No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Demonstration and challenges for joining technology based on direct bonding usable for construction of (large) structures in space
Abstract
A direct bonding process relying on van der Waals forces offers an ideal combination of easiness to assemble and material compatibility. Such a bonding procedure, also denoted as optical bonding,
is already known and used frequently in different applications. However, there are strict requirements to achieve optical bonding: (1) a high level of cleanliness of the surfaces and (2) low roughness (RMS roughness <2 nm and preferably <0.5 nm). The first condition will be possible to realize in space, since the vacuum prevents the absorption of a thin water layer present on all surfaces under atmospheric conditions and forces a desorption of all residual water. Also, particle contamination will be minimized, due to the absence of strong capillary forces which attract particles if present and the absence of particles in space, providing a suitable and without residue removable protection prior to bonding. The second condition is now state of the art of silicon carbide (SiC) polishing and can hence be realized. While meeting the requirements, theoretically very large adhesion forces can be realized in vacuum. Surfaces have been polished according to the requirements. The forces measured on these surfaces are nearly as high as theoretically predicted and demonstrate the proof of principle of direct bonding of SiC under ambient conditions and in vacuum. However, the realization of the required flatness over large contact areas is still a challenge. Furthermore, since the surfaces display a really low roughness, extremely clean handling and bonding conditions need to be realized to avoid the spontaneous adhesion of small particles which would as such prevent direct bonding of larger areas.
... Hence, modules of a larger Fresnel lens could conceivably be constructed and assembled. Direct (optical) bonding is based on high van der Waal forces of adhesion F over a common surface area which requires extreme surface cleanliness and extremely low roughness <2 nm (Fischer et al., 2015): F = (H/6πd 3 ) where H = Hamaker constant∼10 −20 , d = 2 × asperity height. It is particularly suited to glass so this bonding might be exploited by a large Fresnel lens if contacting surfaces are extremely flat and smooth. ...
In the early 1980s, the Sagan-Tipler debate raged regarding the interpretation of the Fermi paradox but no clear winner emerged. Sagan favoured the existence of ETI on the basis of the Copernican principle and Tipler favoured the non-existence of ETI on the basis of the Occam's razor principle. Tipler's stance was an expansion of the similar but earlier Hart declaration. However, crucial to the Tipler argument was the role played by self-replicating interstellar robot probes. Any technologically capable species will develop self-replication technology as the most economical means of exploring space and the Galaxy as a whole with minimal investment. There is no evidence of such probes in our solar system including the asteroid belt, ergo, ETI do not exist. This is a powerful and cogent argument. Counter-arguments have been weak including Sagan's sociological explanations. We present a Copernican argument that ETI do not exist – humans are developing self-replication technology today. We are developing the ability to 3D print entire robotic machines from extraterrestrial resources including electric motors and electronics as part of a general in-situ resource utilization (ISRU) capability. We have 3D-printed electric motors which can be potentially leveraged from extraterrestrial material that should be available in every star system. From a similar range of materials, we have identified a means to 3D print neural network circuitry. From our industrial ecology, self-replicating machines and indeed universal constructors are feasible. We describe in some detail how a self-replicating interstellar spacecraft may be constricted from asteroidal resources. We describe technological signatures of the processing of asteroidal material (which is expected to be common to most star systems), and the excess production of certain types of clay and other detritus materials. Self-replication technology is under development and imminent – if humans are pursuing self-replication technology, then by the Copernican principle, so would any technologically savvy species elsewhere. There is no evidence that they have.
Volume 18 addresses friction and wear from a systems perspective, while providing a detailed understanding of why it occurs and how to control it. It explains the basic theory of friction and wear, and offers valuable insight on the forces, mechanisms, and interactions that are involved. It examines common wear scenarios, including wear by particles or fluids, rolling-contact wear, sliding wear, impact wear, and both chemical and environmentally assisted wear. It also covers operational wear, addressing several cases, including tool and die wear, bearing wear, engine wear, turbine wear, pump wear, and seal wear. In addition, the volume provides information on lubricants and lubrication, coatings, surface treatments and modifications, and the tribology of irons and steels, cobalt-base alloys, titanium, aluminum alloys and composites, cemented carbides, ceramics, polymers, and polymer composites. It also introduces the topic of condition monitoring, addressing wear particle analysis, vibroacoustic monitoring, and motor current signature analysis. For information on the print version of Volume 18, ISBN 978-1-62708-141-2, follow this link.
