WO2025050033A1

WO2025050033A1 - Secondary fuel generating complexing agents for ultra-low temperature metal compound formation via combustion

Info

Publication number: WO2025050033A1
Application number: PCT/US2024/044853
Authority: WO
Inventors: Reinhold H. Dauskardt; Thomas W. COLBURN; Robert D. Miller
Original assignee: Leland Stanford Junior University
Current assignee: Leland Stanford Junior University
Priority date: 2023-09-01
Filing date: 2024-08-30
Publication date: 2025-03-06
Anticipated expiration: 2026-03-01

Abstract

Fabrication of metal compounds by combustion synthesis at lower temperatures is provided by primary fuel species that release a secondary fuel species when heated. Such secondary fuel species can be excellent fuels that reduce the required temperature for combustion synthesis. Secondary fuels produced in this manner are free of many of the constraints that usually govern fuel choice for combustion synthesis. For example, secondary fuels may not be able to form metal-complexes and/or they can be highly volatile.

Description

Secondary fuel generating complexing agents for ultra-low temperature metal compound formation via combustion by

Reinhold H . Dauskardt

Thomas W . Colburn

Robert D . Miller

FIELD OF THE INVENTION

This invention relates to combustion synthesis of metal compounds .

BACKGROUND

Metal compounds , such as metal oxides , are critical chemicals for the global economy, comprising numerous components in applications not limited to batteries , electronics , solar cells , catalysts and barrier films . Often, the fabrication of metal oxides by traditional routes involves two predominant methods : vacuum-based approaches like RE sputtering and open-air processed solution approaches like sol-gel and spray pyrolysis . However, both traditional vacuum-based and solution-based methods struggle with critical requirements for technological translation like low-throughput , high costs , and high temperature annealing steps . Recent developments in combustion synthesis allows for solution-based processing at considerably lower temperatures ( often decreasing from >400 C required in solgel and spray pyrolysis controls to <250 C in combustion processing) ; however, continued innovation is required to ensure compatibility of combustion synthesis with flexible and polymeric substrates that require considerably lower applied temperatures to produce metal oxides via solution methods . Accordingly, it would be an advance in the art to provide lower temperature combustion synthesis of metal compounds .

SUMMARY

In several examples , we present a class of complexing molecules that can be utili zed as fuel species in the combustion synthesis of metal oxides via the production of a highly volatile combustion product ( i . e . , isobutylene , 2- propanimine , acetone , etc . ) from the direct decomposition of the organic-metal complex . This class of molecules are unique because , unlike commonly used complexing fuels like acetylacetone , polyfunctional carboxylic acids ( i . e . , citric, tartaric, or malic acid) , amino acids , or monofunctional carboxylic acids which all trigger combustion by the direct breakdown of the chelating fuel directly, tertiary groups bonded to the heteroatom site of the complexing fuels can undergo a shi ft reaction that generates the highly combustible secondary fuel upon heating .

One prominent example of such a secondary fuel is isobutylene , a highly flammable gas , which would not be a foreseeable potential candidate as a fuel in combustion oxidation due to its non-complexing properties with metal cations and its volatility; however, when generated during the heating preceding the metal-oxide generating combustion event , it is able to further reduce the required heating temperatures for metal oxide formation below 150 C . I sobutylene in this example would be generated directly from t-butyl esters such as from di-t-butyl malonate . Signi ficantly, crystalline indium oxide nanoparticles have been generated at temperatures <80 C without the need for intense UV light that is often required to yield metal oxides below 100 C in the literature . The metal compounds resulting from using this new class of combustion synthesis precursors illustrate the potential compatibility of this synthesis method with a whole array of low-temperature nanofabrication applications and opens the door for further innovation in metal compound synthesis .

The use of this class of molecules for combustion synthesis to achieve substantially reduced temperatures of metal compounds formation could be of imminent use in catalysis , battery material fabrication, aerospace applications , solid-oxide membranes for fuel cells , dielectric materials , optoelectronics , and sensor fabrication . We expect that many fields could benefit from this synthetic approach .

This method to form oxides and other metal compounds using secondary fuel generating complexing fuels has the potential to economically shape the solution-oxide metal oxide landscape by providing an avenue to form a host of metal oxides and compounds and gives the additional possibility of being able to form film layers on substrates , nano-powders , and other oxide products at substantially lower applied temperatures . The reduction of formation temperature to <150 C compared with >300 C for sol-gel and spray pyrolysis is an impactful step forward for demonstrating possible compatibility with polymeric substrates especially those with deep-UV sensitivity ( as deep-UV is often required to success fully cure the film) .

