1. Field of the Invention
The present invention relates to building highly precision monolithic structures comprising components with different Coefficients of Thermal Expansion (CTEs) such as optical assemblies that are highly sensitive to change of environmental temperature, shock and vibration.
2. Background of the Invention
Generally, the existing highly precision assemblies such as optical assemblies can be grouped into two categories:
In this concept, the development process is started with the optical design of a series of discrete optics in free space, then involves the design of a metal structure to connect and align them, and finally moves to fabrication and assembly. Precision optical assemblies, especially Interferometers are very sensitive to environmental conditions. In FT-IR applications, where the Michelson configuration is widely used, the common method of assembly is to mount the mirrors and the beam splitter kinematically, using some kind of screw arrangement, on an aluminum chassis. A significant part of the assembly process is the alignment procedure involved in each assembly, where element positions must be maintained within mil-lionths of an inch and angular orientations within fractions of a second. To enable this alignment, each mirror mount is provided with an X, Y tilting mechanism. This mechanism must be able to lock the optical elements in place without distorting the exiting wavefront of the system. A common problem with this configuration is that the mirror alignment is very sensitive to vibration, shock, and slight metal fatigue conditions. It is also sensitive to changes in ambient temperature because the optical elements are usually made of glass, and the chassis is made of aluminum. These materials have very different coefficients of thermal expansion (CTEs) and conductivities.
The assembly of optical elements and a chassis with different CTEs and conductivities would normally require a flexible member between the mirrors and the aluminum chassis. However, this requirement contradicts a more important one, that is, that the interface between the different components in the assembly be totally stiff. Various technologies have been developed to overcome this problem; nonetheless, maintaining constant alignment is a routine and costly process. To obtain a more accurate and stable interferometer, the monolithic body assembly concept was developed. In the traditional designs, stresses such as those produced by preloaded screws, ball contacts and internal dislocations in the substrate create various problems, including those associated with random relative movement, bulk geometry, birefringence and wavefront. Similarly, as two dissimilar interfaced parts go through thermal cycling, their differential expansion will cause stresses that will lead to misalignment. Monolithic design avoids these issues and further ensures long-term stability by eliminating relative part-to-part movement.
In conclusion, the Pros and Cons of Traditional Compartmentalized Structure are:
Pros:
Cons:
With the emergence of better optical production technologies comes a new assembly concept, monolithic design, an integrated optical solution in which individual optic elements of the same material directly bond to each other or through similar material spacers. As a result the critical alignment between elements is maintained by the parts themselves instead of by an external mechanical assembly. Great care must be taken in the fabrication aligning and bonding of the elements and spacers to achieve the required alignment; however, once successfully assembled, the monolithic structure is extremely robust, nearly impossible to misalign, much less massive, and smaller than a system employing mechanical mounts. The entire face is used in the bond since modern computer numerical control machines can manufacture higher-precision surfaces onto glass, ceramic and single-crystal than on metal. These designs require few or no metal structures; adhesion and external tooling construct the assembly. However metal or other tough ductile materials still affect design, since the assembly eventually will have an interface specifically, the outer face that connects to metal. This type of Monolithic design often relies on completely contiguous glass-to-glass bonding, this changes the optical and the coating design. The changes in coatings are not more difficult, but it should be taken into the design consideration. Some of the advantages over mechanical bonding include no slipping, less machining and lower weight. But this design concept poses several challenges, which are adhesion uncertainty, cure time, thickness uniformity, curing-induced stress, coefficient of thermal expansion mismatch, weaker bond, surface finish issues and outgassing. In high-precision applications, monolithic designs are superior because they stiffen the structure and make it more stable. In commercial products, they are attractive because they enable reduced part counts and less expensive assemblies. Pay attention to the cost and weight of glass, ceramic and single crystal: Parts made with the glass, ceramic and single crystal method are two to three times more expensive than those made with stainless steel and considerably heavier than composite material parts. However, using smaller, lighter assemblies may allow trade-offs in other parts of the system, decreasing overall product cost. As the distances between optic elements grow, the cost, weight, and ability to maintain the required tolerance of spacers become unmanageable. Furthermore, in some applications such as Infrared Interferometer with large aperture, employing optic elements and spacers, all with the same material is impractical and extremely costly; since all transmitting components such as lenses and beam splitters are fabricated Germanium that is expensive, while large mirrors and frame/mounting fixtures can be made with light weight and less expensive composite materials. Another concern is the removable adjustment and alignment fixtures concept: in this structure, monolithic optics are aligned during the bonding process; after the bond is stable, the tooling can be removed, but due to bonding process problems such as adhesion uncertainty, thickness uniformity, curing-induced stress, coefficient of thermal expansion mismatch, and weaker bond, which may cause mechanical drifting after bonding process when the tooling is removed.
