Figure 8) are the reaction bonded metal oxide (RBMO) process, and the volume identical metal oxidation (VIMOX) process. (Note: while the reaction bonded metal oxide, RBMO, process is often referred to as the reaction bonded aluminum oxide, or RBAO, process, in light of the pioneering work of Claussen et al.,^89,93–100 the RBMO label is used herein, as this process may be used to generate a variety of metal oxides.)

Use of Oxidation-induced Volume Changes for Near Net-Shape Processing

For the alkali and alkaline earth elements shown in the right half of Table II, the PBR values are well below unity. Such oxidation-induced reductions in solid volume may be used to accommodate volume expansions resulting from the oxidation of other elements and/or the formation of oxide compounds. Hence, a shaped dense preform containing tailored amounts of alkali or alkaline earth elements and other metallic and/or ceramic phases may be converted, by appropriate oxidation and post-oxidation annealing treatments, into a dense all-oxide body that retains the shape and dimensions of the preform. For example, a compacted green body of Ba, Sr, Al, Al₂O₃, and SiO₂ has been converted into a (Ba,Sr)Al₂Si₂O₈ (celsian) body that retained the preform shape and dimensions (to within 1%).^102,103 This is the basic premise for near net-shape processing by the VIMOX approach. The VIMOX approach has yielded a variety of near net-shaped, functional, alkaline earth oxide-bearing components (e.g., ferroelectric BaTiO₃, PTCR (Ba,Pb)TiO₃, proton-conducting BaCeO₃, ferrimagnetic BaFe₁₂O₁₉, biocompatible Ca₆(PO₄)10(OH)₂, and refractory MgAl₂O₄, BaAl₂O₄, and BaAl₂Si₂O₈).^102–115

Fabrication of Shaped, Metal-bearing Preforms

Oxidation Processing

Post-oxidation Annealing

RBMO preforms have also been converted into dense, nanocrystalline oxide-based materials. With proper use of high-energy milling to obtain a fi nely-divided, well-intermixed assemblage of reactive metallic and ceramic phases, with appropriate conditions of compaction and oxidation, and with the use of fine second phases to inhibit grain growth during post-oxidation sintering, dense RBMO composites have been synthesized with submicrometer grain sizes. For example, Claussen et al.⁹⁷ have reported the syntheses of reaction bonded composites comprised of 55 vol.% mullite, 10 vol.% silicon carbide, 21 vol.% alumina, and 14 vol.% zirconia with phase sizes well below 1 μm. The introduction of fine (0.2 μm dia.) α-Al₂O₃ seeds to preform mixtures to enhance the conversion of γ-Al₂O₃ (a metastable alumina polymorph formed during low-temperature aluminum oxidation) into α-Al₂O₃, and to further refine the grain size of sintered reaction-bonded alumina, has been suggested by Suvaci and Messing.¹²⁸ Mullite crystal seeding to reduce the mullite grain size in reaction-bonded mullite has also been proposed by She et al.¹²⁹ Fine-grained reaction-bonded alumina has been reported to exhibit superplastic-like behavior at 1,250–1,500°C, which may be utilized to generate dense, shaped, nanocrystalline components, via sinter forming or sinter forging, at appreciably lower temperatures than for pressureless sintering.^97,101 Bourz et al.¹³⁰ have generated sinter-forged nanocrystalline (0.63 μm ave. alumina grain size) reaction-bonded alumina-zirconia composites of >95% of theoretical density at only 1,300°C. Such nanocrystalline, reaction-bonded alumina-zirconia composites have exhibited room temperature fracture strengths (in biaxial bending) of 1.1 GPa.¹³⁰ Dense, fine-grained, reaction-bonded alumina-zirconia composites prepared by pressureless sintering at 1,550°C have also exhibited respectable room temperature fracture strength and fracture toughness values (in 4-point bending) of ≥700 MPa and 3.5 MPa·m^1/2.^96,100 Dense, fine-grained, reaction-bonded mullite-silicon carbide- alumina-zirconia composites (with 49–55 vol.% mullite) prepared by pressureless sintering at 1,550°C have exhibited room temperature fracture strength and fracture toughness values (in 4-point bending) of 490–610 MPa and 4.1–4.9 MPa·m^1/2.^97,99

As demonstrated in this overview, nanostructured materials with complex macroscopic-to-microscopic shapes and with tailorable chemistries may be fabricated by first generating a preform with a desired morphology and then chemically transforming the shaped preform into a new material that retains the preform morphology (a type of materials "alchemy"). This fabrication paradigm enables materials that can be readily generated with complex shapes (e.g., formable metal-rich compositions for macroscale components or biologically formed inorganic microscale structures) to be used in the preform fabrication step prior to chemical transformation into the desired material. Under appropriate conditions, gas/solid and liquid/solid reactions can then be used for the shape-preserving conversion of such macroscopic-to-microscopic solid preforms into chemically tailored nanostructures with desired properties. The identification of reaction conditions that allow for such shape-preserving conversion into new nanostructured materials has been significantly aided by utilizing well-known concepts and phenomena from the field of high temperature corrosion (e.g., the Pilling–Bedworth Ratio, active vs. passive corrosion, ignition, internal oxidation). Indeed, by taking the perspective that fluid/solid reactions are a type of "chemical processing", as opposed to a type of "corrosion", this author is confident that scholars who are well-versed in the prior mechanistic knowledge established in the field of high temperature corrosion will continue to develop attractive reaction-based approaches for synthesizing advanced nanostructured materials with tailored morphologies, chemistries, and properties.

ACKNOWLEDGEMENTS

K.H.S. acknowledges the financial support of the Air Force Office of Scientific Research (Dr. Charles Lee, Dr. Hugh DeLong, and Dr. Joan Fuller, program managers) and the U.S. Department of Energy (Dr. Michael Markowitz, program manager).

REFERENCES

Kenneth H. Sandhage is the B. Mifflin Hood Professor in the School of Materials Science and Engineering, and Adjunct Professor in the School of Chemistry and Biochemistry, at the Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA 30332; (404) 894-6882; e-mail: ken.sandhage@mse.gatech.edu.