WO2020037054A1

WO2020037054A1 - Distortion mitigation during sintering of ceramics through the incorporation of ceramic precursor solutions

Info

Publication number: WO2020037054A1
Application number: PCT/US2019/046523
Authority: WO
Inventors: Lynnora Olivia GRANT; Zachary Copoulos CORDERO; Fred C. HIGGS, III
Original assignee: William Marsh Rice University
Priority date: 2018-08-14
Filing date: 2019-08-14
Publication date: 2020-02-20

Abstract

A method according to this application includes binder jet printing of a ceramic powder to form a body; applying a precursor solution to said body; and sintering said body, wherein said application of the precursor solution to the body mitigates distortion of the body during or after the sintering step. A ceramic powder body comprising: a ceramic powder; and a precursor decomposed from a precursor solution, the precursor having a rigid nanoparticle form and coated around the ceramic powder.

Description

DISTORTION MITIGATION DURING SINTERING OF CERAMICS THROUGH THE INCORPORATION OF CERAMIC PRECURSOR

SOLUTIONS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims priority from U.S. Provisional Application No.

62/718,771, which was filed in the United States of America on August 14, 2018.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

[0002] The invention was made with government support under Grant No. DGE-

1842494, awarded by the National Science Foundation Graduate Research Fellowship Program. The government has certain rights in the invention.

BACKGROUND

[0003] During traditional shell molding processes, metal or wax patterns are coated in a refractory slurry. Once the slurry is cured to form a shell, the pattern is eliminated without damaging the shell. The shell mold is then backed with refractory material and molten metal is poured in the cavity. There are a few disadvantages to this process; the cost of machining the metal positive can be expensive and the complexity of the geometry is limited to the geometry achieved from machining the metal. When it comes to wax positives, thermal expansion has been known to destroy the thin shell mold.

[0004] Binder jet 3D printing is an additive manufacturing process that builds a component layer-by-layer by selectively joining powder particles with an organic fluid binder followed by a sintering process, for example, as shown in the schematic of Fig. 1. Sintering is a thermal treatment that may be applied to a powder or compact material to increase the strength of the resulting object formed. Sintering aids in the reorganization of material within the object and should affect the object with a reduced porosity while also resulting in a shrinkage of the material forming the object. Ideally, a component should shrink uniformly in three dimensions when it is sintered; however, high temperature creep can lead to distortion of unsupported features.

SUMMARY

[0005] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0006] In one aspect, embodiments disclosed herein relate to a method that includes binder jet printing of a ceramic powder to form a ceramic body; applying a precursor solution to said ceramic body; and sintering said ceramic body, wherein said application of the precursor solution to the ceramic body mitigates distortion of the ceramic body during or after the sintering step.

[0007] In another aspect, embodiments disclosed herein relate to a ceramic powder body that includes a ceramic powder; a binder; and a precursor decomposed from a precursor solution, wherein the precursor has a rigid nanoparticle form and is coated around the ceramic powder.

[0008] In another aspect, embodiments disclosed herein relate to a method for preparing a positive mold for directionally solidified components that include binder jet printing a ceramic powder to form a ceramic body; applying a precursor solution to said body; and sintering said body, wherein said application of the precursor solution to the body mitigates distortion of the body during or after the sintering step.

[0009] Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

[0010] Fig. 1 shows a schematic for binder jet additive manufacturing.

[0011] Figs. 2A-B show SEM micrographs of (Fig. 2A) a pre-sintered sample and

(Fig. 2B) a TALH-treated sample. Scale bars are 5pm. [0012] Fig. 3 shows the relative density for pre-sintered samples and TALH-treated samples, sintered at 1350 °C under an initial uniaxial stress of 30 kPa.

[0013] Figs. 4A-B show (Fig. 4A) creep strain for pre-sintered samples and TALH- treated samples, sintered at 1350 °C under an initial uniaxial stress of 30 kPa. Fig. 4B shows the creep strain rate for pre-sintered samples and TALH-treated samples as a function of relative density.

[0014] Figs. 5A-5B show a (Fig. 5A) TGA weight loss curve and (Fig. 5B) DSC thermogram during TALH solution thermal decomposition.

[0015] Figs. 6A-6B show XRD patterns of products from TALH solution heated to

(Fig. 6A) 300 °C and (Fig. 6B) 900 °C.

[0016] Fig. 7 shows an SEM micrograph of nanocrystalline Ti0₂ formed by heating

TALH to 900 °C for 5 minutes.

[0017] Figs. 8A-8B show SEM micrographs of (Fig. 8A) green body with the aqueous binder cured by heating to 183 °C for 4 hours and (Fig. 8B) a TALH- infiltrated brown body that had been heated to 700 °C. In (Fig. 8B), note the flaky Ti0₂ nanostructures produced from the thermal decomposition of TALH.