In conclusion, the Pros and Cons of Optically Contact Surface Bonding Monolithic Structure are:
Pros:
Cons:
This invention proposes the utilization of thermal expansion adapter material having varied coefficient of thermal expansion at different points within the adaptor structure, which will mitigate all the cons while retaining all the pros of both above structures. The design paradigm of this concept comprises:
The use of permanent bonding between all components to create a monolithic assembly: As definition, a monolithic structure forms a single body; permanent bonding between several elements meets this criterion. There are several existing bonding techniques, as described by the following excerpts from U.S. Pat. No. 5,846,638:
The heat treatment can be performed at a low enough temperature to prevent interdiffusion between species, thus insuring that the bond is not subjected to excessive mechanical stresses and that the materials do not undergo phase changes. Therefore this technique allows stable bonds to be formed between materials of widely differing physical, mechanical, thermal, optical and electro-optical properties such as different hardness, chemical durability, mechanical strength, coefficients of thermal expansion, thermal conductivity, crystal structure, refractive indices, optical birefringence, nonlinear optical coefficients, electrical conductivity, or semiconducting properties.”
The use of adaptor fixture made of material having varied coefficient of thermal expansion and conductivity: In this concept, two components with different CTEs are interconnected by an CTE adaptor having CTE gradually varied in one direction, which is perpendicular to the flat interfacing planes between the CTE adaptor and the two said components; at each interface, the CTE adaptor has CTE that matches to the adjacent component CTE in certain degree.
Allowing the freedom of material selection for each individual component to obtain best performance, cost, and reliability: With this concept, components can be optimally selected based on function, reliability, cost, manufacturability, weight, adaptation, integration and other parameters.
The use of Optically Contact Surface Bonding Monolithic Structure: This concept is still viable when it is applicable and provides the most optimum solution.
The use of removable adjustment and alignment fixtures: This concept is desirable when the bonding process is predictable and the bond remains stable after the alignment fixture is removed.
The use of built-in adjustment and alignment fixtures: In this concept, the adjustment fixtures preferably made of the same material of the components are bonded to the assembly during the aligning and bonding process.
The proposed concept of this invention is not only applicable to precision monolithic optical assemblies, but to any monolithic structure comprising component with different Coefficient of Thermal Expansion.
The proposed concept comprises the use of adaptor fixture made of material having varied coefficient of thermal expansion and conductivity: Two bodies with different CTEs are interconnected by an CTE adaptor having CTE gradually varied in one direction, which is perpendicular to the flat interfacing planes between the CTE adaptor and the two said bodies; at each interface, the CTE adaptor has CTE that matches to the body CTE in certain degree.
Coefficient of Thermal Expansion (CTE) Adaptor material comprises multi-thin composite material layers, each has a CTE slightly different from its two adjacent layers (or layer at the top and bottom surfaces). All said layers are bond together to form a Coefficient of Thermal Expansion (CTE) Adaptor material having CTE gradually varied in only one direction, which is perpendicular to the said Bonding Interfaces.
These are two proposed techniques to produce the materials:
Bonding technique: Individual thin layers are fabricated with composite material; each has slightly different composition from others to obtain slightly different CTE. The layers are then bonded together in specific order to form a Coefficient of Thermal Expansion (CTE) Adaptor material having CTE gradually varied in only one direction, which is perpendicular to the said Bonding Interfaces.
Vapor deposition technique: Individual thin layer is fabricated with composite material; each has slightly different composition from others to obtain slightly different CTE and the subsequent layer is sputtered directly on top of the previously sputtered adjacent layer in a specific order to form a Coefficient of Thermal Expansion (CTE) Adaptor material having CTE gradually varied in only one direction, which is perpendicular to the said Bonding Interfaces.
The object of this invention is to provide the Thermal Expansion (CTE) Adaptor concept in building highly precision monolithic structures comprising components having different Coefficients of Thermal Expansion (CTEs) such as optical assemblies that are highly sensitive to change of environmental temperature, shock and vibration. Other objects are to provide the preferred makeups of the Thermal Expansion (CTE) Adaptor Material and fabrication techniques of said materials.
These and other objects and advantages of this invention will become apparent through examining the following description of the arrangement, operations and functionalities of the constituent components and appended claims in conjunction with the attached drawings.