The influence of surface roughness on the accuracy with which the Hertz theory of elastic contact predicts the contact pressure and contact area between a sphere and a plane is examined theoretically and experimentally. Statistical theories of surface contact suggest that the influence of surface roughness is governed primarily by a single non-dimensional parameter α defined by where σ is the combined roughness of the two surfaces, R is the radius of the sphere and a0 is the contact radius for smooth surfaces given by the Hertz theory. Experimental measurements of contact area correlate well with this parameter. Provided that the value of α is less than about 0.05, errors in the application of the Hertz theory due to roughness of the surfaces are not likely to exceed about 7%.
The point at which two random rough surfaces make contact takes place at the
contact of the highest asperities. The distance upon contact d_0 in the limit
of zero load has crucial importance for determination of dispersive forces.
Using gold films as an example we demonstrate that for two parallel plates d_0
is a function of the nominal size of the contact area L and give a simple
expression for d_0(L) via the surface roughness characteristics. In the case of
a sphere of fixed radius R and a plate the scale dependence manifests itself as
an additional uncertainty \delta d(L) in the separation, where the scale L is
related with the separation d via the effective area of interaction L^2\sim\pi
Rd. This uncertainty depends on the roughness of interacting bodies and
disappears in the limit L\to \infty.
The influence of random surface roughness of Au films on the Casimir force is explored with atomic force microscopy in the plate-sphere geometry. The experimental results are compared to theoretical predictions for separations ranging between 20 and 200 nm. The optical response and roughness of the Au films were measured and used as input in theoretical predictions. It is found that at separations below 100 nm, the roughness effect is manifested through a strong deviation from the normal scaling of the force with separation distance. Moreover, deviations from theoretical predictions based on perturbation theory can be larger than 100%. Comment: 18, 5 figures
Calculations of Hamaker constants using Lifshitz theory require the availability of accurate dielectric data, especially in the ultraviolet spectral region, and the use of a convenient and appropriate mathematical representation. In this review, a multiple oscillator model - the so-called Ninham-Parsegian (N-P) representation - has been used and spectral parameters for 31 different inorganic materials (including diamond) have been generated from critically evaluated optical data or collected from the literature. For most materials, a two-oscillator model (one UV and one IR term) was used but more detailed representations were included when available. The spectral parameters presented here can be combined with previous data, mainly focused on hydrocarbon and organic systems, to yield an extensive spectral data base for both solids and liquids enabling Lifshitz calculations of Hamaker constants for many materials combinations. Non-retarded Hamaker constants for symmetric material combinations across vacuum (A 1v1) and water (A 1w1) have been calculated for the different materials; these calculations were performed using the full Lifshitz theory. Asymmetric combinations, A 1v3 and A 1w3, against four commonly used materials in atomic force microscopy studies: silica, amorphous silicon nitride, sapphire, and muscovite mica, have also been covered. The use of a new dielectric representation for water resulted in significantly lower values of A 1w1 compared to previous calculations. Analytical approximations to the full Lifshitz theory were evaluated and found to give surprisingly accurate results (the Tabor-Winterton approximation) for A 1v1 when the IR contribution is of minor importance. An attempt to make the TW approximation more general by establishing some scaling relationship between n 0 and ω UV was met with little success; only the UV spectral parameters of the covalent oxides, sulphides and nitrides may be fitted to a simple power law relation. The Lifshitz calculations in this study were compared with an alternative method where a more detailed dielectric representation in the visible-ultraviolet spectral range was obtained through Kramers-Kronig (K-K) transformation of reflectivity data over a broad frequency range. Despite the difference in dielectric information, the two methods generally yield non-retarded Hamaker constants which do not differ significantly. This is not true for all materials, e.g. water, where a more detailed representation using either an N-P representation with several oscillators or the K-K representation must be used. It was shown that the omission of the static and low frequency contribution in the latter method may result in a significant underestimation of the value for A 1w1 when the dispersive contribution becomes very small.