[0018] Fig. 9 shows TMA results showing engineering strain for treated and untreated cylinders sintered under a uniaxial pressure. Cylinder dimensions are shown in Fig. 9 inset.

[0019] Figs. 10A-10F show in situ images of (Figs. 10A-10C) untreated and (Figs.

10D-10F) TALH-treated cantilevered beams during sintering. Scale bars are 5 mm. Figs. 10A and 10D show in situ images of untreated and TALH treated cantilevered beams at 1008 °C, respectively; Figs. 10B and 10E show in situ images of untreated and TALH treated cantilevered beams at 1271 °C, respectively; and Figs. 10C and 10F show in situ images of untreated and TALH treated cantilevered beams at 1420 °C, respectively.

[0020] Figs. 11A-11B show (Fig. 11A) a temperature ramp protocol that corresponds to (Fig. 11B) a plot of the relative density as function of time for comparative samples with TALH and samples without TALH. DETAILED DESCRIPTION

[0021] Directionally solidified components, such as single crystal turbine blades, are typically formed using shell molds, which may be prepared using a lost wax process that begins with injection molded wax positives. These positives have complex designs and are manufactured in low volume while further requiring expensive tooling. Typical shell molds are often prepared by the lost wax process, in which layers of ceramic slurry consisting of ceramic powder and silicate -based liquid binder are applied to a fugitive pattern, which is then burned out. One or more embodiments herein may relate to the preparation of shell molds that may be used in the process of growing metallic crystals.

[0022] Embodiments disclosed herein relate generally to methods of binder-jet printing and sintering ceramic materials. When using binder jet printing, there is no need for a positive and, therefore, material and tooling costs of the pattern may be eliminated. Likewise, the shell design is not limited to the pattern’s geometry as it is with traditional shell casting methods.

[0023] Sintering may enhance the mechanical properties of binder-jet printed parts by densifying the material, but often comes at the cost of distortion due to material creep. Creep related distortion is detrimental to the dimensional accuracy of the final part and may result in the binder-jet printed parts being limited in their application in advanced technologies or even relatively common technologies.

[0024] One or more embodiments presented herein may be directed towards an approach for mitigating such distortion. Such an approach may be to incorporate additives into the green body that form interparticle necks during the early stages of sintering. Several different types of distortion-mitigating additives, including metallic salts, nanoparticle dispersions, and metallo-organic inks may be incorporated.

[0025] Disclosed herein is a method to mitigate the distortion during sintering wherein ceramic precursors such as: metal alkoxides and dissolved metallic salt solutions (acetates, sulfates, nitrates) are decomposed to form solid bridges between particles during the early stages of sintering binder jet printed parts. Without being bound by theory, the inclusion of ceramic precursors with the binder-jet printed bodies prior to sintering increases the contacts between particles at temperatures lower than the sintering temperature, and in turn may prevent distortion due to creep during sintering.

[0026] For example, one or more embodiments presented herein may include methods to mitigate distortion of a ceramic (Ti0₂) using a water soluble ammonium titanium lactate complex, titanium (IV) bis(ammoniumlactato)dihydroxide (TALH). In the present work, a dilatometry may be employed to characterize the mechanism by which the precursor-derived interparticle necks mitigate creep in Ti0₂ components.

[0027] Terms such as“approximately,”“substantially,” etc., are intended to mean that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.

[0028] Similarly, the terms“can” and“may” and their variants are intended to be non-limiting, such that recitation that an embodiment can or may comprise certain elements or features does not exclude other embodiments of the present technology that do not contain those elements or features.

[0029] As referred to herein, all compositional percentages are specified as being by weight (wt%) or by moles (mol%) of the total composition, unless otherwise disclosed. Disclosures of ranges are, unless specified otherwise, inclusive of endpoints and include all distinct values and further divided ranges within the entire range.

[0030] In accordance with one or more embodiments, the following disclosure details methods and products formed from such methods, wherein the produced printed body demonstrates decreased distortion at elevated temperatures that may be experienced during processes typical to its use. The methods and the product components and properties are detailed as follows.

[0031] COMPONENTS

[0032] Ceramic Powder [0033] In one or more embodiments, the produced body may comprise a ceramic powder that may be titanium dioxide, silicon carbide, zirconia or combinations thereof.

[0034] In some embodiments, the average particle diameter of the ceramic powder may range from a lower limit selected from 1, 5, 10, 15, and 20 pm, to an upper limit selected from 50, 55, 60, 65, 70 pm, where any lower limit may be paired with any mathematically-compatible upper limit.

