WO2019121827A1

WO2019121827A1 - Use of filtration media for purification of nanowires and process for the purification of nanowires

Info

Publication number: WO2019121827A1
Application number: PCT/EP2018/085709
Authority: WO
Inventors: Philippe Carvin; Ana HIPOLITO
Original assignee: Rhodia Operations
Priority date: 2017-12-19
Filing date: 2018-12-18
Publication date: 2019-06-27

Abstract

A process for the separation of nanowires with an aspect ratio of at least 10 from nanoparticles with an aspect ratio of less than 10.

Description

Use of filtration media for purification of nanowires and process for the purification of nanowires

[0001] Cross reference to a related application: the present application claims priority to European application No. 17306833.9 -filed on 19 Dec 2017-, the whole content of this application being incorporated herein by reference for all purposes.

[0002] The present invention relates to filter media for the purification of

nanowires and a process for the purification of nanowires.

[0003] Transparent conductive electrodes are commonly used in organic

electronic devices such as organic light emitting diodes, displays and photovoltaic cells.

[0004] At present mainly indium tin oxide (ITO) is used for the manufacture of such electrodes, but ITO has some drawbacks like its brittleness and the need of high processing temperatures to achieve the desired properties. The use of ITO in new and flexible organic electronic devices is thus difficult.

[0005] In the recent past nanoparticulate products with a high aspect ratio such as nanowires and nanotubes have been investigated as a replacement material for ITO. Such high aspect ratio materials have the benefit compared to spherical or close to spherical particles (i.e. particles with low aspect ratio) that the amount needed to achieve a percolation network (which is necessary to achieve sufficient conductivity) is significantly lower. This is an economic as well as a technical advantage as the transparency of an electrode deteriorates with increasing content of nanoparticulate material. Thus, because of the need of higher amounts of low-aspect nanoparticles the transparency of respective electrodes made therefrom is worse than of respective products obtained from nanowires and

nanotubes.

[0006] Among the nanowires and nanotubes silver nanowires offer a good

potential as silver is the metal with the highest metal conductivity.

[0007] However, in many synthesis processes for nanowires (which hereinafter are generally to be understood as nanoparticulate products with aspect ratios of at least 10, cf. later), nanoparticles with aspect ratios of less than 10 are produced as by-products in significant amounts. These

nanoparticles have to be removed to restore transparency and they don’t provide a beneficial effect in terms of conductivity (they do not improve the electrical percolation).

[0008] Centrifugation has been described e.g. in Y. Sun et al., Nano Letters 2(2), 2002, 165-168 or Balberg et al., Phys. Rev. Lett. 52(17), 1984, 1465 et seq. as a purification method for nanowires. However, centrifugation has a number of disadvantages as it usually promotes the formation of entangled agglomerates which are difficult to redisperse. Furthermore, centrifugation is expensive and time consuming.

[0009] Cross-flow filtration where a cross-flow filter utilizes the nanowire

alignment with flow direction to separate nanowires from nanoparticles has been described by Pradel et al. Angew.Chem. Int. Ed. 50(15), 2011 , 3412- 3416. However, cross-flow filtration is an expensive method and is very sensitive to variations in process conditions.

[0010] Jarret and Crook, Materials Research Innovations 20 (2016), Issue 2, 86- 91 describe the purification of silver nanowires by size and shape using multi-pass filtration.

[0011] Filtration as a purification method for nanowires to remove nanoparticles as previously described was heretofore suitable for lab-scale operations only and clogging of the filtration medium is a common problem which significantly impedes the use of filtration in this regard. On the other hand, classical dead-end filtration would be an economic way for purification if the clogging and other problems could be overcome.

[0012] There thus exists a need for suitable processes for the purification of

nanowires.

[0013] It was thus an object of the present invention to provide a process for the purification of nanowires by dead-end (u ltra)fi Itration .

[0014] This object is achieved with the process in accordance with claim 1.

[0015] Preferred embodiments are set forth in the dependent claims and in the detailed specification hereinafter.