Calculations of Hamaker constants using Lifshitz theory require the availability of accurate dielectric data, especially in the ultraviolet spectral region, and the use of a convenient and appropriate mathematical representation. In this review, a multiple oscillator model — the so-called Ninham-Parsegian (N-P) representation — has been used and spectral parameters for 31 different inorganic materials (including diamond) have been generated from critically evaluated optical data or collected from the literature. For most materials, a two-oscillator model (one UV and one IR term) was used but more detailed representations were included when available. The spectral parameters presented here can be combined with previous data, mainly focused on hydrocarbon and organic systems, to yield an extensive spectral data base for both solids and liquids enabling Lifshitz calculations of Hamaker constants for many materials combinations.
The tendency of smooth surfaces to stick spontaneously to each other is becoming a serious problem, with: a) the increasing quality in surface finish for many components and systems, b) on miniaturization in mechanical components, and c) in demanded precision of positioning of parts in high-end equipment machines and systems. Surfaces tend to be made smoother in order to gain flatness or in order to fulfill the need for more precise and reproducible positioning of parts. Adhesion or even sticking of the surfaces is a major showstopper for these applications.
There are several measures that can be taken in order to reduce spontaneous adhesion. Quantification of the effectiveness of the chosen solution is most often done using an AFM with probes varying from 1 nm to 8 micron of contact diameter. A serious disadvantage in measuring adhesion by sharp tips is the wear of the tips. Sharp tips wear easily, resulting in undefined contact areas. When the real area of contact is not well defined, the quantification of the adhesion force is not significant.
In the current study results of AFM measurements from literature with different tip diameters of colloidal probes are compared with measurements we performed using AFM cantilevers with a plateau tip and using probes from large spheres using an alternative setup (UNAT). These methods give results that are in good agreement with values found in literature. Large contacting surface enhance the quality of the measured adhesion values.
Another part of the study deals with a deliberately roughening of smooth surfaces to minimize (spontaneous) adhesion. Good agreement has been found with existing results.
For the use of larger surfaces it is important that the surfaces to be tested are extremely clean. Particles on smooth surface do influence the measurements quite easily. Especially for larger areas, the possibility of encountering particles on the surface are more likely, when particles are present. For the measurements in this study a lot of care has been taken therefore to remove contamination: particles as well as contamination of organic origin.
The evolving use of large space systems, with accompanying large truss-like structures, creates the need to develop an on-orbit fabrication and assembly capability for producing structural elements and structural subsystems. Such a capability is a basic necessity for extremely large structural subsystems because of the economy of operations resulting from the on-orbit conversion of high-density material stock into low-density structural elements. This paper discusses the development of large space structural subsystems; in particular the role a generic structure plays in that development. This generic structure embodies the key issues of the structural subsystems of various classes of large space systems and as such, when its development is completed a proof-of-capability will have been accomplished for a wide variety of large space structures.
A preliminary study of advanced composites and truss-type designs for application to large space structures is discussed. The benefits of advanced composite materials for large space structures and examples of large space structure design concepts and construction techniques are presented. Criteria are developed to guide the selection of structural elements, joints, and on-orbit fabrication techniques for truss-type structures and a ranking of candidate on-orbit fabrication techniques is accomplished in accordance with the criteria developed. On the basis of study results, recommended research and development goals for design, construction, and on-orbit fabrication of large space structures are established.
A preliminary design concept for a weldable joint for on-orbit assembly of large space structures is described. The joint was designed for ease of assembly, for structural efficiency, and to allow passage of fluid (for active cooling or other purposes) along the member through the joint. The members were assumed to consist of graphite/epoxy tubes to which were bonded 2219-T87 aluminum alloy end fittings for welding on-orbit to nodes of the same alloy. A modified form of gas tungsten arc welding was assumed to be the welding process. The joint was designed for the thermal and structural loading associated with a 37 m diameter tetrahedral truss intended as an aerobrake for a mission to Mars. It was concluded that the assembly process could lock large loads into the truss members and that the assembly robot could be required to exert large forces while aligning pairs of nodes during assembly. It was also concluded that the connections between the composite struts and the aluminum fittings will be subjected to very high service stresses due to the effects of differential thermal expansion.
A theoretical investigation of van der Waals forces acting between two solid silicon surfaces at separations from zero to approximately 20nm is presented. We focused our efforts on the analysis of different factors that can cause deviations from the classical pressure–distance dependence p∼1/D3. It is demonstrated that a layer (oxide or water) at any of the surfaces influences the pressure up to distances, which are an order of magnitude larger than its own thickness. A jump on the p(D) curve is expected at contact of the adsorbed liquid layers. The retardation of van der Waals forces at 5
Adhesion measurements were performed by AFM (Atomic Force Microscopy). It was shown that many parameters need to be controlled in order to provide reproducible and quantitative results. Adhesion forces were shown to depend on combination of materials characteristics and testing geometry as well as experimental protocol (contact time, contact force and contact area). This contact area was modified by means of FIB (Focused Ion Beam) milling and deliberate abrasion. As a result, a drastic change in adhesion could be observed. Still, those are problems connected to adjustment of interacting surfaces.
Bonding is a subject matter, which on the one hand is at least as old as written history, and on the other hand is as modern as ultrahigh-vacuum (UHV) technology. In this paper, we present the main threads of its historical evolution and modern evaluation. Bonding has always been a high-tech technology, which used to be governed by an ‘object in view,’ and nowadays is governed by the ‘state-of-the-art.’ Direct-bonding, i.e. the glueless joining of two solid bodies, is more or less embodied in what we have called ‘contact bonding,’ i.e. a large variety of bonding and annealing techniques. Reasonably weak van der Waals attractions are transferred into strong chemical bonds by annealing. Sir Isaac Newton was the first to see direct-bonding, as testified by his famous central black spot surrounded by ‘Newton rings,’ established between an optical contact of a flat and a convex optical surface. Before World War II, direct-bonding was mainly applied in classical optical instruments (such as interferometers); after World War II it was primarily applied in semiconductor technology, optoelectronics, micromechanics and microelectromechanics. This leads to the need for the thinning of one of the wafers for appropriate applications, such as silicon-on-insulator (SOI). More recently, direct-bonding has been investigated for a large variety of materials, thus leading to significant upgrades in terms of flatness, smoothness and cleanliness. A polishing strategy is one consequence of this, which we will deal with in some detail. During the last decade of the 20th century, great progress was made in UHV-bonding, a technology comparable to lateral solid-phase epitaxial growth (SPEG). Bonding and crystal growing have, therefore, become united disciplines. Wafer thinning now has a new impact, for example, by dedicated ion implantation and low-temperature annealing, called ‘smart-cut.’ A great deal of effort has been exerted to master lattice mismatch in the form of dislocations, i.e. compliant layers. The outlook of these technologies is promising, to say the least, and might one day surpass the physical limits of those of bulk monocrystalline materials such as silicon. All these subject matters are treated step-by-step in this paper. We take a phenomenological approach, sometimes alone or in combination with other disciplines, but not specifically application-directed. The paper covers pragmatic issues and also treats know-how.
As the performance of space based astrophysics observatories is directly limited by the size of the spacecraft and the telescope it carries, current missions are reaching the limit of the launchers’ capabilities. Before considering to develop larger launchers or to implement formation flying missions or in orbit assembly, the possibility of deploying structures once in orbit is an appealing solution. This paper describes the different technologies currently available to develop deployable structures, with an emphasis on those that can allow achieving long focal lengths. The review of these technologies is followed by a comparison of their performance and a list of trade-off parameters to be considered before selecting the most appropriate solution for a given application. Additionally, a preliminary structural analysis was performed on a typical deployable structure, applied to the case of a mission requiring a 20 m focal length extension. The results show that by using several deployable masts, it is possible to build stiff deployed structures with eigen frequencies over 1 Hz. Finally, a discussion on metrology concepts is provided, as knowledge of the relative position between the telescope and the deployed focal plane instruments is critical.
This paper describes a program aimed at the early on-orbit demonstration of a large-space-structure fabrication and assembly capability. Requirements for the demonstration concept have been formulated. The concept selected to meet these requirements is a Large Space Structure Platform consisting of a triangular prism of 31.5 m length. Sensors can be mounted on this platform to perform earth-observation measurements from space. Structural elements of the platform are fabricated using an automated beam builder in the Shuttle Orbiter payload bay. Special fixtures are designed to assemble the structure with the aid of the remote manipulator system and two astroworkers in an EVA mode. Results of the platform preliminary design are presented in terms of a design layout with related structural, thermal, mass-properties, and control-dynamics data. The assembly scenario is described. Estimates of the total construction time and Orbiter support requirements are also presented.