[0035] Binder

[0036] In one or more embodiments, a binder may be added to the ceramic powder before, during or after the ceramic powder has been formed into a powder body. In some embodiments the binder may be added before, during or after the precursor solution has been added to the powder body. In one or more embodiments, the binder may be an organic binder, including one or more of paraffin, polyethylene glycol, vinyl polypyrrolidone, polyacrylic acid, and polyvinyl alcohol.

[0037] In some embodiments, the binder and the precursor solution may be combined and simultaneously added to the ceramic powder. For example, in one embodiment presented herein, the binder and the precursor solution may be combined into liquid that is ink jet printed onto successive layers of ceramic powder to form the powder body. In yet another embodiment, the precursor solution may be infiltrated throughout a formed powder body comprising the ceramic powder and the binder.

[0038] Precursor Solution

[0039] In one or more embodiments, a precursor solution may be combined with binder jet 3D printing to overcome the disadvantages of shell casting. Ceramic precursors may be incorporated to prevent potential creep of the material during sintering, as decomposition of the precursor may increase the contacts between particles at temperatures lower than the sintering temperature.

[0040] In one or more embodiments, the precursor may be decomposed, and it may form rigid coatings around the particles comprising the printed body. In some embodiments the precursor may be decomposed to form nanoparticles that may or may not be the same material as the powder of the powder body. In some embodiments, the precursor solution may be decomposed to form a ceramic metal oxide coating on the ceramic powder. In one or more embodiments, a decomposed precursor product may form a rigid coating around ceramic powder particles and may densify the powder body, which may mitigate distortion due to creep and shrinkage during sintering and densification and may help strengthen the resulting sintered body.

[0041] In one or more embodiments, the precursor may be selected for specific compatible use with a ceramic powder. In one or more embodiments herein, the precursor solution may comprise at least one selected from the group of metallic salts, metal oxides, metal alkoxides, sol-gels, polymer-derived ceramics, and co precipitation/calcination solutions. In some embodiments, the precursor solution may comprise a polymeric additive. In some embodiments, the precursor solution may comprise an aqueous solution.

[0042] In some embodiments, the precursor solution may be a metallic salt solution comprising titanium (IV) bis (ammonium lactato) dihydroxide (TALH) and deionized H2O. In one or more embodiments, the aqueous solution may comprise water in an amount that ranges from at least 30% to at most 70%.

[0043] In one or more embodiments, TALH may begin to decompose at about 150° C to form solid TiC . In some embodiments, at elevated temperature, TALH may decompose as demonstrated in chemical Equation 1:

Equation 1:

C 4H 10 N 2O 6 Ti(OH) '2 + 502—_► TiO 2(s) + 2NH 3(g) + 4CO 2(g) + 6H 2 <T (g)

[0044] METHODS

[0045] Binder Jet Printing

[0046] In one or more embodiments, binder jet printing may refer to an additive manufacturing process during which a powder is first spread on a platform. In one or embodiments, a binder may then be deposited on to the powder corresponding to a cross section of the desired part geometry that may be acquired from a computer- aided design (CAD) file. In some embodiments, the spreading step and binding step are repeated until the green part is complete. In one or more embodiments, the desired material properties may be obtained when the green part is sintered. The green part may represent the powder body prior to being subjected to a sintering step.

[0047] One or more embodiments disclosed herein may relate to improvements related to the sintering step. Whereas in conventional sintering processes, creep resulting in distortion may occur which is detrimental to the dimensional accuracy of the final part, methods of sintering disclosed herein may reduce or inhibit distortion during sintering, which may result in improved dimensional accuracy of the final part. In accordance with one or more embodiments presented herein, the sintered powder bodies formed may advantageously provide for refractory ceramic molds to be used in hybrid- additive manufacturing processes in which the printed molds are resistant to high temperature creep during sintering due to the added precursor material. The molds may then be used to directionally solidify metal crystals of the subsequent hybrid additive manufacturing processes. This method combined with other methods of additive manufacturing and casting may lend itself to achieving superior control over the manufacturing of formed objects through control of the object’s crystalline microstructure.

[0048] Precursor Solution Application

[0049] In one or more embodiments herein, the steps relating to the application of the precursor solution may comprise at least one selected from the group consisting of, applying the precursor solution layer by layer in conjunction with binder jet printing a green body, immersing a green body (which may be formed by 3D printing, as described above) in the precursor solution, and infiltrating a green body with the precursor solution. In some embodiments, the precursor solution may be decomposed after it is applied to the green body.