[0016] The process in accordance with the present invention comprises the following steps:

a) providing a filter medium selected from non-woven stainless steel fibres and multifilament polyolefin fabrics,

b) providing a slurry comprising nanowires with an aspect ratio of at least 10 and nanoparticles with an aspect ratio below 10,

c) filtering the slurry at a ratio of volume of liquid to filtration surface of from 10 to 150 L/m² by dead-end filtration with a pressure drop of 1000 to

15000 kPa until a filter cake with a height in the range of from 0.5 to 5 mm is obtained,

d) re-slurrying the filter cake in water,

e) filtering the slurry in accordance with step d) and

f) recovering the filter cake and re-slurrying same in water to obtain a purified nanowire dispersion.

[0017] In step a) of the process in accordance with the present invention a filter medium selected from non-woven stainlesssteel fibres and multifilament pololyefin fabrics is provided.

[0018] The first type is a non-woven stainless steel fiber material. These filter media are multi-layered structures of non-woven sintered fibers of stainless steel, the fibers being in some cases more densely packed in the direction of flow. This results in progressively decreasing pore sizes which are the interstitial spaces between the fibers.

[0019] The preferred filter material is sintered non-woven stainless steel fibers.

The particular stainless steel is not critical to the Invention, and a variety of different stainless steel alloys can be used. Austenitic stainless steels, i.e. , those whose chief alloying elements are chromium and nickel, are preferred. A particularly preferred stain-less steel is 316L stainless steel, whose composition is approximately 0.03 % carbon, 2.00% manganese, 1.00% silicon, 16.0-18.0% chromium, 10.0-14.0% nickel, 0.45%

phosphorus, 0.03% sulfur, and 2.0-3.0% molybdenum (all percentages by weight). Examples of other useful stainless steels are 304, 304H, 304L, 304 LN, 316, 316F, 316H, 316LN, 316N, 317, 317L, 321 , 321 H, 347,

347H, 348, 348H, and 384.

[0020] A currently preferred filter medium that meets the parameters of this invention is BEKIPOR® ST filter medium, and in particular BEKIPOR® ST 3AL3, a product of NV Bekaert SA of Belgium, available through Bekaert Fibre Technologies Europe, Zwevegem, Belgium, and Bekaert

Corporation, Atlanta, Georgia, USA. This medium is made of 316L stainless steel fibers, randomly compressed in a non-woven structure and sintered, and is supplied in sheets, with typical lateral dimensions of 1180 mm X 1500 mm and 0.35 mm in thick. This particular product has an absolute filter rating of 3 microns, a bubble point pressure of 12,300 Pa (ASTM E 128061 , equivalent ISO 4003), an average air permeability of 9 L/dm²/min at 200 Pa (NF A 95-352, equivalent IOS 4022), a permeability factor k of 4.80 X 10^-13, a weight of 975 g/m², a porosity of 65%, and a dirt holding capacity of 6.40 mg/cm² according to Multipass method ISO 4572 with 8" initial differential pressure. Other media of similar characteristics and made of similar materials can also be advantageously used.

[0021] A second group of preferred filter media are based on multifilament fabrics preferably based on polyolefins or polyesters, in particular based on polypropylene fibers.

[0022] For some applications it has been found that multifilament yarns based on staple fibers with a weight in the range of from 300 to 1000 g/m², preferably of from 450 to 750 g/m² and a liquid permeability of from 50 to 200, preferably of from 70 to 150 L/m²/min at 20 kPa can be preferably used.

[0023] The tensile strength in warp direction normally exceeds the tensile

strength in weft direction, typical values being in the range of from 1000 to 2000 N/cm for the warp direction and 500 to less than 1000 N/cm in the weft direction.

[0024] The use of mixed yarns (i.e. different types of yarn for warp and weft) can provide benefits such as facilitating of cake discharge, improving fabric strength, durability and filtration efficiency.

[0025] Surface coatings applied to filter fabrics can enhance one or more of the filtration properties of the fabric. Microporous polymer coatings may be used to provide a smoother and finer aperture size to the fabric surface, which may enebale easier detachment of the cake and prolong the lifetime of the medium.

[0026] The skilled person knows such materials and will, depending on the

specific case, select the suitable material.

[0027] Just by way of one example the Azurtex® materials may be mentioned here, which are commercially available from Clear Edge company. Azurtex is a microporous polyurethane impregnation of a woven polyester or polypropylene substrate for use in fine particle filtration in solid/liquid separation. The coating lies predominantly on the surface of the fabric, which is thereby smoothed. Only limited polyurethane enters the space between the fibres. The product can be described as a microporous polymer membrane integrated into a woven polypropylene structure.

