Patent Application
20200407565
This disclosure relates to the field of intense pulse light (IPL) sintering. In particular, it relates to the IPL sintering of inks on substrates with low-temperature stability that are coated with boron nitride nanotubes.
Intense pulsed light (IPL) sintering is a photonic sintering technique that provides an effective way of sintering materials. The technique is well-suited for printable electronics since it can sinter material on a timescale of milliseconds and therefore readily integrates with roll-to-roll processes. IPL sinters material by generating heat through the absorption by an ink of broadband, high-intensity pulsed light. Heat can be localized to the ink layer if the pulses of light are short enough that thermal equilibrium is not established between the ink and substrate layers. This unique feature of IPL allows processing of inks that require high sintering temperatures on substrates that have low heat stabilities but are generally favored due to their low cost. The pulsed light however, must be carefully optimized for energy density, duration and frequency in order to generate heat that is sufficient to sinter the ink but low enough that the substrate is not significantly damaged.
Currently, metallic inks can be processed using IPL on temperature-sensitive substrates, such as plastic substrates. However, the underlying substrate is often warped due to the high energies required to process the samples. One solution is to provide a coated substrate. As an example, Novele™ is a commercially-available substrate made of polyethylene terephthalate (PET) coated with a relatively thick layer of silica particles. The silica coating minimizes the amount of heat that reaches the substrate, thereby mitigating the damage incurred by the substrate when sintering using IPL.
One of the disadvantages of Novele™ is that it is opaque. As a result, it is not ideal for many printable electronics applications. In addition, metallic traces processed on Novele™ show very poor mechanical stability (e.g. poor adhesion characteristics).
There have been few attempts to address the issue of damage to underlying substrates during IPL sintering of inks.
US Pat. Application No. 2015/0187983 discloses the use of IPL sintering of Ag or Cu nanowires on a substrate (e.g. PET) to improve conductivity of the nanowires without damaging the substrate. The surface of the nanowire may be coated with a metal selected from Au, Pt, Ti, Ni, Al, W and Co.
U.S. Pat. No. 9,972,742 B2 discloses a production method for a transparent electrically-conductive film that includes a step of coating an electrically-conductive nanowire dispersion containing electrically-conductive nanowires with an optical activity and an organic binder onto a substrate; and a step of irradiating a pulse-type white light onto the electrically-conductive nanowires. The substrate includes polycarbonate and polyethylene terephthalate.
WO 2016/100715 discloses the incorporation of aligned high quality boron nitride nanotubes (BNNTs) into groups and bundles that can be placed in electronic and electrical components (ECs). Such use of high quality BNNTs enhances the heat removal and diminishes heat production and dielectric heating. High quality BNNTs are electrically insulating and excellent conductors of heat at the nanoscale level.
US Patent Application No. 2005/0116336 discloses nano-composite materials for thermal management applications. One type of nano-composite material disclosed has a base material and nanostructures (e.g., boron nitride nanotubes) dispersed in the base material.
Kell A, et al. (ACS Applied Materials & Interfaces. May 3, 2017, DOI: 10.1021/acsami.7b02573) discloses the use of intense pulsed light (IPL) sintering to sinter silver-based molecular inks on plastic substrates (e.g. Kapton™).
A composite comprising an electrical insulator layer comprising unmodified or modified boron nitride nanotubes (BNNTs) supported on a plastic substrate will first be described in its general form, and then its implementation in terms of embodiments will be detailed hereafter. Modified BNNTs include both i) BNNTs that are modified covalently with functional groups (OH or NH2) and ii) BNNTs that are modified by wrapping them in either in a polymer, a ligand or a surfactant. In the following description and in the appended claims, the term “BNNT” is intended to include any unmodified BNNT or modified BNNT material with properties that make it suitable for forming a composite with a plastic substrate (i.e., forming a coating or a film on a surface of a plastic substrate), supporting a metallic ink trace, and reducing heat transfer to the substrate during sintering of the metallic ink trace, relative to heat transfer to the substrate that would otherwise occur absent the BNNT.
Disclosed herein is the use of BNNT coatings on substrates with low-temperature stability (e.g. plastic substrates), to reduce heat transfer to the substrate during intense pulsed light (IPL) sintering of printed metallic molecular ink traces on the substrate. Reducing heat transfer from a printed trace to the substrate reduces damage to the substrate during IPL sintering. This increases the trace quality, which in turn improves the electrical and mechanical properties of the traces.
Furthermore, compared to the commercially-available Novele™ product, BNNTs provide higher transmission of light, and yield printed films with significantly improved mechanical and electrical properties. Finally, the use of BNNT coatings may mitigate the necessity for careful optimization of energy density, duration and frequency of the light used in the IPL sintering process.