[0050] In some embodiments, the precursor solution may be applied to a ceramic powder prior to, during, or after a powder body forming process is conducted with the powder. In one or more embodiments, the precursor solution may be applied after a powder body has been pre-sintered. In accordance with certain embodiments presented herein, after the application of the precursor solution and the completion of the powder body forming process, the powder body may be subjected to a heat treatment under a specified atmosphere condition or in vacuum. In some embodiments, the heat treatment may be conducted in stages and with or without additional forming operations taking place between the stages. In certain embodiments, the heat treatment may result in the formation of a ceramic coating around the particles throughout the powder body from the decomposed precursor solution material. As the heat treatment progresses, the ceramic coating may bind the ceramic powder particles, thus enhancing the at-temperature mechanical strength of the powder body during the remainder of the heat treatment to help avoid slumping.

[0051] In some embodiments, the precursor solution may be decomposed by heating a formed green body comprising ceramic powder and the precursor solution to a temperature of 10° C to 1000° C and for a specific period of time. In some embodiments, precursor solution may be decomposed by heating a formed green body comprising a ceramic powder and the precursor solution to a temperature of 100° C to 600° C. In some embodiments, precursor solution may be decomposed by heating a formed green body comprising a ceramic powder and the precursor solution to a temperature of 250° C to 500° C. In one or more embodiments, the precursor solution may be decomposed by heating a formed green body comprising ceramic powder and the precursor solution for a period of time ranging from 1, 3, 5, and 10 min. to 15, 20, 25, and 30 min. where any lower limit maybe combined with any mathematical compatible upper limit.

[0052] In some embodiments, a green body may be pre- sintered at the temperatures described above and subsequently held at a temperature for a range of time, wherein such range may vary from any of 20, 30, 40, and 60 min. to 90, 100, 110, and 120 min.

[0053] In one or more embodiments, the decomposition of the precursor solution may yield ceramic nanoparticles dispersed throughout the ceramic powder body to which it was applied. The precipitated nanoparticles may have an average grain size that ranges from any one of 25, 50, 75, or 90 nm to 110, 130, 150, and 200 nm.

[0054] Heat Treatment

[0055] In one or more embodiments, a printed ceramic body comprising at least one of a powder, a binder and a precursor material may be subjected to heat treatment steps including one or more of curing, pre- sintering, and sintering. In some embodiments, sintering may include heating a ceramic body up to 80 percent of its melting point to densify the resulting product. In one or more embodiments herein a sintering step may be accomplished through at least one selected from the group consisting of thermal treatment, chemical treatment, and UV treatment of the body.

[0056] In one or more embodiments, a ceramic body printed using an aqueous binder may be subjected to a curing step prior to sintering, wherein the aqueous binder may be cured at a temperature between 170° C and 200° C for 2 to 6 hours.

[0057] In some embodiments, the sintering step may be after a pre-sintering step. In some embodiments, the sintering step may occur after a pre-sintering step and a decomposition step, wherein the precursor solution is decomposed at a temperature that is less than the temperature that would decompose the binder. In some embodiments, a printed ceramic body may be subjected to pre-sintering conditions, wherein the ceramic body is heated to a temperature between 1000° C and 1200° C for 1 to 2 hours. In other embodiments, a printed ceramic body may be subjected to pre- sintering conditions, wherein the ceramic body is heated to a temperature between about 50 and 70% of the melting temperature of the ceramic powder for 30 min. to 180 min.

[0058] In some embodiments, a green body comprising ceramic powder, binder, and precursor solution may be sintered by heating the body to a temperature of 1000° C to 1500° C and for a specific period of time. In some embodiments, a green body comprising the ceramic powder, binder and the precursor solution may be sintered by heating the body to a temperature of 1200° C to 1500° C. In some embodiments, a green body comprising the ceramic powder, binder and the precursor solution may be sintered by heating the body to a temperature of 1300° C to 1400° C. In some embodiments, the green body may be sintered at the temperatures described above and subsequently held at the temperature for a range of time, wherein such range may vary from any of 2, 4, 6, 8, and 10 hours to 12, 16, 20, 24, and 28 hours, where any lower limit may be combined with any mathematically compatible upper limit. In one or more embodiments, longer sintering times may be associated with a higher relative density in the resultant sintered body. [0059] Physical Characterization of a Sintered Body

[0060] In one or more embodiments, a sintered body comprising the yielded precursor precipitate may have a relative density that ranges, relative to the theoretical density of the material, from any one of 40, 45, 50, to 55, 60, and 65%, where any lower limit may be combined with any mathematically compatible upper limit.

[0061] In one or more embodiments, the decomposition of the precursor solution may yield ceramic nanoparticles dispersed throughout the ceramic powder body to which it was applied. The precipitated nanoparticles may have an average grain size that ranges from any one of 1, 5, 10, 25, 50, 75, or 90 nm to 110, 130, 150, 170, 190, and 200 nm, where any lower limit may be combined with any mathematically compatible upper limit.

