WO2011019725A2

WO2011019725A2 - Article and method of making a ceramic article and composites

Info

Publication number: WO2011019725A2
Application number: PCT/US2010/045029
Authority: WO
Inventors: Eugene A. Olevsky
Original assignee: San Diego State University Research Foundation
Priority date: 2009-08-10
Filing date: 2010-08-10
Publication date: 2011-02-17
Also published as: WO2011019725A3

Abstract

The present invention is a technological sequence which is composed of uniaxial freezing with control solution composition, followed by freeze-drying, followed by sintering, and followed by infiltration. The invention enables the manufacturing of functionally graded composites. These composites can include spatially distributed phases of ceramics, metals, polymers, glasses, and their combinations as long as the composite matrix material is originally in the form of powders. During uniaxial freezing, the diameter of crystals growing from the suspension (and, thus, the diameter of the voids formed after freeze-drying) is dependent on the controllable rate of cooling, pressure and the changeable chemical composition of the suspension liquid phase. Thereby a structure with conic voids can be created. After following sintering and infiltration, the produced composite (e.g., ceramic-metal composite) will have a specially distributed concentration of the second phase in the composite matrix, that is, it will have a functionally-graded structure.

Description

ARTICLE AND METHOD OF MAKING A CERAMIC ARTICLE AND COMPOSITES

The application claims priority of US provisional application number 61/232,589 filed on August 10, 2009 and is included herein in its entirety by reference.

COPYRIGHT NOTICE

A portion of the disclosure of this patent contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever.

BACKGROUND OF THE INVENTION

Field of the Invention

[001] The present invention relates to composites and methods of making composites comprising a ceramic with a second composition such as a metal. The method produces conical voids in ceramics which are filled with the second composition.

Description of Related Art

[002] Freeze-drying has been utilized to obtain SisN₄ and AI₂O₃ preforms with aligned channels by Fukasawa et al. [1 ,2]. By controlling the temperature gradient in the crystallization process within powder slurry, this approach provides a convenient method for the fabrication of ceramic preforms with controlled pore alignment.

Generally, this methodology does not involve chemical reactions; therefore, it is applicable to any material as long as it is in the form of a powder. By controlling the solidification parameters, such as the chemical composition of the slurry, temperature gradient, solidification rate, etc., the structure can be controlled.

[003] The US Patent 5,628,938 by Sangeeta, et al. describes the method for making a ceramic composite by a similar technological sequence comprising directional solidifying (uniaxial freezing), freeze-drying, sintering, and infiltration; it does not, however, include the crystal morphology-controlling conditions at the uniaxial freezing and freeze-drying steps thereby leading to the fabrication of non- functionally-graded composites. There is no teaching of a method or the ability to produce a cone shaped pore with this method.

BRIEF SUMMARY OF THE INVENTION

[004] The present invention relates to a method of preparing a ceramic with conical voids and a composite wherein the voids are formed by uniaxial freezing of a ceramic suspension based on a salt solution subjecting the suspension to the freezing process, then freeze-drying, sintering, and the infiltration of a second phase.

[005] Accordingly, one embodiment of the present invention relates to a method for the preparation of a ceramic article with conical voids comprising:

a) uniaxially freezing a liquid medium containing a dispersion of ceramic particles wherein a salt solution is added to the liquid media subjected to freezing for a time and temperature sufficient to cause directional solidification and formation of conical shaped area of frozen crystal liquid in the frozen medium; b) removing the solidified liquid by freeze-drying to form a conical pore structure; and

c) sintering the freeze dried article.

BRIEF DESCRIPTION OF THE DRAWINGS

[006] FIG. 1 is a diagram of unidirectional freezing of a powder slurry/suspension.

[007] FIGS. 2a and 2b are SEMs of sintered ZrO₂ porous preforms.

DETAILED DESCRIPTION OF THE INVENTION

[008] While this invention is susceptible to embodiment in many different forms, there is shown in the drawings and will herein be described in detail specific embodiments, with the understanding that the present disclosure of such

embodiments is to be considered as an example of the principles and not intended to limit the invention to the specific embodiments shown and described. In the description below, like reference numerals are used to describe the same, similar or corresponding parts in the several views of the drawings. This detailed description defines the meaning of the terms used herein and specifically describes

embodiments in order for those skilled in the art to practice the invention.