[0028] Azurtex^® materials usually have nominal area weights in the range of from 620 to 750 g/m2 and average pore sizes of appr. 6 pm. The liquid permeability is in most cases around 110 L/m²/min (measured at 20 kPa Hg pressure).

[0029] One specific example of a suitable filter material is Azurtex® 28730A.

[0030] In step b) of the process of the present invention a slurry comprising

nanowires with an aspect ratio of at least 10 and nanoparticles with an aspect ratio of less than 10 is provided.

[0031] The term“nanowires”, as used herein, is intended to denote elongated nanoparticulate materials having an aspect ratio of at least 10, preferably in the range of from 10 to 10000, more preferably in the range of from 10 to 1000 and even more preferably in the range of from 25 to 500.

[0032] In some cases it has shown to be advantagoeus if the aspect ratio of the nanowires differs form the aspect ratio of the nanoparticles by at least a factor of 2, preferably by at least a factor of 5 and particularly preferred by at least a factor of10.

[0033] The term“aspect ratio” of a geometric shape, as used herein, is the ratio of its sizes in different dimensions, i.e. the ratio of the largest diameter of a particle to the smallest diameter orthogonal to the largest diameter (or, generally speaking the ratio of length to width). Thus, the aspect ratio is a shape factor numerically describing the shape of a particle independent of its absolute size. The aspect ratio of an ideal sphere is 1 (the size in all dimensions is equal). Fibres and wires have high aspect ratii, i.e. their size in one dimension exceeds the size in other dimensions significantly. An aspect ratio of at least 10 means a size of the particle in one axis being at least 10 times the size in another axis.

[0034] The aspect ratio in accordance with the present invention is preferably determined by photographic imaging with a scanning electron microscope. The major axis length and the minor axis length of the particles can thus be determined and through a suitable algorithm the mean values are available, thus providing the value for the aspect ratio.

[0035] In accordance with a preferred embodiment of the present invention, the nanowires to be purified have an average diameter of from 5 to 50 nm, preferably of from 10 to 40 nm and particularly preferred of from 15 to 30 nm. The average length is in the range of from 1 to 100 pm, preferably of from 2 to 75 pm and particularly preferred of from 5 to 60 pm.

[0036] In step c) of the process in accordance with the prsent invention, the slurry provided in step b) is filteredd by dead-end filtration.

[0037] The term“dead-end filtration”, as used herein, is intended to denote a

filtration method where the feed moves toward the filter medium wherein all the particles that can be filtered settle on the filter surface. Since the filtration is not sustainable forever without removing accumulated solids, backwashing is performed periodically and/or filter medium is replaced. In contrast, in so called crossflow filtration, feed moves parallel to the filter medium to generate shear stress to scour the surface.

[0038] Dead end-filtration, compared to cross-flow filtration is cheaper in the

equipment and it uses less liquid volume which is advantageous.

[0039] In the course of the present invention the ratio volume of liquid / filtration surface is within the range of form 10 to 150 L/m², i.e. there are10 to 150 L of slurry filtered per square meter of filtration suzrface. A range of from 30 to 100 L/m² is beneficial for a good separation efficiency, a range of from 30 to 75 L/m² being particularly preferred.

[0040] The pressure drop in filtration step c) (dead end filtration) is in the range of from 1000 to 15000 kPa, preferably in the range of form 2000 to 12000 kPa and particularely preferable in the range of form 2000 to 8000 kPa. [0041] It is preferable to carry out the filtration with a pressure drop as low as possible to avoid the risk of structural damage to the nanowires during filtration (which will increase the losses in the course of the filtration). On the other hand a certain minimum pressure drop helps to achieve satisfactory process times.

[0042] The skiled person will adjust the best suitable pressure drop in accordance with the special application case..

[0043] The filtration is carried out util a height of the filter cake of from 0.5 to 5 mm, preferably in the range of from 0,75 to 4 mm and perticularly preferred in the range of from 1 to 3 mm with heights of the filter cake in the range from 1 to 2 mm being especially advantageous in certain cases.