Additional metal ions can easily be screened with the new fuel molecules and are expected to be functional with a host of metals such as Mn, Al , Fe , Cu, Zn, Sn, La, Zr, etc . These metals will need to be tested in the application as both powders and thin films .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG . 1 shows some examples of release of a secondary fuel species from a primary fuel species .

FIG . 2 shows an exemplary proposed reaction sequence for release of a secondary fuel species from a primary fuel species .

FIGs . 3A-B shows thermogravimetric analysis results for combustion synthesis of indium oxide .

FIGs . 4A-D show thermogravimetric analysis results for combustion synthesis of several metal compounds .

FIGs . 5A-D show X-ray photoelectron spectroscopy results for combustion synthesis of several metal oxide and metal compounds .

FIGs . 6A-B show a comparison of X-ray characteri zation spectra for combustion synthesis of indium oxide as compared to reference spectra from the literature .

FIG . 6C shows a comparison of X-ray characteri zation spectra for combustion synthesis of a nickel compound as compared to reference spectra from the literature .

DETAILED DESCRIPTION

Section A describes general principles relating to embodiments of the invention . Section B considers some speci fic examples . A) General principles

An exemplary embodiment of the invention is a method of combustion synthesis of a metal compound, the method including :

1) forming a mixture including at least one metal species, an oxidant, and at least one primary fuel species. Here the primary fuel species includes a complexing agent having a linking carbon atom (e.g., 102 in the examples of FIG. 1) bonded to a primary fuel heteroatom (e.g., X in the examples of FIG. 1) . Here, examples of oxidants such as nitrates, perchlorates, peroxydisulfates, peroxides, superoxides, and permanganates can be used as the oxidant source. Here a linking carbon atom is a carbon atom in a molecule that has 4 single bonds to any combination of other atoms which promotes heterolytic or homolytic dissociation of the X-102 bond. For example, this promotion of dissociation can be via a stabilization of the proposed cyclic mechanism transition state either by raising the ground-state energy and/or lowering the transition state energy. The linking carbon can have substitutions of any combination of heteroatoms, carbon groups, and/or hydrogen if there is promotion of dissociation of the secondary fuel. An atom bonded to the linking carbon atom can contain a hydrogen capable of engaging in the proposed cyclic mechanism described in FIG. 2; and

2) combusting the mixture with a combustion reaction to form a metal compound. Here the complexing agent thermally decomposes during the combustion reaction to release a secondary fuel species (e.g., 104 in the examples of FIG. 1) from the linking carbon atom, and the secondary fuel species acts as a co-fuel in the combustion reaction. Practice of the invention does not depend critically on the physical form of the resulting metal compound . One option is to deposit the mixture on a substrate prior to combustion to provide a precursor layer, so that combustion of the precursor layer forms a metal compound layer . Alternatively, the metal compound can be formed as a powder . Such a powder can be of nanoparticles or other unbound nanomaterials . Here nano-particles/nanomaterials have at least one dimension that is 1 m or less in extent .

The linking carbon atom can be bonded to the metallic site via various species including but not limited to : oxygen, nitrogen, sul fur, or phosphorus . Such species are shown as X in the examples of FIG . 1 .

Although the examples considered here relate to synthesis of metal oxides , it is expected that other metal compounds can be similarly formed, e . g . by making suitable replacements of the metallic salt in the reactants , adj usting the complexing fuel to oxidant , and including heteroatoms in the co- fuel or as solution additives . Such other compounds include , but are not limited to mixtures containing : metal carbides , metal phosphides , metal sul fides , and metal nitrides .

The complexing agent can form a neutral or anionic complex with the metal ion . This complex can arise from the presence of oxygen, sul fur, nitrogen, or other metal complexing heteroatoms in the primary fuel molecular structure .

There are various options for the secondary fuel generating component of the complexing agent . One simple option is that the linking carbon atom is part of a t-butyl group . The complexing agent can includes one or more of : t- butyl acetoacetate, t-butyl malonate, t-butyl acetate, and t-butyl esters.

The complexing agent can include a substituent on the linking carbon atom selected from the group consisting of: aryl substituents, alkyl substituents, cycloalkyl substituents, heteroalkyl substituents, and heteroaryl substituents .

The linking carbon atom can be part of a compound selected from the group consisting of: esters, urethanes, aldehydes, carbonates, amides, and ethers.