[009] The terms "a" or "an", as used herein, are defined as one or as more than one. The term "plurality", as used herein, is defined as two or as more than two. The term "another", as used herein, is defined as at least a second or more. The terms "including" and/or "having", as used herein, are defined as comprising (i.e., open language). The term "coupled", as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. [010] Reference throughout this document to "one embodiment", "certain

embodiments", and "an embodiment" or similar terms means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of such phrases or in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments without limitation.

[011] The term "or" as used herein is to be interpreted as an inclusive or meaning any one or any combination. Therefore, "A, B or C" means any of the following: "A; B; C; A and B; A and C; B and C; A, B and C". An exception to this definition will occur only when a combination of elements, functions, steps or acts are in some way inherently mutually exclusive.

[012] The drawings featured in the figures are for the purpose of illustrating certain convenient embodiments of the present invention, and are not to be considered as limitation thereto. Term "means" preceding a present participle of an operation indicates a desired function for which there is one or more embodiments, i.e., one or more methods, devices, or apparatuses for achieving the desired function and that one skilled in the art could select from these or their equivalent in view of the disclosure herein and use of the term "means" is not intended to be limiting. [013] As used herein "ceramics" can include all ceramic materials capable of pore formation. In one embodiment, the ceramics are the thermo-isolative ceramics, such as oxide and nitride ceramics including alumina, zirconia, titania, aluminum nitride and the like.

[014] As used herein "infiltrate phase" refers to any second compositions which are capable of being added to the pores of a ceramic material by infiltration such as other ceramics, metals, polymers and glasses. In one embodiment the infiltrate phase is a metal such as copper, iron, nickel and the like. By addition of the second infiltrate phase, a functionally graded composite material can be produced for a variety of purposes such as articles used in thermal management systems, bio- implant components and the like, where a ceramic composite would be useful.

[015] Freeze-drying has been used mainly in the food industry and pharmacology, and has some history in materials research [3,4], but has been gaining frequency of use in this field only in the past decade. [5,6]

[016] Others have reported on the morphology that uniaxial freeze-drying (a combination of uniaxial freezing and freeze-drying) can create, mainly

macroscopically aligned channels. The orientation of the channels may be

controlled, depending on the placement of the freezing surface, the temperature gradient between the freezing surface and the top surface of the material, and the quality of the aqueous solution. These channels can be oriented in different directions, parallel or perpendicular to the substrate, depending on the shape of the cooling plate, and its contact with the glass substrate. One of the main factors controlling the growth of these channels is the temperature gradient between the conductive plate immersed in a refrigerant, and the medium surrounding the upper surface of the material. The temperature gradient is the driving force for ice crystal growth and causes the ice crystals to form in a direction parallel to the heat flow and perpendicular to the cooling surface. Figure 1 shows this in detail.

[017] Freeze-drying is the sublimation of a frozen, aqueous solution, usually at a low temperature and decreased pressure for water-based solutions (parameters vary depending on the liquid being used). Freeze-drying is necessary to compliment the uniaxial freezing process, as normal melting of the water in the slurry destroys the fragile structure of the pores in the material.

[018] Controlling the uniaxial freezing conditions by changing the composition of the suspension liquid phase, pressure, and temperature allows the dynamic change of the growing crystal sizes (effective diameters). Controlling temperature and pressure conditions during freeze-drying also allows the formation of graded pore

morphologies. This, in turn, leads to the creation of the quasi-cone shape crystals whose cross-section increases from one side of the powder component to another. This leads to the creation of graded pores after freeze-drying and, ultimately, to the functionally-graded structure of the two-phase (matrix and an infiltrating second phase) composite. In functionally-graded materials the concentration of different phases changes directionally in the component volume. EXAMPLE

[019] A sample experiment includes the zirconia slurry preparation, unidirectional freezing and freeze-drying, sintering of zirconia pre-forms, and infiltration of copper. The details of the experimental procedures are described below.