In one aspect, there is provided a composite comprising: a plastic substrate; and an electrical insulator layer disposed on the plastic substrate and comprising boron nitride nanotubes (BNNTs).
In another aspect, there is provided a process for printing electronics, the process comprising sintering a metallic ink on the composite.
In another aspect, there is provided the use of a coating on a plastic substrate for reduction of heat transfer to the substrate during sintering, relative to heat transfer to the substrate that would otherwise occur absent the coating, wherein the coating comprises BNNTs.
The BNNTs may be single-walled or multi-walled. Modified BNNTs may comprise pendant hydroxyl (OH) and amino (NH2) functional groups covalently bonded to an outer surface of the BNNT. The ratio of pendant OH groups to NH2 groups may be about 2:1, while the number of pendant OH groups may be between about 1 OH per 6 BN-units and about 1 OH per 240 BN-units, or between about 1 OH per 12 BN-units and about 1 OH per 124 BN-units, or about 1 OH per 18 BN-units. Alternatively, modified BNNTs may be wrapped with a polymer or coated with a ligand or a surfactant. Where a polymer is used, the polymer may comprise a conjugated polymer, such as (but not limited to), a polythiophene comprising at least one alkyl side chain, a polyfluorene comprising at least one alkyl side chain, a poly(fluorene-co-pyridine) comprising at least one alkyl side chain, or a poly(thiophene-co-fluorene) comprising at least one alkyl side chain.
Additional examples include:
wherein Ra is C4-24 alkyl and n is an integer from 10-200;
wherein each Rb is independently C6-24 alkyl and m is an integer from 10-100; or
wherein each Rc is independently C6-24 alkyl and q is an integer from 10-100.
In the above, Ra may be n-hexyl or n-octadecyl, Rb may be n-octyl or n-octadecyl; and RC may be n-octadecyl.
Furthermore, the substrate can comprise polycarbonate (PC), polyethylene terephthalate (PET), polyethylene terephthalate glycol (PETG), polyethylene naphthalate (PEN), polyimide, or a polyester. Plastic materials with low glass transition temperatures, e.g., below about 200° C., such as for instance PETG, PET, PEN and PC, form particularly useful composites when coated with BNNT.
The metallic ink can comprise silver, copper or nickel. As to types of inks, the metallic ink can be a molecular ink, a particulate ink or an ink comprising flakes, or any mixture thereof. Where the metallic ink is a particulate ink, the particulate ink can comprise nanoparticles comprising a metal. In addition, the metallic ink may be a mixture of: a particulate ink of nanoparticles comprising a metal and a molecular ink. With regards to the nanoparticles comprising a metal, the metal may be silver, copper or nickel. In some cases, the metallic ink can comprise flakes of silver, or may be a mixture of an ink comprising flakes of silver and a molecular ink.
A molecular ink is a flake-less ink that does not make use of metallic flakes to provide conductivity. Instead, the molecular ink typically comprises a metal precursor molecule, for example, a metal carboxylate such as a silver carboxylate or bis(2-ethyl-1-hexylamine) copper (II) formate, bis(octylamine) copper (II) formate or tris(octylamine) copper (II) formate. Because the inks do not contain flakes, conductive traces formed from the inks comprise interconnected metal nanoparticles, which permit the formation of very thin and narrow conductive traces. Some molecular inks are described, for example, in WO 2015/192248, WO 2015/192248, WO 2016/197234, WO 2018/018136, WO 2018/146616, WO 2018/146617, WO 2018/146619, etc.
With respect to the process, the metallic ink may be sintered by photonic or thermal sintering. In some embodiments intense pulsed light sintering (IPL) is used. The metallic ink may be applied to the BNNT surface of the composite by screen, inkjet, microplotting, flexure-gravure, offset printing or stencil printing.
In another aspect there is provided a process, comprising: providing a plastic substrate; forming on at least a portion of a surface of the plastic substrate a layer comprising boron nitride nanotubes (BNNTs); forming a metallic ink trace on a portion of the layer, such that the metallic ink trace is spaced-apart from the substrate; and sintering the metallic ink trace using photonic or thermal sintering techniques.
Wherever ranges of values are referenced within this specification, sub-ranges therein are intended to be included, unless otherwise indicated. Where characteristics are attributed to one or another variant of the composite, the process, or the use described above, unless otherwise indicated, such characteristics are intended to apply to all other variants where such characteristics are appropriate or compatible with such other variants.
Further features will be described or will become apparent in the course of the following detailed description.