In designing a monolithic structure having all discrete bodies with different Coefficient of Thermal Expansion (CTE) permanently bonded together, the following principles must be followed to avoid internal stress caused by changing of temperature:
Bodies having identical Coefficient of Thermal Expansion (CTE) may be directly bonded together
The Coefficient of Thermal Expansion (CTE) Adaptor must be bonded between two bodies having different Coefficient of Thermal Expansion (CTE).
The Bonding Interfaces (the interconnection interfaces) must be parallel.
The Coefficient of Thermal Expansion (CTE) Adaptor is made of material having varied CTE, the variation must be gradual and in only one direction, which is perpendicular to the said Bonding Interfaces.
At each Bonding Interface, the CTE of the CTE Adaptor must match the CTE of bonding body in certain degree to avoid built up internal stress due to temperature variation.
The real world is almost always more complex: The CTE can vary with temperature, so that the amount of expansion not only depends upon the temperature change but also upon the absolute temperature of the material. Some materials are not isotropic and have a different value for the coefficient of linear expansion dependent upon the axis along which the expansion is measured. For instance, with increasing temperature, calcite (CaCO3) crystals expand along one crystal axis and contract along another axis. In some applications, it is desirable to have the Coefficient of Thermal Expansion (CTE) Adaptor made of non-isotropic material, which has near zero CTE directional component in direction perpendicular to the Bonding Interfaces while CTE directional components in directions parallel to the Bonding Interfaces still gradually varied in the direction perpendicular to the said Bonding Interfaces; at each Bonding Interface, the CTE directional components of the Coefficient of Thermal Expansion (CTE) Adaptor only needs to match the CTE directional components of the adjacent body in all directions parallel to the Bonding Interfaces.
Example of a monolithic structure following these principles is illustrated in
A lens assembly 1 having Coefficient of Thermal Expansion (CTE) value of CTE1;
A spacer 2 having Coefficient of Thermal Expansion (CTE) value of CTE2;
A beam bender/splitter 3 having Coefficient of Thermal Expansion (CTE) value of CTE3;
A spacer 4 having Coefficient of Thermal Expansion (CTE) value of CTE4;
A beam bender/splitter 5 having Coefficient of Thermal Expansion (CTE) value of CTE5;
A spacer 6 having Coefficient of Thermal Expansion (CTE) value of CTE6;
A lens assembly 7 having Coefficient of Thermal Expansion (CTE) value of CTE7;
A spacer 8 having Coefficient of Thermal Expansion (CTE) value of CTE8;
A lens assembly 9 having Coefficient of Thermal Expansion (CTE) value of CTE9;
A Coefficient of Thermal Expansion (CTE) Adaptor 10 that interconnects the lens assembly 1 to the spacer 2 has CTE gradually varied from CTE1 at its interface with the Lens assembly 1 to CTE2 at its interface with the spacer 2.
A Coefficient of Thermal Expansion (CTE) Adaptor 11 that interconnects the spacer 2 to the beam bender/splitter 3 has CTE gradually varied from CTE2 at its interface with the spacer 2 to CTE3 at its interface with the beam bender/splitter 3.
A Coefficient of Thermal Expansion (CTE) Adaptor 12 that interconnects the beam bender/splitter 3 to the spacer 4 has CTE gradually varied from CTE3 at its interface with the beam bender/splitter 3 to CTE4 at its interface with the spacer 4.
A Coefficient of Thermal Expansion (CTE) Adaptor 13 that interconnects the spacer 4 to the beam bender/splitter 5 has CTE gradually varied from CTE4 at its interface with the spacer 4 to CTE5 at its interface with the beam bender/splitter 5.
A Coefficient of Thermal Expansion (CTE) Adaptor 14 that interconnects the beam bender/splitter 5 to the spacer 6 has CTE gradually varied from CTE5 at its interface with the beam bender/splitter 5 to CTE6 at its interface with the spacer 6.
A Coefficient of Thermal Expansion (CTE) Adaptor 15 that interconnects the spacer 6 to the lens assembly 7 has CTE gradually varied from CTE6 at its interface with the spacer 6 to CTE7 at its interface with the lens assembly 7.
A Coefficient of Thermal Expansion (CTE) Adaptor 16 that interconnects the beam bender 3 to the spacer 8 has CTE gradually varied from CTE3 at its interface with the beam bender 3 to CTE8 at its interface with the spacer 8.