[0062] Casting

[0063] According to embodiments of the present disclosure, a mold may be binder jet printed using the methods described above, including printing a ceramic powder and organic binder into a green body and treating the green body with one or more precursors (e.g., during printing or in a subsequent treatment step). The green body mold including one or more precursors may be sintered, which may densify and strengthen the mold. As described above, one or more additional heat-treating steps may be performed before or after sintering.

[0064] A sintered mold formed using one or more precursor materials may be used for forming molded bodies, e.g., for casting metallic bodies or infiltrating bodies. Thus, the sintered mold may be printed to have a negative shape of the subsequently formed molded body. Further, because the sintered mold may be smaller than the green body from which it is formed (from shrinkage during sintering), the green body may be designed to have the negative shape of the planned molded body, but in a larger size corresponding to the amount of expected shrinkage from sintering.

[0065] A sintered ceramic mold formed according to methods disclosed herein may be used, for example, in casting metal parts. By casting metal parts (as opposed to, for example, 3D printing metal parts) in sintered ceramic molds with precursor material disclosed herein, which may have improved dimensional accuracy over printed molds formed without precursor material, the resulting casted metal part may have a single crystal micro structure. Metal parts having a single crystal microstructure without grain boundaries may be more resistant to creep at higher temperatures than metal parts having a polycrystalline microstructure (for example, resulting from forming the metal parts by sintering). Thus, metallic parts casted in molds formed according to embodiments disclosed herein may be particularly desired for applications in which the casted metal part is exposed to high temperatures and/or high stress, such as engine parts. However, molds formed according to embodiments disclosed herein may be used in a wide range of industries, including for example, goods ranging from jewelry to turbines.

[0066] EXAMPLES

[0067] Example 1

[0068] In this first Example, an ExOne Innovent binder jet 3D printer was used to print green cylinders from rutile Ti0₂ powder (Saint-Gobain S.A.) with an average particle size of 16 pm, and an aqueous binder with a 90% saturation and a layer thickness of 100 pm. The green cylinders had an initial radius (ro) of 3.15 mm, an initial height (ho) of 6.3 mm, and an average relative density of 0.42. To increase the handling strength of the as-printed cylinders, the green cylinders were pre-sintered via heating to 1100 °C at a rate of 6 °C/min and then held at this temperature for 1 hr. For comparison, additional green cylinders were prepared as described in Example 1 without the addition of the TALH solution.

[0069] Characterization and Analysis

[0070] Fig. 2A shows a neck that formed during this pre-sintering process. After pre sintering, select cylinders were vacuum infiltrated with an aqueous TALH solution (50 wt% water, Sigma- Aldrich). The TALH was thermally decomposed by heating the infiltrated cylinders to 600 °C at a rate of 6 °C/min and holding for 5 min, forming secondary structures around the printed powder. SEM investigations on specimens heated to 600 °C revealed a TALH-derived nanocrystalline Ti0₂ coating on the printed powder particles that increased the inter-particle contact area and filled pores as seen in Fig. 2B. The added Ti0₂ coating increased the average relative density of the samples to 0.44. [0071] To understand how the TALH-derived Ti0₂ affects the creep behavior of the powder aggregates, the cylinders were sintered for 2 hours at 1350 °C under a uniaxial load using a thermomechanical analysis (TMA) system. The ramp rate to the sintering temperature was 6 °C/min, and a static force of 0.02 N was applied before the sintering temperature was reached. Once the sintering temperature was reached, a load of 0.1 or 1 N, corresponding to initial applied pressures of 3 kPa and 30 kPa respectively, was applied during the isothermal dwell. The load was returned to 0.02 N during cooling.

[0072] To characterize the strain due to creep, a series of interrupted experiments were conducted, where the radial and axial lengths were measured using an optical microscope and a micrometer, and where the relative density was determined using the mass volume method. The measurements were taken on samples that had been held at the sintering temperature for different time intervals, with a maximum total sintering time of 135 minutes. The relative density measurements plotted in Fig. 3 show that the initial 2% difference between the untreated and the TALH-treated samples is maintained over the course of the sintering experiment.

[0073] The axial strain (eh) and radial strain (er) were calculated using the following equations.

Formula 1:

Formula 2:

[0074] In Formula 1, h and r are the instantaneous height and radius of the cylindrical specimens. With these measurements, the creep strain (ec) was determined by deconvoluting the strain component due to densification from distortion, as shown in Formula 2. [0075] Fig. 4A shows creep strain values for the TALH-treated and untreated samples. These data reveal that while both samples experience high temperature creep, the untreated samples creep to a greater extent than do the TALH-treated samples. Fig. 4B shows the creep strain rate plotted against relative density and reveals that the creep rates for TALH-treated and untreated samples are comparable at a given relative density. This finding suggests that, although the untreated sample had a larger distortion due to creep, the creep mechanism in both the untreated and the TALH-treated samples are similar.