[0044] The flow resistance of the filter cake with the enriched nanowires

increases significantly with increasing thickness of the filter cake and it is preferable to avoid filter cake thicknesses exceeding a certain height. This means that the time limit of a filtration step is basically determined by the thickness of the filter cake. Once the said thickness has reached the value as described above, the filter cake is resuspended and, if necessary, subjected to one or more additional filtration steps c).

[0045] The reslurrying once a thickness in the given range is obtained on one hand is beneficial for maintaining the structural integrity of the nanowires and it also reduces the extentt of entanglement and agglomeration of the nanowires. Entanglement and agglomeration are undesired effects.

[0046] Summarizing the foregoing, the most preferred process in accordance with the present invention works with different and subsequent filtration steps where the filtration cake is reslurried once the filter cake has reached a thickness of 1 mm or slightly less or more.

[0047] In accordance with a preferred embodiment of the present invention, steps d) (re-slurying) and e) (filtration) are repeated 2 to 5 times to achieve a certain degree of purification.

[0048] After each filtration step, the degree of purification achieved can be

monitored by either UV absorption spectroscopy or by SEM as described hereinbefore. [0049] In some cases it has been found advantageous if the filter medium used has a positive zeta potential.

[0050] Zeta potential is the electrical potential that exists at the shear plane of a particle, which is some small distance from the surface. Colloidal particles dispersed in a solution are electrically charged due to their ionic characteristics and dipolar attributes. The development of a net charge at the particle surface affects the distribution of ions in the neighboring interfacial region, resulting in an increased concentration of counter ions (ions of charge opposite to that of the particles) close to the surface.

[0051] When a voltage is applied to a solution in which particles are dispersed, particles are attracted to the electrode of the opposite polarity,

accompanied by the fixed layer and part of the diffuse double layer. The potential at the boundary between this unit, that is to say at the above- mentioned shear plane between the particle with its ion atmosphere and the surrounding medium, is known as the zeta potential.

[0052] Zeta potential is a function of the surface charge of a particle, any

adsorbed layer at the interface and the nature and composition of the surrounding medium in which the particle is suspended.

[0053] Most materials when immersed in water exhibit a zeta potential. The

majority of demineralized water contaminants, including most colloids, particles, bacteria, and pyrogens (bacterial fragments), are negatively charged. Filter media can be chemically modified to give them a positive zeta potential.

[0054] Methods for modifying the zeta potential of surfaces are known per se to the skilled person and the skilled person will, depending on the actual specific circumstances choose a suitable modifying agent to achieve the desired zeta potential.

[0055] In accordance with a preferred embodiment of the present invention, the surface of the filter medium prior to use is modified by treating the filter medium with an aqueous solution or dispersion of a heavy metal oxide.

[0056] The heavy metal oxide is preferably an oxide of a metal having an atomic number in the periodic system of at least 24, and zirconium oxide may be mentioned here as particularly preferred. The skilled person will select the appropriate metal oxide based on the individual application case using his professional experience. The treatment can be simply achieved by filtering a solution or dispersion of the heavy metal oxide through the filter medium in a step preceding the actual purification filtration.

[0057] As a result of the treatment with a heavy metal oxide, the filter medium will usually have a positive zeta potential.

[0058] In a further embodiment of the process in accordance with the present invention the water used for reslurrying in step e) comprises

polyvinylpyrrolidone and/or a heavy metal oxide. Preferred heavy metal oxides are the ones mentioned hereinabove for the treatment of the surface of the filter medium with a heavy metal oxide. To avoid

unnecessary repetitions, reference is made to the foregoing description in this regard.

[0059] Nanowire dispersions, in particular silver nanowire dispersions obtained by the polyol method usually contain polyvinylpyrrolidone to stabilize the nanowires. PVP is also beneficial in promoting the unilateral growth of nanoparticles to obtain nanowires and is therefore already for this reason usually present in nanowire dispersions used in the process in accordance with the present invention.

[0060] In accordance with another preferred embodiment of the present

invention, the filter medium used has a mean flow pore size in the range of from 0.5 to 10 pm, preferably of from 1 to 8 pm, even more preferably of from 2 to 7 pm.

[0061] The mean flow pore size is the pore diameter at which the flow through a wetted medium is 50% of the flow through the dry medium at the same pressure drop.lt is not necessarily the mean pore size because the flow through large diameter pores can be dispoportionately larger than the flow through smaller diameter pores.