The secondary fuel species generated can include one or more compounds selected from the group consisting of: isobutylene, 2-propanimine, and acetone.

Preferably, the formation temperature of the metal compound is 300 °C or less. More preferably, this formation temperature is 200 °C or less. Even more preferably, this formation temperature is 130 °C or less. It is generally expected that rare earth compounds may require higher formation temperatures than transition metal compounds.

B) Examples

Standard routes for fabricating metal oxides, oxide nanostructures, and metal compounds span a wide array of processing techniques including vacuum-based and solutionbased methods. Vacuum-based methods include sputtering, e- beam evaporation, and atomic layer deposition while solution methods include sol-gel, spray pyrolysis, and hydrothermal synthesis .

The traditional methods can struggle from negatives including low-throughput depositions on often small deposition areas for vacuum methods and the listed wet- syntheses often demand high temperatures >300 ° C with long sintering processes often on the order of hours to generate dense , polycrystalline or crystalline nanomaterials ( i . e . , thin films and nanoparticles ) . Additionally, sol-gel processes , in particular, often rely on unstable metal alkoxide precursors or demand rigorous control of pH, humidity, and temperature during deposition to avoid premature hydrolysis of the sol and precipitation of metal hydroxides . The existing challenges with manufacturing, cost , and energy requirements have driven the investigation into alternative synthesis techniques for metal compounds .

Solution combustion synthesis ( SCS ) is an alternative method that can produce an array of metal compounds owing to its rapid formation mechanisms and high local temperatures during combustion . The reaction mixtures involve three vital components : a metal cation source , an organic fuel that complexes the metal cation, and an oxidant . Upon heating, the mixture reacts to allow for the generation of heat and, in turn, the conversion of the metal cation to its product compounds . In addition to oxides , metallic compounds have been demonstrated including pure metals , carbides , and nitrides . This process occurs in a single rapid event which is characteri zed by the release of carbon dioxide , nitrogen, water, and other combustion products .

The chemical properties of the organic complexing agent can have a dramatic ef fect on the both the morphological properties of the compounds generated as well as the purity, crystallinity, and chemical properties . For example , a combustion event that is fuel-rich or fuel-poor may yield increased amounts of undesirable phases due to a poor progression of the combustion event .

The literature currently has many examples of forming metal oxide nanoparticles , thin films , and other nanomaterials at temperatures generally ranging from 200 - 400 ° C depending on the metal cation' s propensity for oxidation and organic complexing fuel used . The metal compounds are formed in two maj or geometries : a bulk combustion where the reaction mixture is unconfined to a substrate and a film case where the mixture forms a layer thinner than approximately 1 pm . In the bulk case , a critical thermal mass of precursor is allowed to undergo rapid combustion generally accompanied by the evolution of heat , gas , light , and sound . In the film geometry, a layer is deposited via a traditional method such as spin, blade , or spray coating and then heated to T>Ti_gnition to cure the precursor .

SCS holds substantial potential to both reduce applied curing temperatures required to form metal compounds but also generate complex mixtures of metal compounds that would be di f ficult to achieve using standard, equilibrium-based annealing techniques . However, one avenue for improvement in SCS is the use of non-complexing fuels that commonly would be too volatile to participate in the rapid combustion process . Many small molecule fuels like isobutylene cannot be introduced into the solution and interact with the metal cation to drive combustion processes .

As such, we present t-butyl ester containing complexing agents capable of generating isobutylene at low applied temperatures as a class of fuels for SCS . We demonstrate combustion reactions with four sought-after metal materials : In, Mn, Ni , and Co at applied temperatures <150 ° C including the formation of InOx at <80 ° C which are among the lowest purely heat-initiated SCS reactions observed to-date . This work is a demonstration of the utility of isobutylene and other co- fuel generators to enable a wider temperature window accessible to SCS processes . FIG. 1 shows several examples of secondary fuel generator molecules and secondary fuels having a metal center (shown here as M⁺ coordinated to 0) along with a linking carbon atom 102 connected to a heteroatom (X) . Rh can be an alkyl, aryl, cycloalkyl, heteroalkyl, or heteroaryl group, R1 and R2 can be alkyl, aryl, heteroalkyl, or heteroaryl groups, R3 and R4 can be H or similar alkyl, aryl, heteroalkyl, or heteroalkyl groups. In these examples, gentle heating is sufficient to break the chemical bonds cut by the vertical dashed lines on the left, thereby liberating the secondary fuel species 104 on the right.