For clearer understanding, preferred embodiments will now be described in detail by way of example, with reference to the accompanying drawings, in which:
Samples of PET (Melinex™) substrates coated with a film of polymer-wrapped modified BNNTs were fabricated using a combination of electrophoretic deposition and dry transfer, as described below with reference to
A clear solution (see
A set of silver traces with nominal widths varying from 1 to 20 mil were prepared by printing a silver ink on the untreated and coated Melinex™ substrates by screen printing using a screen with a mesh count of 360 counts/inch and emulsion thickness of 7 μm. The silver ink contained 20 g of silver neodecanoate, 3.9 g of 2-ethyl-2-oxazoline and 0.25 g of Rokrapol™ 7075 resin. The samples were dried at 150° C. for 30 minutes prior to sintering by IPL (Novacentrix™ Pulse Forge 1300). The pulse length of the pulsed light was 1500 μs and the voltage ranged from 260V to 340V producing pulses with energies ranging from 1.06 J/cm2 to 1.43 J/cm2. Resistances across silver lines were measured using a voltmeter and their sheet resistance values were calculated based on the lengths and widths of the respective lines, as obtained from optical profilometry.
In
In
Scanning electron microscopy (SEM) images were obtained for silver traces with nominal widths of 2 mils and sintered with an energy of 1.18 J/cm2.
Since lower sintering energies can be used to prepare conductive silver traces on the coated samples, there is less warpage of the substrate in the coated samples, and less cracking of the silver film compared to the untreated samples. This can be seen in the SEM image of the BNNT-coated substrate (
In
As shown in
In summary,
A Melinex™ substrate coated with polymer-wrapped BNNTs was prepared as follows: a 0.1 mg/mL solution of regiorandom poly(3-hexylthiophene-2,5-diyl) (rra-P3HT) (PEHT-87K) wrapped BNNT in chloroform solvent was spray coated on Melinex™ using an Iwata Air Brush Studio Series Smart Jet air compressor, generating a film of 36mg/m2 of polymer-wrapped BNNTs on Melinex™.
An ink consisting of 15 g of silver neodecanoate, 3.2 g of 2-ethyl-2-oxazoline, 0.8 g of 1-octanol and 0.185 g of Rokrapol 7075™ was printed using a screen with a mesh of 360 counts/inch and 7 μm emulsion on polymer-wrapped BNNT-coated Melinex™ and on untreated Melinex™ (uncoated). The printed samples were dried in a convection oven for 15 minutes at 160° C. The samples were sintered using pulse light of 1500 μs and voltages ranging from 290V to 320V producing pulses with energies ranging from 1.30 J/cm2 to 1.73 J/cm2. Resistances across silver lines were measured using a voltmeter and their sheet resistance calculated based on the lengths and widths of the lines obtained from optical profilometry.
As shown in
The current carrying capacity of silver lines printed on BNNT-coated PET and untreated PET are shown in Table 1. For traces with nominal widths of 3 and 20 mil, IPL sintering energies are 1.43 and 1.73 J/cm2, respectively.
Table 2 shows the average change in resistance of silver lines after undergoing the compressive test ASTM F1683-02. The bend and crease test shows that the silver traces have very poor mechanical properties on Novele™, improved mechanical properties when printed on PET, and excellent mechanical properties when printed on BNNT-coated PET:
Using slightly different conditions than were described above for Example 2a, Melinex™ (ST505) substrates were coated with polymer-wrapped BNNTs as follows: a 0.2 mg/mL solution of regiorandom poly(3-hexylthiophene-2,5-diyl) (rra-P3HT) (PEHT-87K) wrapped BNNT in chloroform solvent was spray coated on Melinex™ using an Iwata Air Brush Studio Series Smart Jet air compressor.
The light transmission of the BNNT films on Melinex™ were measured using a Cary 5000 spectrometer, with an uncoated Melinex sample acting as a background for the measurement.
An ink containing 25 g of silver neodecanoate, 3.28 g of 2-ethyl-2-oxazoline, 1.97 g of 1-octanol and 0.30 g of Rokrapol 7075™ was printed using a screen with a mesh of 360 counts/inch and 7 μm emulsion onto polymer-wrapped BNNT coated Melinex™ substrates and onto untreated Melinex™ substrates (uncoated). The printed samples were dried in a convection oven for 15 minutes at 155° C., sintered using pulse light of 3000 μs and voltages ranging from 270V to 330V producing pulses with energies ranging from 1.96 J/cm2 to 3.41 J/cm2.