[0076] These experiments serve to elucidate the method by which precursor derived secondary structures (e.g., the ceramic coating formed around the ceramic powder particles of a printed body) aid in mitigating distortion due to high temperature creep. In the observed range of relative densities, the primary effect of TALH addition on the samples is an increase in relative density that remains constant throughout the sintering process.

[0077] The material contributing to the 2% increase in relative density is visible in

SEM micrographs as Ti0₂ that coats the printed particles and fills the pores. This increases the inter-particle contact area within the samples, raising the effective viscosity, and making the part more resistant to creep. The extent of the resistance to creep was determined using loading dilatometry. Although untreated samples experienced more creep, the finding that both TALH-treated and untreated samples creep at the same rate gives reason to believe the creep mechanism may be comparable.

[0078] Structural Evolution of Reactive Binder

[0079] Example 2

[0080] A TALH solution was isolated so that its thermal behavior could be individually studied.

[0081] In this example, an aqueous titanium (IV) bis(ammoniumlactato) dihydroxide

(TALH) 50 wt% solution was applied to binder jet printed Ti0₂ parts. TALH is a water soluble chelate material. In accordance with one or more embodiments, aqueous TALH solutions may be decomposed to form titanium dioxide (Ti0₂) nanoparticles through hydrothermal reaction, chelate destabilization reactions using acidic solutions, and photodecomposition reactions. The decomposition of the TALH solution is demonstrated using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and x-ray diffraction (XRD).

[0082] Fig. 5A is a TGA weight loss curve showing the change in weight of the

TALH solution as a function of temperature while heating at a rate of 20 °C/min. There is a 50% reduction in mass around 100 °C, corresponding to water evaporation. A second period of mass loss occurs in the 250 to 500 °C temperature range, corresponding to decomposition of organic compounds. At temperatures greater than 500 °C, the weight percent stabilizes around 17%, indicating the amount of Ti0₂ obtained through decomposition. Fig. 5B is a DSC thermogram that was collected during the TGA experiment. There is an endothermic peak around 100 °C due to water evaporation and an exothermic peak in the 250 to 500 °C range indicative of organic decomposition of the binder.

[0083] Figs. 6A-6B show results from interrupted XRD measurements of the TALH solution after it had been heated to 300 °C and 900 °C at a rate of 6 °C/min and held for five minutes. A phase transformation from anatase to rutile was observed during heating, demonstrating the decomposition of the TALH solution to Ti0₂.

[0084] Additional XRD measurements revealed that this transition occurred in the range of 500 to 900 °C with both phases present at 700 °C. The decrease in the background noise from Fig. 6A to Fig. 6B denotes the removal of chemical impurities such as residual organic compounds. This observation shows that at temperatures above 500 °C, the TALH decomposition reaction changes to yield gaseous ammonia, water, and carbon dioxide; and these chemical impurities are thus removed from the system, leaving behind a Ti0₂ residue. A third observation from the XRD plots is the sharpening of the diffraction peaks from Fig. 6A to Fig. 6B, which corresponds to grain coarsening. In hydrothermal reactions, decomposition of the TALH solution begins at temperatures as low as 120 °C, forming anatase nanoparticles with a diameter of about 3 nm. When the sample was heated to 900 °C, coarser grains on the order of 100 nm were present. This grain structure is evident in Fig. 7, which shows an SEM micrograph of the Ti0₂ produced by heating the TALH solution to 900 °C for five minutes.

[0085] By isolating the TALH solution and investigating its thermal behavior, it can be concluded that at sintering temperatures above 900 °C, rigid structures of rutile Ti0₂ with a grain size on the order of 100 nm will be present in the system.

[0086] Reactive Binder Evolution in 3D Printed Green Bodies

[0087] Example 3

[0088] Green bodies were printed using a polydisperse rutile Ti0₂ powder with a jagged particle shape and a mesh size of -230 (sourced from Saint-Gobain). The powder is 99% Ti0₂ with 0.12% Ah03 and trace amounts of other oxides. The parts were printed with an aqueous binder (sourced from ExOne, item number 7100037CL) with a saturation level of 90% and a 100 pm layer thickness. The micro structure of a printed and cured green body is shown by an SEM micrograph in Fig. 8A. The solid material which forms a bond between the Ti0₂ particles is produced by heating the aqueous binder solution. Through TGA of a ceramic powder body (ramp to 900 °C at 20 °C/min), it was determined that the binder burnout process begins at 300 °C and finishes around 600 °C.