FIG. 2 shows an exemplary proposed mechanism whereby the secondary fuel is generated from the metal complex to assist in the formation of metal compounds. More specifically, 202 shows a metal oxidant and a fuel as reactants. Next, 204 shows formation of a metal complex from these reactants. After that, 206 schematically shows a proposed cyclic electron transfer reaction that ends up liberating the secondary fuel as shown at 208.

FIG. 3A shows an exemplary thermogravimetric analysis showing the combustion event well below 100 °C at two heating rates for the preferred embodiment of InO_x. FIG. 3B shows thermogravimetric analysis with differential scanning calorimetry showing a large exotherm at the point of combustion for the preferred embodiment of InO_x.

FIGs. 4A, 4B, 4G, 4D show exemplary thermogravimetric analysis results for oxide synthesis as described above for In, Co, Ni, Mn, respectively. The role of the addition of a co-complexing and pH modifying base (ammonia) is demonstrated. Ammonia acts to enhance combustion in a variety of combustion synthesis reactions and assists especially in driving combustion for Ni and Mn. FIGs. 5A, 5B, 5C, 5D show X-ray photoelectron spectroscopy results for oxide synthesis as described above for In, Co, Ni, Mn, respectively. The spectra show the atomic surface states of the nanomaterial products of these metal compounds indicating a largely oxide phase with nitrogen and carbon present.

FIGs. 6A and 6B show X-ray Absorption Near Edge Spectroscopy (XANES) (FIG. 6A) and Extended X-ray Absorption Fine Structure (EXAFS) (FIG. 6B) spectra for indium oxide prepared as described above compared to reference spectra. These results show similarity between InO_x generated from secondary-fuel combustion compared with an indium oxide reference .

FIG. 6C shows XANES spectra of a combustion generated nickel compound showing a mixture of likely phases including oxide, carbide, and metallic species. The reference nickel carbide spectrum was provided from an author of: J. Phys. Chem. C 2009, 113, 6, 2443-2451.

Claims

1 . A method of combustion synthesis of a metal compound, the method comprising : forming a mixture including at least one metal species and at least one primary fuel species , wherein the at least one primary fuel species includes a complexing agent having a linking carbon atom bonded to a primary fuel heteroatom; and combusting the mixture with a combustion reaction to form a metal compound, wherein the complexing agent thermally decomposes during the combustion reaction to release a secondary fuel species from the linking carbon atom, and wherein the secondary fuel species acts as a cofuel in the combustion reaction .

2 . The method of claim 1 , further comprising depositing the mixture on a substrate prior to combustion to provide a precursor layer, whereby combustion of the precursor layer forms a metal compound layer .

3 . The method of claim 1 , wherein the bond between the primary fuel heteroatom and linking carbon is weakened toward dissociation to facilitate release of the secondary fuel species via thermal decomposition .

4 . The method of claim 1 , wherein the metal compound is formed as a powder .

5 . The method of claim 1 , wherein the metal compound is formed as nanoparticles or formed as unbound nanomaterials .

6. The method of claim 1, wherein the linking carbon atom is bonded to the metallic site via a species selected from the group consisting of: oxygen, nitrogen, sulfur, or phosphorus .

7. The method of claim 1, wherein the metal compound is selected from the group consisting of: metal oxides, metal carbides, metal phosphides, metal sulfides, and metal nitrides .

8. The method of claim 1, wherein the linking carbon atom is part of a t-butyl group.

9. The method of claim 1, wherein the complexing agent includes one or more compounds selected from the group consisting of: t-butyl acetoacetate, di-t-butyl malonate, mono-t-butyl malonate, t-butyl acetate, and t-butyl esters.

10. The method of claim 1 wherein the complexing agent can form a neutral or anionic complex with the metal species.

11. The method of claim 1, wherein the complexing agent includes a substituent on the linking carbon atom selected from the group consisting of: aryl substituents, alkyl substituents, cycloalkyl substituents, heteroalkyl substituents, and heteroaryl substituents.

12. The method of claim 1, wherein the linking carbon atom is part of a compound selected from the group consisting of: esters, urethanes, aldehydes, carbonates, amides, and ethers .

13. The method of claim 1, wherein the secondary fuel species includes one or more compounds selected from the group consisting of: isobutylene, 2-propanimine, and acetone .

14. The method of claim 1, wherein a formation temperature of the metal compound is 300 °C or less.

15. The method of claim 13, wherein a formation temperature of the metal compound is 130 °C or less.