[020] 35% vol. concentration slurry is made by dispersing ZrO2 powders (TZ-3YS Tosoh, Japan, specific surface area 6.6 m2/g) in de-ionized water. Commercial dispersant (Duramax D-3005, Rohm and Hass, USA) is added at the ratio of 2.2 mg per square meter of total particle surface area. The slurry is milled in a planetary ball mill (Fritsch LC-106A) for 30 minutes prior to freezing. The slurry is further de- aerated in a vacuum chamber before freezing. After the above-mentioned

processing, the slurry is stable and homogeneous based on visual inspection.

[021] The unidirectional freezing configuration is similar to that of Fukasawa except that the conditions of uniaxial freezing are changed through pumping a NaCI (or other salt) water solution into the suspension subjected to freezing. The diameter of the cylindrical mold is 12.5 mm and the height is 12.5 mm. The ZrO2 slurry is smoothly poured into the mold. Then the bottom part of the aluminum rod is dipped into liquid nitrogen. After the entire slurry was frozen, it is removed from the mold and dried in a freeze-dryer (Labconco FreeZone) for 24 hours to sublimate the ice.

[022] Freeze-dried samples are partially sintered in a tube furnace in air at 1400⁰C for 2 hours. The heating rate is 10°C/min. After partial sintering, the preforms are cooled down to room temperature at the rate of 10°C/min. The partially sintered ZrO2 preforms are cut perpendicular to the axis into 2mm thick slices for copper infiltration. A typical SEM image of a freeze dried specimen with aligned pores is shown in Figure 2a and b. It is anticipated that the said suspension liquid phase as well as pressure and temperature control will provide the cone-like pore shape in the preform structure.

[023] The ZrO2 preforms are subjected to molten copper infiltration in a furnace with controlled atmosphere.

References

1. T. Fukasawa, Z. Deng, M. Ando, T. Ohji, and S. Kanzaki, Synthesis of porous silicon nitride with unidirectionally aligned channels using freeze-drying process, J. Am. Ceram. Soc, 85 (9), 2151 (2002)

2. T. Fukasawa, M. Ando, T. Ohji, and S. Kanzaki, Synthesis of porous ceramics with complex pore structure by freeze-dry processing, J. Am. Ceram. Soc, 84 (1 ), 230 (2001 )

3. F. K. Roehrig, T. R. Wright, "Carbide Synthesis by Freeze-Drying," J. Am.

Cer. Soc. 55, 58 (1972)

4. K. H. Song, H. K. Liu, S. X. Dou, and C. C. Sorrell, "Rapid Formation of the 110 K Phase in Bi-Pb-Sr-Ca-Cu-O through Freeze-Drying Powder Processing," J. Am. Cer. Soc, 71 , 1771-73 (1990)

5. E. A. Olevsky, X. Wang, E. Bruce, M. B. Stern, S. Wildhack, and F. Aldinger, "Synthesis of gold micro- and nano-wires by infiltration and thermolysis", Scripta Mater., 56, 867-869 (2007).

6. K. Araki, and J. W. Halloran, "Porous Ceramic Bodies with Interconnected Pore Channels by a Novel Freeze Casting Technique," J. Am. Ceram. Soc, 88, 1108-1114 (2005)

Claims

What is claimed is:

1. A method for the preparation of a ceramic article with conical voids

comprising:

a) uniaxially freezing a liquid medium containing a dispersion of ceramic particles wherein a salt solution is added to the liquid media subjected to freezing for a time, pressure and temperature sufficient to cause directional solidification and formation of conical shaped area of frozen crystal liquid in the frozen medium;

b) removing the solidified liquid by freeze-drying to form a conical pore structure; and

c) sintering the freeze dried article.

2. A method according to claim 1 which further comprises infiltrating the conical pore with an infiltrate phase.

3. A method according to claim 2 wherein the infiltrate phase is a metal.

4. A method according to claim 1 wherein the salt solution is a sodium chloride salt solution.

5. A method according to claim 1 wherein the ceramic is ZrO2.

6. A ceramic article with conical voids.

7. An article according to claim 6 wherein the void is filled with a second phase.

8. An article according to claim 7 wherein the second phase is a metal.