[0062] The measurement of the parameter is as follows: The dry filter medium is placed in a suitable holder and then air pressure is applied in increasing increments. The flow is measured at each pressure and the flow-pressure relationship is plotted. The filter medium is then completely wetted with a liquid of known surface tension and density or specific gravity. The flow- pressure plot is repeated with the liquid medium. The method is described in ASTM Standard ASTM F316-03 (2011).

[0063] In accordance with another preferred embodiment of the present

invention, the filter medium has a bubble point pressure in the range of from 3000 to 30 000 Pa, preferably of from 5000 to 20 000 Pa.

[0064] The bubble point method is the most widely used method for pore size determination. It is based on the fact that, for a given fluid and pore size with a constant wetting, the pressure required to force an air bubble through the pore is inverse proportional to the size of the hole. The procedure for determining the bubble point pressure is described in ASTM E 128061 (equivalent to ISO 4003). The top of the filter is placed in contact with the liquid, the bottom with air, the filter holder is connected to a source of a regulated pressure. The air pressure is gradually increased and the formation of bubbles on the liquid side is noted. At pressures below the bubble point, gas passes across the filter only by diffusion, but when the pressure is high enough to dislodge liquid from the pores, bulk flow begins and bubbles will be seen.

[0065] One of the great advantages of the bubble point test is that it can be

performed with filters under actual use conditions and with any filter. It is a non-destructive test, thus it does not contaminate the filter and so can be used to determine the integrity of a filter at any time, as well as

establishing the absolute rating.

[0066] The wetted filter is placed in the appropriate housing and the outlet fitting from the compressed air pressure regulator is connected to the upstream side of the test filter. The outlet fitting from the compressed air pressure regulator is connected to the upstream side of the test filter and a piece of flexible tubing is connected from the downstream port of the test filter into a beaker filled with water. Starting from zero pressure, the pressure is gradually increased to the test filter using the pressure regulator. The submerged end of the tubing is examined for the production of bubbles as the upstream pressure is slowly increased. The bubble point of the test filter is reached when bubbles are produced from the tube at a steady rate.

[0067] While the process in accordance with the present invention may be applied to any nanowire materials, it is preferably used for metal nanowire purification, in particular for silver nanowire purification as silver nanowires are in terms of conductivity and availability amongst the most interesting metal nanowires.

[0068] The following information on the preparation of suitable silver nanowire manufacturing methods to obtain the starting materials for the use in accordance with the present invention are given for silver nanowires but it is apparent for the skilled person that the process may be applied to other nanowire compositions as well.

[0069] The nanowire dispersions suitable for the use in accordance with the

invention can be obtained by a variety of different processes which are known to the skilled person and which have been described in the prior art. The process of manufacture of the nanowire dispersion or suspension is not critical.

[0070] The suspensions or dispersions of nanowires obtained after the synthesis have usually a solids content in the range of from 0.05 to 10 % by weight, preferably of from 0.05 to 5 % by weight, based on the entire weight of solids and solvent. Preferred metal concentrations in the slurry used for purification are in the range of from 0.001 to 1 mol %, preferably in the range of from 0.01 to 0.5 mol%.

[0071] The viscosity of the nanowire suspension or dispersion is usually in the range of from 0.5 to 10 mPas, preferably in the range from 1 to 5 mPas and particularpy preferred about 1.5 to 3 mPas. Visciositiy is determined with a capillary viscosimeter (Ubellohde viscosimeter) at room

temperature.

[0072] The weight ratio of nanowires with an aspect ratio of at least 10 to

nanoparticles with an aspect ratio of less than 10 is not subject to particular limitations and is generally in the range of from 1 :1 to 10:1 , preferably in the range of from 1.5:1 to 5:1. After the filtration with the filter medium in accordance with the present invention, the said ratio is higher than before filtration since nanoparticles with a low aspect ratio are eluted in the course of the filtration with the solvent and pass the filter medium thereby being enriched in the filtrate. This effect can be easily followed by scanning electron microscopy (SEM) where the relative proportions of nanoparticles with low aspect ratio can be distinguished from elengated nanowires. SEM is thus a suitable method to monitor the degree of purification achieved with the process in accordance with the present invention and also helps in the decision how many filtrations followed by reslurrying would be necessary to reduce the nanoparticle content to the desired level.