A CT-100 optical profilometer from Cyber Technologies was used to measure the width and roughness of the resulting sintered traces. Table 3 shows the roughness of the silver traces with a nominal width of 20 mil prepared on an uncoated Melinex™ substrate and on a polymer-wrapped BNNT coated substrate. Using three different measures of roughness (Rq, Rt, Rz), the roughness of the traces decreases with increasing concentration of surface concentration of BNNT. More particularly: Rq is defined as the root mean square average of the profile height deviations from the mean line, recorded within the evaluation length; Rt is defined as the difference between the height of the highest peak and the depth of the deepest valley within the evaluation length; Rz is defined as the sum of the height of the highest profile peak and the depth of the deepest profile valley, relative to the mean line, within a sampling length.
SEM images of the silver lines were acquired. The bright areas in the images are the silver traces. As shown in
The current carrying capacities shown in
PETG substrates were coated with polymer-wrapped BNNTs as follows: a 0.2 mg/mL solution of regiorandom poly(3-hexylthiophene-2,5-diyl) (rra-P3HT) (PEHT-87K) wrapped BNNT in chloroform solvent was spray coated on PETG using a Iwata Air Brush Studio Series Smart Jet air compressor, generating a films 142 mg/m2 of BNNTs on PETG.
An ink was prepared by mixing, in a weight to weight ratio, 0.6385 of silver oxalate, 1.000 of an amine mixture, 0.13067 di(propylene glycol) methyl ether, 0.01596 of lactic acid, 0.035250 of 0.4% solution of antifoam 204 and 0.05612 of BYK-R605™. The amine mixture was a mixture of, weight to weight, 2.67 amino-2-propano1, 1.000 of 2-amino-1-butanol, and, 0.0494 hydroxyethylcellulose (90 000 g/mol). The ink was printed using a screen with a mesh of 360 counts/inch and 7 μm emulsion on uncoated PETG substrates and on PETG substrates coated with polymer-wrapped BNNT. The samples were thermoformed into a half cylinder shape with a height of 1.5 cm using a FormTech450DT thermoformer with heating at 50% power for 53 seconds. The printed sample was positioned such that the printed line pattern covered the half cylinder. Following thermoforming, the samples were sintered using pulse light of with a voltage of 250V 3000 μs generating 2.72 J/cm2 of energy. Optical microscope images of the traces were acquired as shown in
The resistances of printed, thermoformed and photosintered silver traces 10 cm in length and with varying nominal widths were measured. The silver traces were measured using an ohm meter. Table 4 compares the resistance of traces with 5, 10, 15 and 20 mil in nominal widths printed on uncoated PETG with those on polymer-wrapped BNNT coated PETG. The results show that the traces with narrow line widths (5, 10, 15 mil) on polymer-wrapped BNNT coated PETG substrates are conductive while those on uncoated PETG are non-conductive (NC) on average. The polymer-wrapped BNNT coating allows conductive silver traces with narrow widths to form after printing, thermoforming and photosintering.
The silver traces were printed on uncoated PET and BNNT-coated PET with varying concentrations of BNNT, printed and photosintered with different sintering light energy.
It should be noted that while BNNTs wrapped in regiorandom poly(3-hexylthiophene-2,5-diyl) (rra-P3HT) were used in a number of the examples, the following conjugated polymers presented in Table 5 may also be used to wrap BNNTs.
Specific examples have been provided in order to facilitate a better understanding of the claimed invention, and in particular the examples relate to polymer-wrapped (modified) BNNT coated onto polyethylene terephthalate or polyethylene terephthalate glycol substrates. As described with reference to Table 5 a variety of other polymers may be used to produce polymer-wrapped (modified) BNNT. In addition, the substrates are not limited to the polyethylene terephthalate or polyethylene terephthalate glycol substrates that were used in the examples but may also be selected from at least the following: polycarbonate, polyethylene naphthalate, polyimide, or a polyester. Further, in addition to polymer-wrapped (modified) BNNTs the invention may also be practiced using unmodified BNNTs or BNNTs that are modified covalently with functional groups (OH or NH2).
The novel features will become apparent to those of skill in the art upon examination of the description. It should be understood, however, that the scope of the claims should not be limited by the embodiments but should be given the broadest interpretation consistent with the wording of the claims and the specification as a whole.
This application claims the benefit of U.S. Provisional Application No. 62/578,003 filed Oct. 27, 2017 and it also claims the benefit of U.S. Provisional Application No. 62/650,091 filed Mar. 29, 2018. The entire contents of U.S. Provisional Applications No. 62/578,003 and 62/650,091 are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2018/051357 | 10/26/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62578003 | Oct 2017 | US | |
| 62650091 | Mar 2018 | US |
