HIGH STRENGTH CERAMICS WITH NOVEL FRACTURE MODE

Abstract

The present invention provides a method for making a high strength, small grain size ceramic having a trans-granular fracture mode by rapid densification of a green body and subsequent cooling of the densified ceramic. The ceramic may include dislocations, defects, dopants, and/or secondary phases that are formed as a result of the process and resulting in stress fields capable of redirecting or arresting cracks within the material. This ceramic can maintain transparency from ultraviolet to mid-wave infrared.

Claims

1. A method for making a high strength, small grain size ceramic having trans-granular fracture behavior, comprising: forming a green body comprising a ceramic powder; densifying the green body using microwaves to heat the green body at a rate of 100° C./min to 1450° C. and then holding at 1450° C. for about 15 minutes, wherein a ceramic is formed; and cooling the ceramic to room temperature, resulting in a ceramic having a trans-granular fracture mode.

2. The method of claim 1, wherein said ceramic powder comprises magnesium aluminate spinel.

3. The method of claim 1, wherein said green body is formed by cold isostatic pressure, slip casting, tape casting, or any combination thereof.

4. The method of claim 1, wherein said green body is densified using a cavity designed to produce uniform heating throughout said green body.

5. The method of claim 1, wherein the microwave frequency is greater than 30 GHz.

6. The method of claim 1, wherein said microwave beam is used without the use of a susceptor or indirect heating.

7. The method of claim 1, wherein said ceramic powder is doped with calcium prior to green body formation.

8. A method for making a high strength, small grain size ceramic having trans-granular fracture behavior, comprising: forming a green body comprising lutetium oxide powder; densifying the green body using microwaves to heat the green body at a rate of 100° C./min to 1500° C. and then holding at 1500° C. for about 15 minutes, wherein a ceramic is formed; and cooling the ceramic to room temperature, resulting in a ceramic having a trans-granular fracture mode.

9. The method of claim 8, wherein said green body is formed by cold isostatic pressure, slip casting, tape casting, or any combination thereof.

10. The method of claim 8, wherein said green body is densified using a cavity designed to produce uniform heating throughout said green body.

11. The method of claim 8, wherein the microwave frequency is greater than 30 GHz.

12. The method of claim 8, wherein said microwave beam is used without the use of a susceptor or indirect heating.

13. The method of claim 8, wherein said ceramic powder is doped with calcium prior to green body formation.

14. A method for making a high strength, small grain size ceramic having trans-granular fracture behavior, comprising: forming a green body comprising barium titanate or yttrium oxide powder; densifying the green body using microwaves to heat the green body at a rate of 100° C./min to 1100° C. and then holding at 1100° C. for about 15 minutes, wherein a ceramic is formed; and cooling the ceramic to room temperature, resulting in a ceramic having a trans-granular fracture mode.

15. The method of claim 14, wherein said green body is formed by cold isostatic pressure, slip casting, tape casting, or any combination thereof.

16. The method of claim 14, wherein said green body is densified using a cavity designed to produce uniform heating throughout said green body.

17. The method of claim 14, wherein the microwave frequency is greater than 30 GHz.

18. The method of claim 14, wherein said microwave beam is used without the use of a susceptor or indirect heating.

19. The method of claim 14, wherein said ceramic powder is doped with calcium prior to green body formation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1A is a diagram illustrating the difference between the atomic arrangement in a perfect crystal versus a crystal with an edge dislocation. FIG. 1B is a diagram illustrating the difference between the atomic arrangement in a perfect crystal versus a crystal with a screw dislocation.

[0020] FIG. 2 is a diagram illustrating a mixed dislocation containing one edge and one screw component.

[0021] FIG. 3 is a scanning electron microscope image of the fracture surface of a spinel window sintered using 83 GHz microwave.

[0022] FIG. 4 is a scanning electron microscope image of the fracture surface of a spinel window sintered using conventional techniques.

[0023] FIG. 5 is a scanning electron microscope image of demonstrated small grain size spinel window with novel fracture.

DETAILED DESCRIPTION OF THE INVENTION

[0024] As described herein, a high strength, transparent spinel ceramic with small grains and trans-granular fracture behavior is created through rapid densification of a green body and subsequent cooling of the densified ceramic. The ceramic may include dislocations, defects, dopants, and/or secondary phases that are formed as a direct result of the described process and resulting in stress fields capable of redirecting or arresting cracks within the material.

[0025] The green body may be of any size and/or shape. The green body may be comprised of powder and material necessary to maintain its shape prior to densification. Here particles are defined as the individual masses of matter that make up a powder. These masses may be single crystals or polycrystalline. A grain is defined here as an individual single crystal contained within a polycrystalline material. A polycrystalline material is by definition made up of multiple grains that are bonded or fused together. The green body may be formed by any traditional ceramic forming method or derivative thereof that may be conceived by a person of ordinary skill in the art. In the embodiments, green bodies were formed using a combination of uniaxial and cold isostatic pressing. Green bodies may be formed using any ceramic powder with particle size less than 5 μm. The powder may be of any morphology and should have a surface area greater than 5 m.sup.2/g.