A Coefficient of Thermal Expansion (CTE) Adaptor 17 that interconnects the spacer 8 to the lens assembly 9 has CTE gradually varied from CTE8 at its interface with the spacer 8 to CTE9 at its interface with the lens assembly 9.
directly if components have identical CTE to that of the enclosure. Reflective components such as mirrors can fabricated with the same material of the enclosure.
or via Coefficient of Thermal Expansion (CTE) Adaptor.
Preferred Frame and Spacer material:
Advanced composites utilize a combination of resins and fibers, customarily carbon/graphite, kevlar, or fiberglass with an epoxy resin. The fibers provide the high stiffness, while the surrounding polymer resin matrix holds the structure together. The fundamental design concept of composites is that the bulk phase accepts the load over a large surface area, and transfers it to the reinforcement material, which can carry a greater load. The significance here lies in that there are numerous matrix materials and as many fiber types, which can be combined in countless ways to produce just the desired properties. These materials were first developed for use in the aerospace industry because for certain applications they have a higher stiffness to weight or strength-to-weight ratio than metals. This means metal parts can be replaced with lighter weight parts manufactured from advanced composites. Generally, carbon-epoxy composites are two-thirds the weight of aluminum, and two and a half time as stiff. Composites are resistant to fatigue damage and harsh environments, and are repairable. Metal Matrix or Alloy Composite material such as Albite Particle in Al6061 having high wear resistance, dimensional stability, thermal conductivity and low CTE is good candidate for frame/part material; furthermore, its CTE can be tailored by varying the Albite composition, this Particulate-Reinforced composite can be utilized as Coefficient of Thermal Expansion (CTE) Adaptor material.
Its has been shown that the advanced composite materials, which can be obtained by either simple pressing of the powders (metal, ceramic, polymer) and sintering or by a wet-chemical sol-gel process are well suited for stable space structures due to their low Coefficient of Thermal Expansion (CTE), high stiffness and light weight. For a given design application, composite hardware can be tailored for strength, stiffness, CTE, and Coefficient of Moisture Expansion (CME). The Particulate-Reinforced composites, which not only have high specific strengths and modulus at room and elevated temperatures but also have excellent wear resistance, high thermal conductivity, low thermal expansion and good dimensional stability can be a great candidate for the Coefficient of Thermal Expansion (CTE) Adaptor material; especially since its elastic modulus and CTE may be tailored by varying the ceramic particle content in the matrix.
The sol-gel process is a process for making glass/ceramic materials. The sol-gel process involves the transition of a system from a liquid (the “sol”) into a solid (the “gel”) phase. The sol-gel process allows the fabrication of materials with a large variety of properties: ultra-fine powders, monolithic ceramics and glasses, ceramic fibers, inorganic membranes, thin film coatings and aerogels. The sol is made of solid particles of a diameter of few hundred nm, usually inorganic metal salt suspended in a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent.
In practice, the Coefficient of Thermal Expansion (CTE) Adaptor material comprises multi-thin composite material layers, each has a CTE slight different from its two adjacent layers (or layer at the top and bottom surfaces). All said layers are bonded together to form a Coefficient of Thermal Expansion (CTE) Adaptor material having CTE gradually varied in only one direction, which is perpendicular to the said Bonding Interfaces. These are two proposed techniques to produce the materials:
Bonding technique: In practical manufacturing processes, the Coefficient of Thermal Expansion (CTE) Adaptor material can be produced by the following concepts: Individual thin layer is fabricated with composite material; each has slightly different composition from others to obtain slightly different CTE. The layers are then bonded together in specific order to form a Coefficient of Thermal Expansion (CTE) Adaptor material having CTE gradually varied in only one direction, which is perpendicular to the said Bonding Interfaces. Each layer can be produced by either simple pressing of the powders (metal, ceramic, polymer) and sintering them together or by a wet-chemical sol-gel process. The layers can be bonded together by either simple pressing of the layers and sintering them together or by a wet-chemical sol-gel process.
Vapor deposition technique: Individual thin layer is fabricated with composite material; each has slightly different composition from others to obtain slightly different CTE and the subsequent layer is sputtered directly on top of the previously sputtered adjacent layer in a specific order to form to form a Coefficient of Thermal Expansion (CTE) Adaptor material having CTE gradually varied in only one direction, which is perpendicular to the said Bonding Interfaces.
In commercially viable practice, the Coefficient of Thermal Expansion (CTE) Adaptor materials are pre-fabricated in slab form with predetermined specific thicknesses and Extremity CTE determined by material commonly found in specific industry; for example, in Optical Industry, Quartz, other ceramic glass, aluminum, stainless steel, invar and granite are common. Parts are then finally fabricated from these slabs.