[0089] For select ceramic powder bodies, TALH solution was directly applied to the samples via pipetting. The amount of TALH applied was equal to the theoretical volume of the pores in the green body, as determined by the mass-volume method. The green body treated with TALH was then heated to precipitate the Ti0₂ nanoparticles. Fig. 8B shows a TALH infiltrated sample that was heated to 700 °C for five minutes. Ti0₂ derived from the decomposition of the TALH is evident as smaller particles covering most of the surface of the powder, revealing that at higher temperatures, after the binder has burned out, the precipitated Ti0₂ nanoparticles can serve as a binding agent that strengthens the ceramic powder printed parts. Fig. 8B further shows that the Ti0₂ coating partially fills the pores between particles, indicating a mechanism for shrinkage mitigation. These observations suggest that the Ti0₂ nanostructures can serve as a secondary binding agent that strengthens the printed parts during and following binder burnout. [0090] Sintering Studies

[0091] Example 4

[0092] Binder jet 3D printed cylindrical samples, 6.5 mm tall and 6.5 mm wide, were sintered under a uniaxial pressure in a NETZSCH thermomechanical analysis system (TMA). This TMA system is capable of measuring changes in sample height throughout the sintering process. For select cylinders, the applied TALH solution was decomposed by heating to 300 °C prior to sintering. The cylinders were sintered by heating to 1420 °C at a rate of 6 °C/min, then holding at 1420 °C (a homologous temperature of 0.8) for ten hours. The pre-specified uniaxial pressures of 0.3 and 3 kPa were applied continuously during sintering. A thin layer of soda-lime glass powder was applied between the alumina pushrod and the sample to prevent frictional adhesion.

[0093] Fig. 9 shows engineering strain measurements as a function of temperature during the TMA experiments. These measurements were collected for four samples: TALH-treated samples under pressures of 0.3 kPa and 3 kPa, and untreated samples under the same pressures. When the temperature reached 500 °C, there was an abrupt increase in the shrinkage rate of the samples without TALH, presumably due to particle rearrangement during binder burnout. This behavior was not observed in the TALH-treated samples, suggesting that T1O2 from decomposed TALH adds strength to the finally formed ceramic powder body (i.e., after binder burnout process). When the applied pressure was increased from 0.3 to 3 kPa, the compressive strain increased significantly in the untreated sample during the final stages of sintering. Thus, the untreated samples have a higher stress sensitivity: creep and densification occur faster in the untreated samples than in those treated with TALH. This difference in stress dependence reveals that the T1O2 nanoparticles derived from the TALH have increased the effective viscosity of the cylinders.

[0094] TALH treatment also had the effect of increasing the relative density of the samples. For example, Fig. 11A shows a temperature ramp protocol that corresponds to a plot shown in Fig. 11B of the relative density as function of time for comparative samples with TALH and samples without TALH. The initial average relative density for all the samples was around 39%. TALH-treated samples, after heating to 300 °C, had an average relative density of 48%. After sintering, the average relative density of the TALH-treated samples increased to 55%, whereas the untreated samples had a final relative density of 48%, as shown in Fig. 11B.

[0095] Monitoring of Cantilevered Beams During Sintering

[0096] Example 5

[0097] To achieve a uniform densification rate that is independent of position, beams were designed so that sintering stress was greater than maximum hydrostatic stress. The sintering stress was approximated by dividing the surface energy by the average particle size while the hydrostatic stress was calculated using elementary beam bending theory. The overhang of the symmetric beam was designed with a length of 10 mm, a height of 2.5 mm, and a width of 6 mm. A pre-sintering heat treatment was used to increase the handling strength of the specimens. During this step, the beams were heated to 1100 °C (homologous temperature of 0.65) for one hour in a supported position to initiate neck growth. After the pre-sintering step, select beams were treated with TALH which was then decomposed by heating to 300 °C. Specimens were placed in a tube furnace, heated to the sintering temperature of 1420 °C over the course of 11 hours, then held at 1420 °C for 10 hours.

[0098] The beam specimens were monitored using a 22.3 megapixel Cannon SLR camera equipped with a 200 mm lens and 2x extender. Photographs were taken at five second intervals. Figs. 10A-10C show three optical images of an untreated beam specimen, each at a different stage in the sintering process. In the first image, taken 7 hours into the sintering process, no deflection is observed. In the second and third images, taken after 10 and 21 hours respectively, deflection due to creep can be observed. A majority of the creep-induced deflection occurs above 1180 °C (homologous temperature of 0.68). Figs. 10E-10F show similar optical images for a TALH-treated sample, taken at 7 hours, 10 hours, and 21 hours. Deflection in the TALH-treated beam is not visible in the image taken 10 hours into the experiment, and the deflection visible in the image taken at 21 hours is significantly less than that in the untreated beam. The images reveal that the TALH treatment mitigated gravity- induced creep during sintering. The total deflection of the beams after sintering was measured through image analysis: the final deflection of the untreated beam was 4 mm whereas the deflection of the TALH treated beam was 1 mm, a 75% reduction of distortion.