[0026] A preferred embodiment includes the use of volumetric heating by interaction of the ceramic with microwaves to facilitate rapid densification uniformly throughout the material without the use of a susceptor or indirect heating. The frequency of the microwave should be sufficient to ensure uniform heating of the ceramic from room temperature to the maximum sintering temperature. The power of the microwave source should be adequate to ensure the necessary heat transfer to the ceramic. Microwaves would be made to interact with the material through the use of a microwave beam with or without beam optics and/or a cavity designed to provide the appropriate thermal profile for the required sample size and geometry. In embodiments, the materials were processed using a single fixed frequency 83 GHz microwave beam with a maximum power of 15 kW without the use of a designed cavity or specialized beam pattern.

[0027] Contemplated herein are processes for making high strength, small grain ceramics with trans-granular fracture in large sizes and/or complex shapes. Ceramic material may be opaque or transparent. The microwave frequency must be greater than 30 GHz where the distance between node and anti-node will be a few mm or less. The microwave setup may include multiple sources. The microwave sources do not need to be of the same frequency and/or power. They also do not need to be of fixed frequency and/or power. For example, a green body with a curved shape can be heated using a single fixed 100 GHz source positioned at the top and three 60-90 GHz variable frequency sources positioned around the green body. The sources may be independently or collectively controlled.

[0028] The type of fracture mode and mechanical properties can be tailored to optimize the ceramic by changing the heating/cooling rates, maximum temperature, and time at temperature. The grain morphology and size can be modified based on particle size, powder morphology, and microwave frequency. The microwave frequency dictates the spacing between hot and cold nodes in the material which dictates the degree of uniformity within the densified ceramic as well as manipulating grain growth directions.

[0029] The various embodiments disclosed herein may be combined as evident to a person of ordinary skill in the art.

Example 1

[0030] Spinel powder with a grain size of 200 nm and a 30 m.sup.2/g surface area is uniaxially pressed into a 12 mm cylinder with 4 mm thickness then cold isostatically pressed (pressure is applied to a fluid and the fluid creates pressure on all of the material's geometric exterior surfaces) as a means to consolidate the powders into a green body shape with a density of greater than 50%. The green body is then heated to 600° C. for 6 hours in air to remove any organic matter from the green body.

Example 2

[0031] Spinel powder from Example 1 is slip cast (powder is suspended in a fluid then poured into a porous mold that removes the liquid and leaves behind consolidated particles in the shape of the mold) into an 8″ dome shape and cold isostatically pressed to a green density of greater than 50%. Organics are removed similar to Example 1.

Example 3

[0032] Similar to Example 2, spinel powder from Example 1 is slip cast into a 1 m×1 m×0.025 m window and cold isostatically pressed to a green density of greater than 50%. Organics are removed similar to Example 1.

Example 4

[0033] Spinel powder from Example 1 is tape cast (powder is suspended in a viscous fluid and then shaped into thin sheets or tapes using a blade with a fixed height that the suspended powder must pass under) into a 130 mm wide×50 μm thick sheet with a green density greater than 40%. Organics are removed similar to Example 1.

Example 5

[0034] The green bodies from Examples 1-4 are densified to >95% of theoretical density using a 6 kW, 83 GHz microwave beam shaped to evenly heat their geometry at a rate of 100° C./min to 1450° C. and held there for about 15 minutes causing the formation of dislocations within the ceramic. The ceramic is then air cooled to room temperature to trap dislocations into the crystal structure.

Example 6

[0035] The green bodies from Examples 1-4 are densified using a cavity designed to produce uniform heating throughout the green body geometry using an 83 GHz microwave source based on the schedule in Example 5.

Example 7

[0036] Spinel powder as in example 1 is intentionally doped with calcium prior to green body formation as in Examples 1-4. The resulting green body is processed similar to Examples 4 and 5. The rapid diffusion of the calcium dopant within the ceramic disrupts the normal spinel structure by creating stress fields, dislocations, and defects within the crystal lattice to steer cracks as they pass through the grains during fracture (see FIG. 5).

Example 8

[0037] The same procedures as in Examples 1-7 but using barium titanate. The green bodies are densified at 1100° C. for 15 minutes to obtain a high strength, small grain ceramic (15 μm or less) with trans-granular fracture mode. Traditional processes typically yield grain sizes of 20 μm and larger indicating the trans-granular fracture mode dominates the strength behavior of the material.

Example 9

[0038] The same procedures as in Examples 1-7 but using Lutetium oxide. The green bodies are densified at 1500° C. for 15 minutes to obtain a high strength, small grain ceramic with trans-granular fracture mode.

Example 10

[0039] The same procedures as in Examples 1-7 but using Yttrium oxide. The green bodies are densified at 1100° C. for 15 minutes to obtain a high strength, small grain ceramic with trans-granular fracture mode.

Example 11

[0040] Similar to Examples 8-10, this method is applicable to other doped and undoped ceramics where higher mechanical strength is needed. Other microwave frequencies greater than 30 GHz may be used to tailor the coupling of microwaves to the ceramic green body.

Example 12

[0041] The ceramics made using this process exhibit strengths that are more than 5 times higher than glass. This higher strength is attributed to the novel transgranular fracture mechanism observed in spinel made using high frequency microwaves.

[0042] The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular.