The present invention relates to a method for manufacturing a composite material, and a composite material.
A method of manufacturing a composite material is described in WO 2008/029178. Two or more layers of carbon nanotubes (CNTs) are grown in-situ; and each layer is impregnated with a matrix before growing the next layer.
Another method of manufacturing a composite material is described in WO 2009/019510. In this case the CNTs are grown ex-situ and dipped into a liquid layer of matrix in a previously impregnated layer of CNTs.
A limiting factor for the usefulness of the apparatus and techniques described in the above documents is the natural volume of fraction of reinforcement material, approximately 2 vol %, and the difficulty of creating architectures where reinforcements in adjacent layers overlap each other with a sufficient aspect ratio.
A method of capillary forming CNTs is described in Diverse 3D Microarchitectures Made by Capillary Forming of Carbon Nanotubes; by De Volder, Michael; Tawfick, Sameh H.; Park, Sei Jin; Copic, Davor; Zhao, Zhouzhou; Lu, Wei; Hart, A. John; Advanced Materials; 2010; volume 22; pages 4384-4389.
A first aspect of the invention provides a method of manufacturing a composite material, the method comprising:
In WO 2008/029178 and WO 2009/019510 the proportion of the deposition area occupied by the catalyst regions is relatively low, and hence the overall density of CNTs is low. The method of the first aspect of the invention enables an improved composite material to be manufactured with a higher density of filaments than is possible using the methods described in WO 2008/029178 and WO 2009/019510.
Preferably the proportion of the deposition area occupied by the catalyst regions is greater than a proportion of the deposition area occupied by the gaps between the catalyst regions.
Preferably each catalyst region has a solid shape with no internal voids which are substantially free of catalyst material. However if such internal voids are present then the proportion of the deposition area occupied by the catalyst regions is preferably greater than the proportion of the deposition area occupied by the gaps and the voids summed together.
Typically the matrix material forms a continuous structure which substantially completely fills the space in the composite material which is not occupied by the filaments, thereby fixing the filaments in space relative to each other.
Typically the matrix material is capable of transferring load from each array to adjacent arrays, from each bundle to adjacent bundles and from each filament to adjacent filaments.
Typically the matrix material is formed for a material which is less strong and less stiff than the filaments.
The composite material may be impregnated with matrix material in a single step. However more typically the matrix material is applied as a series of layers, each layer being applied at a different time. In this case the first array is impregnated with a first layer of matrix material in a first impregnation step, and the second array is impregnated with a second layer of matrix material in a second impregnation step.
Typically the matrix material is cured as a series of layers, each layer being cured at a different time. The benefit of such a layer-by-layer curing approach is that each cured matrix layer may have a different cross-sectional shape, size, or pattern, enabling a “net shape” part to be grown by additive fabrication. However, the invention also extends to processes in which all of the matrix material in the composite is cured at the same time. That is, each successive layer of matrix material remains uncured until the part is complete, and the part is then heated to cure the matrix throughout in a single curing step.
The matrix material may cured by exposure to electromagnetic radiation, such as a scanning laser beam or other radiation beam such as an electron beam. This enables the matrix to be cured selectively—that is with a desired shape, size or pattern which may vary layer-to-layer.
The bundles of the second array may be positioned in a separate step e. by dipping the free tips of the second array into a layer of liquid matrix material in the inter-bundle gaps of the first array, as described in WO 2009/019510. This enables the second array (and optionally also the first array) to be grown ex-situ, that is remotely from the liquid matrix material, and optionally on the same substrate as the first array. This allows the filaments to be grown at high temperatures, up to ˜1400° C., which is significantly higher than the temperatures required to cure certain types of liquid matrix material such as liquid epoxy resin. In this case the method typically further comprises: growing the first array on a substrate; transferring the first array onto a build platform with the tips adjacent the build platform and the bases remote from the build platform; and impregnating the inter-filament gaps and inter-bundle gaps of the first array with the layer of liquid matrix material after it has been transferred to the build platform. The composite material may have only two layers, or steps a. and b. (and optionally also step c.) can be repeated to provide a third array of bundles of filaments; impregnating the inter-filament gaps and inter-bundle gaps of the second array with a second layer of liquid matrix material; and dipping the free tips of the third array into the second layer of liquid matrix material in the inter-bundle gaps of the second array. Typically the first layer of liquid matrix material is cured before the second layer of liquid matrix material impregnates the inter-filament gaps and inter-bundle gaps of the second array, for instance by scanning a radiation beam across it.
Alternatively the inter-filament gaps and inter-bundle gaps of the first array may be impregnated with a matrix layer before growth of the second array; and at least part of the second array grown in the inter-bundle gaps of the first array by depositing a catalyst material on the matrix layer in the inter-bundle gaps of the first array and growing the second array of bundles of filaments on the catalyst regions in the inter-bundle gaps of the first array.
Optionally only the first array may be formed by drawing the free tips of the filaments together within each bundle. However more preferably the method further comprises drawing the free tips of the filaments of the second array together within each bundle, so that the inter-filament gaps become smaller at the tip of each bundle than at the base of each bundle where the filaments remain attached to the catalyst region.
The free tips of the filaments may be drawn together in step c. by an electrostatic forming method, or more preferably by a capillary forming method involving impregnating the bunches of filaments with a capillary forming liquid and evaporating the capillary forming liquid.
Preferably the number of catalyst regions in the first array is different to the number of catalyst regions in the second array. This enables a series of reinforcement layers to be grown with different shapes under the control of a three-dimensional computer model of a part so that a “net shape” part is grown by additive fabrication.
A further aspect of the invention provides a composite material comprising:
Typically the bases of the first array of bundles lie in a plane, and the bases of the first array of bundles overlap with the second array of bundles when viewed at a right angle to said plane.
The filaments may comprise single walled CNTs; multi-walled CNTs, carbon nanofibres; CNTs coated with a layer of amorphous carbon, or any other suitable filament material. Typically the filaments have an aspect ratio greater than 100, preferably greater than 1000, and most preferably greater than 106.
Typically the matrix material forms a continuous structure which substantially completely fills the space in the composite material which is not occupied by the filaments, thereby fixing the filaments in space relative to each other.
Typically the matrix material is capable of transferring load from each array to adjacent arrays, from each bundle to adjacent bundles and from each filament to adjacent filaments.
Typically the matrix material is formed for a material which is less strong and less stiff than the filaments.
Possible applications for the composite material include flat lenses, or three-dimensional meta-materials with negative refractive index.
Embodiments of the invention will now be described with reference to the accompanying drawings, in which:
Referring to
The regions 12 have a diameter L and the gaps 13 have a width I. For the regions shown in
After the catalyst has been deposited, and subsequent conditioning of the catalyst by a combination of heat and oxidation and reduction using oxygen and hydrogen gases, a layer of carbon nanotubes (CNTs) is grown on the catalyst regions 12 by a chemical vapour deposition process. Carbonaceous gas is introduced into the CVD-CNT chamber via the gas input 7 and the substrate 4 is heated locally by the electrical heating circuit 6. More specifically, the circuit 6 induces an electrical current in the transfer body 4 which heats it resistively. Growth of CNTs is enhanced by generating a plasma in the chamber using an electrode 20 powered by a power supply 21.
After the CNTs have been grown, they are capillary formed by the process shown in
After the first layer of CNTs has been grown and densified, the doors 9,10 between the chambers are opened to allow the substrate 4 to be decoupled from the resistive heating circuit 6, rotated by 180 degrees, and moved into the ALM chamber.
Referring to
In the next step the substrate 4 is removed. The CNTs remain embedded in the polymer layer 19 due to the surface interactions. The substrate 4 is then returned to the CVD-CNT chamber and the doors 9, 10 are closed.
A laser 11 is then activated and scanned over the surface of the layer of CNTs to selectively cure areas of resin, resulting in a cured base layer 31 (
The process is then repeated to provide a second array of CNTs on the substrate 4 with narrow stalks 32 which are dipped into a layer of liquid matrix material 33 in the inter-bundle gaps of the first array as shown in
The bases 14 of the first array of bundles are spaced apart from each other by inter-base gaps 36 (one of which is labelled in
In the next step the substrate 4 is removed and the CNTs 32 remain embedded in the polymer layer 33 due to the surface interactions. The substrate 4 is then returned to the CVD-CNT chamber and the doors 9, 10 are closed.