[0099] In accordance with one or more embodiments, a method for mitigating distortion during sintering of binder jet printed ceramic parts, using as a model system TiCte powder and the reactive liquid precursor TALH is demonstrated to effectively reduce distortion by as much at 75% relative to samples not treated with the precursor solution. Through characterization using TGA, DSC, XRD, and SEM, the product of heating TALH to temperatures exceeding 900 °C was determined to be rutile T1O2 with a grain size on the order of 100 nm. The reactive precursor may provide structural support in the brown body (after pre-sintering heat treatments). This capability is useful when sintering ceramics, which generally require higher sintering temperatures than metallic engineering alloys and are therefore more susceptible to failure immediately after binder burnout.

[00100] Infiltration of porous cylindrical specimens with TALH led to lower uniaxial strains during sintering as measured by TMA. A dramatic decrease in deflection of TALH-treated cantilever beams was observed during sintering by means of in situ monitoring. The mechanism for this mitigation, as revealed by SEM, is the nanoparticle structure which results from the decomposition of TALH, coating the printed powder and filling the interparticle pores. Due to the lower sintering temperatures of nanoscale particles, such a coating has the potential to form a rigid, reinforcing structure early in the sintering cycle, thus strengthening the sintering body, increasing its viscosity, and enhancing its resistance to creep.

[00101] Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.

Claims

CLAIMS What is claimed:

1. A method comprising:

binder jet printing of a ceramic powder to form a ceramic body;

applying a precursor solution to said body; and

sintering said body, wherein said application of the precursor solution to the body mitigates distortion of the body during or after the sintering step.

2. The method of claim 1, wherein the ceramic power is combined with a binder during the binder jet printing of the body.

3. The method of claim 2, wherein the ceramic body is pre- sintered before applying the precursor solution.

4. The method of claim 1, wherein the body comprising the applied precursor solution is subjected to a decomposition reaction at a temperature that ranges from 200° C to 600° C, before the sintering.

5. The method claim 4, wherein the decomposition of the precursor solution yields a ceramic nano-particulate.

6. The method of claim 1, wherein the ceramic powder is one or more selected from the group consisting of titanium dioxide, silicon carbide, zirconia, and aluminum oxide, or a mixture thereof.

7. The method of claim 1, wherein the sintering of said body is conducted at a temperature that ranges from 1000° C to 1500° C.

8. The method of claim 1, wherein the precursor solution comprises at least one selected from the group consisting of metal alkoxides, metallic salt solutions, sol-gels, polymer-derived ceramics, and co-precipitation/calcination solutions.

9. The method of claim 1, wherein the applying comprises at least one selected from the group consisting of: applying the precursor solution layer by layer in conjunction with the binder jet printing, immersing the body in the precursor solution, and infiltrating the body with the precursor solution.

10. The method of claim 1, wherein the sintering step comprises at least one selected from the group consisting of thermal treatment, chemical treatment, and UV treatment of the body.

11. The method of claim 1, wherein the ceramic powder comprises Ti0_2.

12. The method of claim 8, wherein the precursor solution comprises titanium (IV) bis(ammoniumlactato)dihydroxide.

13. A ceramic powder body comprising:

a ceramic powder; and

a precursor decomposed from a precursor solution, the precursor having a rigid nanoparticle form and coated around the ceramic powder.

14. The ceramic powder body of claim 13, wherein the ceramic powder body is formed by binder jet printing.

15. The ceramic powder body of claim 13, wherein the ceramic powder body comprises a ceramic powder that is one or more selected from the group consisting of titanium dioxide, silicon carbide, zirconia, and aluminum oxide, or a mixture thereof.

16. The ceramic powder body of claim 13, wherein the ceramic powder body further comprises a binder.

17. The ceramic powder body of claim 13, wherein the precursor solution comprises at least one selected from the group consisting of alkoxides, sol-gels, polymer-derived ceramics, and a co-precipitation/calcination solution.

18. The ceramic powder body of claim 13, wherein the average relative density of the sintered body is from 50 to 60%.

19. The ceramic powder body of claim 13, wherein the precursor solution is decomposed to yield a ceramic nanoparticle coating of the same ceramic material of the ceramic powder.

20. A method for preparing a positive mold for directionally solidified components comprising:

binder jet printing a ceramic powder to form a ceramic body;

applying a precursor solution to said body; and