[0073] Another possibility to monitor the degree of the purification is UV

absorption spectroscopy. Nanowires normally have an absorption maximum at a lower wavelength than nanoparticles with a low aspect ratio. E.g. for silver nanowires the absorption maximum is below 400 nm (around 370 nm) whereas silver nanoparticles with low aspect ratii have an absorption maximum at appr. 420 nm. Thus, the UV absorption spectra are an indicator of the content of nanoparticles with low aspect ratii. When comparing the UV absoprtion spectra of purified cake slurry with the filtrate the two different maxima can be readily seen.

[0074] The duration of the filtration in step c) or in any subsequent filtration step e) is not subject to particular limitations, but extended filtration times increase the risk of nanowire breakage and subsequent increased loss of nanowires. If the nanowires during filtration are broken to shorter length, this for obvious reasons increases the risk that the shortened nanowires will pass through the filter. It is therefore preferred if the filtration time per step does not exceed 12, more preferably does not exceed 6 h.

[0075] In some cases it has been found advantageous if the diesperion or

suspension to be filtrated is subjected to stirring as this may incerease the productivity. To avoid mechaniucal destruction of the nanowires, the mixer tip speed when stirring is used should not exceed 0.5 m/s, and should preferably be in the range form 0.2 to 0.4 m/s.

[0076] The nanowire dispersions used as starting material are not subject to

particular limitations. It has been found, however, that dilution of the dispersion before filtration is beneficial if the concentration of nanowires in the starting solution exceeds a certain limit.

[0077] Typically, the slurry used for filtration is characterized by the measurement of certain parameters by UV analysis, SEM analysis and ICP analysis (to determine initial concentration).

[0078] Hereinafter a preferred process for the purification of silver naowires in accordance with the present invention is described; the skilled person will know how to modify the process parameters to adopt same to other nanowire dispersions so that further details need not to be given here.

[0079] Process embodiment for purification of silver nanowires

[0080] A silver nanowire dispersion used as starting material is first characterized through its pH-value, its UV absorbance spectrum, SEM analysis and ICP analysis. The data obtained are then used as basis to determine the efficiency of the process and the degree of purification after a filtration cycle.

[0081] Before starting a complete filtration cycle, a decantation step is preferably performed to separate big AgBr particles (which are present in most silver nanowire dispersions from the synthesis in accordance with the polyol process). The container with the slurry can e.g. decant during one night or appr. 6 to 12 h.

[0082] If the slurry has been stored for a longer period, the container with the slurry should be shaken in order to avoid any fine particles segregation.

[0083] The slurry, eventually after decantation, is then, if necessary, diluted with a certain volume of water, preferably deionized water. The final slurry used as starting material should have an UV absorption in the range of 400 to 450 nm of less than 2.The water used for dilution, in accordance with a preferred embodiment, may contain a certain amount of poly vinyl pyrrolidone (PVP) and/or a certain amount of a heavy metal oxide to stabilize the dispersion and to achieve a positive zeta surface potential of the filter medium (prior to the first filtration step with the nanowire dispersion, the filter medium is conditioned, preferably with a heavy metal oxide).

[0084] During the filtration, the evolution of filtrate volume with time is monitored and provides an indication on the flow resistance of the filter cake over time. Once the filtration volume per time is below a certain threshold, the filtration should be stopped, the filter cake reslurried with water (preferably deionized water, eventually containing additives as mentioned above) and subjected to another filtration step.

[0085] This sequence of steps is repeated for a certain number of cycles with intermediate characterization of the reslurried filter cake before staring the next filtration to be able to estimate the degree of purification achieved.

[0086] The process in accordance with the present invention provides an easy and economic route to the purification of nanowires on an industrial scale. Due to the use of specific filter media and certain process parameters (volume ratio of liquid to filter surface) it is possible to significantly reduce the filter clogging and the breaking of nanowires.

[0087] The filter media used in the process of the present invention are

compatible with industrial filter technology operations (in terms of mechanical resistance and processability).

[0088] The filter media used allow an automatic cake removal and by associating specific filter media interactions with the cake and the controlled operating conditions described hereibefore, cake remoival and re-slurryining is possible without or with significantly reduced damage to the nanowires thereby increasing the yield of the desired nanowires.

[0089] The amount of water needed for removal and re-slurrying of the cake is minimized in accordance with the process of the present invention which is beneficial for productivity of the process.