The laser 11 is then activated and scanned over the surface of the second layer of CNTs 32 to selectively cure areas of resin, resulting in a cured layer 34 (
The process can then be repeated any number of times. For example
The first and second arrays in
The first and second arrays 40,41 are both densified, and
A heated hopper 58 and a cooled ink-jet printing head 59 are mounted on a transport mechanism (not shown) which can move the hopper 58 and printing head 59 from left to right in
In a first process step, the hopper 58 is filled with a polymer powder such as polyetheretherketone (PEEK). The hopper 58 is moved across the negative plasma source 52, and a dispensing orifice (not shown) in the hopper 58 is opened to deposit a layer 60 of polymer powder. Thus the source 52 also acts as a bed or platform for the additive layer manufacturing process. The orifice is then closed. The inert gas prevents oxidation of the polymer. The laser 55 is turned on and the raster mechanism scans the beam across the layer 60 to consolidate the layer 60. The heating effect of the laser beam causes the polymer layer 60 to melt. A shutter (not shown) in the path of the laser beam is opened and closed selectively to modulate the beam as it is scanned over the layer 60. Thus the layer 60 is consolidated only in the areas required to form a desired shape. More specifically, the shutter is opened and closed in accordance with a computer-aided design (CAD) model which defines a series of slices through the desired three-dimensional shape.
In a second process step, the printing head 59 is moved across the layer 60 to deposit a patterned array of catalyst particles. The printing head 59 sprays an array of colloid drops onto the layer 60, and as the colloid evaporates in the high temperature inert gas environment, metal catalyst particles suspended in the colloid drops are deposited. The catalyst particles may be, for example a metal, preferably transition metals Fe, Ni or Co, or alloys thereof; and the colloid liquid may be, for example alcohol, water, oil, or a mixture thereof. A fluid-based cooling system (not shown) cools the printing head 59 and a reservoir (not shown) containing the printing fluid to prevent the colloid liquid from boiling before it is printed. The printing orifice of the printing head 59 (which emits the spray of droplets) is positioned sufficiently close to the layer 60 to ensure that the colloid liquid does not evaporate deleteriously in flight, before hitting the layer 60.
Optionally the catalyst material may be conditioned as part of the second process step, through a process of spherulisation and/or oxidation and/or reduction, depending on the catalyst type. This conditioning is performed by the combination of heating and supply of an oxidising and/or reducing gas, depending on the catalyst type
In a third process step, carbonaceous feed stock is introduced from the gas supply 56 and the power source 54 is turned on to generate a plasma between the electrodes 52, 53. This causes the in-situ growth of a layer of nanofibres, aligned with the direction of the electromagnetic field between the electrodes 52,53. The growth mechanism is as described by Baker (Baker, R. T. K., Barber, M. A., Harris, P. S., Feates, F. S. & Waire, R. J. J J Catal 26 (1972). It is generally accepted that the carbonaceous gas is dissociated on the surface of the metal catalyst particle and carbon is adsorbed onto the surface where it is then transported by diffusion to the precipitating face forming a carbon filament with the catalyst particle at the tip. Discussion is ongoing with regards to whether this diffusion is through the bulk of the catalyst or along its surface(s) and to whether the diffusion is driven by a carbon concentration or thermal gradient. Thus when the growth process is complete, a “forest” of nanofibres bundles 62 is produced, each nanofibre carrying a catalyst particle at its tip.
The catalyst particles and plasma enable the nanofibre growth to occur at a relatively low temperature, lower than the melting point of the matrix.
Once nanofibres of a suitable length have been grown, the plasma power source 54 and gas supply 56 are turned off, the inert gas is purged, and in a fourth process step the CNT bundles 62 are densified by the capillary forming method described above, reducing the diameter of the CNT bundles as shown in
Next the platform 52 is lowered and the hopper 58 is moved along the layer of nanofibres to deposit a further layer 63 of polymer powder.
In a fifth process step, the laser 55 is turned on and the raster mechanism scans the beam across the layer 63 to form a consolidated layer 63′ shown in
The thickness of the unconsolidated polymer layer 63 is selected so that the layer of CNTs is only partially impregnated with the matrix through a lower part of its thickness, leaving an upper part of the layer of CNTs exposed as shown in
Next a patterned layer of catalyst is deposited on the cured matrix layer 63′ in the gaps 65 between the bundles 62, and a second array of CNT bundles 66 is grown on the catalyst regions as shown in
Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.
Number | Date | Country | Kind |
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1104130.8 | Mar 2011 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2012/050512 | 3/8/2012 | WO | 00 | 11/18/2013 |