[0090] The process in accordance with the present invention also allows to

minimize the filtration area needed to purify a given quantity of liquid slurry. The combination of the specific filter material with the specific operating conditions (diilution, pressure and hydrodynamic properties allows to reach productivities compatible with induszrial filtration processes which was not possible with the nown process for nanowire purification.

[0091] Up to 15 g nanowires per square meter of filter materia, in some cases even 20 g of nanowires per square meter of filtration medium can be obtained. The quantity of nanowires can be measured by inductively coupled plasma mass spectrometry (ICP).

[0092] The present invention thus makes available silver nanowire dispersions having a desired degree of purity and a certain concentration on an industrial scale at economically feasible conditions.

[0093] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0094] The following example describes a preferred embodiment of the invention.

[0095] Example 1

[0096] A silver nanowire dispersion having a silver concentraion of 2073 ppm was filtered through a filter medium Bekipore ST 3AL3 (as characterized hereinbefore). Once the thickness of the filter cake had reached about 1 mm, filtration was stopped and the filter cake was removed from the filter medium with a squeeze bottle. To recover the cake, the media surface was sprayed with deionized (Dl) water, without coming too close to the surface, since this can damage the media. During this step, the quantity of water used was minimized.

[0097] Thereafter, deionized water was added to reslurry the cake to obtain the starting medium for the next filtration step. The amount of Dl water added was chosen so that the volume of the dispersion for the second filtration was the same as for the first filtration. The deionized water used for reslurrying contained PVP 40(40 stands for the molecular weight in kDa, 0.5 vol %, based on total dispersion volume, with a concentration of 10 wt%) and 0,5 vol % of a 20 wt% zirconium dioxide solution.

[0098] Between the different filtration steps, the filter medium was rinsed with deionized water.

[0099] This cycle was repeated until a total number of five filtrations was reached.

After the fifth filtration step, the washing was carried out with deionized water only (no additives present).

[00100] For each filtration step, the ratio of the volume of liquid to the filtration

surface was 47,8 L/m²

[00101] After each filtration step, the degree of purification achieved was

qualitatively determined by UV absorption spectroscopy and SEM analysis to evaluate the ratio of nanowires to nanoparticles after each filtration step. [00102] After the last filtration step, the nanowire slurry obtained was concentrated. An increase in concentration was achieved by filtering a volume of slurry and reslurrying with a lower amount of liquid. E.g. if 600 ml of slurry are filtered and the filter cake thereafter is reslurried in 60 ml, the concentraion of the silver nanowires should increase by a factor of around 10 (provided there are no significant losses of nanowires during filtration).

Claims

1. A process for the separation of nanowires with an aspect ratio of at least 10 from nanoparticles having an aspect ratio of less than 10 in a slurry comprising the following steps:

c) filtering the slurry at a ratio of volume of liquid to filtration surface of from 10 to 150 L/m² by dead-end filtration with a pressure drop of 1000 to 15000 kPa until a filter cake with a height in the range of from 0.5 to 5 mm is obtained, d) re-slurrying the filter cake in water,

e) filtering the slurry in accordance with step c) and

2. The process of claim 1 wherein the steps d) and e) are repeated 2 to 5 times.

3. The process in accordance with claim 1 or 2 wherein the water used for re- slurrying in step d) comprises poly vinyl pyrrolidone and/or a heavy metal oxide.

4. The process in accordance with any of claims 1 to 3 wherein the mean flow pore size of the filter medium is in the range of from 0.5 to 10 pm.

5. The process in accordance with any of claims 1 to 4 wherein the bubble point pressure of the filter medium is in the range of from 3000 to 30 000 Pa.

6. The process in accordance with any of claims 1 to 5 wherein the nanowires are metal nanowires.

7. The process in accordance with claim 6 wherein the metal nanowires are silver nanowires.

8. The process in accordance with any of claims 1 to 7 wherein the metal

concentration in the slurry is in the range of rom 0.001 to 1 mol-%.

9. The process in accordance with any of claims 1 to 8 wherein the pressure drop in step c) is in the range or from 2000 to 12000 kPa.

10. The process in accordance with any of claims 1 to 9 wherein the volume of liquid to filtration surface in step c) is in the range of from 30 to 100 L/m².