A METHOD, A SYSTEM AND A PACKAGE FOR PRODUCING A THREE DIMENSIONAL OBJECT, AND A SENSING DEVICE COMPRISING A 3D OBJECT MANUFACTURED WITH THE METHOD

Abstract

The present application relates to a method for producing a three-dimensional object, comprising:providing a first material (A) and, thereon, a second material (B) which is a reversible chromic material;applying a stimulus to the second material (B) to change its optical properties from non-strong optical or substantially non-strong optical absorption properties to strong optical absorption properties, regarding a specific wavelength, andexposing the second material (B) to electromagnetic radiation to be absorbed thereby to photothermally fuse portions of the first material (A) in thermal contact with the second material (B). A second aspect of the application relates to a system adapted to implement the method of the first aspect. A third aspect of the application concerns a kit of materials for producing a three-dimensional object. In a fourth aspect, the application relates to a sensing device comprising a three-dimensional object manufactured according to the method presented in the application.

Claims

1. A method for producing a three-dimensional object, comprising: providing a first material in a non-continuous solid form; providing a second material on at least a region to be at least partially fused of said first material, wherein said second material exhibits strong optical absorption properties at a specific wavelength which make the second material be a strong optical absorber; and exposing said second material to electromagnetic radiation having said specific wavelength, to be absorbed thereby to photothermally generate heat to fuse at least those portions of the first material in thermal contact with the second material, wherein the method further comprises: providing as said second material a reversible chromic material which changes its optical properties induced by a stimulus, from non-strong optical absorption properties or substantially non-strong optical absorption properties, at said specific wavelength, to said strong optical absorption properties at said specific wavelength; and applying at least said stimulus to the second material to temporarily change its optical properties to said strong optical absorption properties at said specific wavelength, wherein at least said stimulus is applied before and/or during at least part of the time during which the second material is exposed to said electromagnetic radiation.

2. The method according to claim 1, wherein said strong optical absorption properties are optical resonant properties, said strong optical absorber is an optical resonance absorber that optically resonates when exposed to said electromagnetic radiation to produce said photothermal heat generation, said non-strong optical absorption properties are optical non-resonant properties, and said substantially non-strong optical absorption properties are optical substantially non-resonant properties.

3. The method according to claim 1, wherein said strong optical absorption properties are optical polaronic properties, said strong optical absorber is an optical polaronic absorber, said non-strong optical absorption properties are non-strong optical polaronic properties, and said substantially non-strong optical absorption properties are substantially non-strong optical polaronic properties.

4. The method according to claim 1, wherein said strong optical absorption properties comprise optical resonant properties and optical polaronic properties, said strong optical absorber is both an optical resonance absorber and an optical polaronic absorber, said non-strong optical absorption properties comprise optical non-resonant properties and non-strong optical polaronic properties, and said substantially non-strong optical absorption properties comprise optical substantially non-resonant properties and substantially non-strong optical polaronic properties.

5. The method according to claim 1, wherein said reversible chromic material is one of: a thermochromic material, and said stimulus is a thermal stimulus; and a photochromic material, and said stimulus is an electromagnetic radiation stimulus.

6. (canceled)

7. The method according to claim 1, wherein said reversible chromic material is excitable by different types of stimuli and/or comprises a combination of chromic materials differing in that they are excitable by different types of stimuli, wherein said step of applying at least said stimulus to the second material comprises applying, sequentially or simultaneously, stimuli of said different types to the second material.

8-11. (canceled)

12. The method according to claim 1, comprising applying the stimulus or stimuli during all of the time during which the second material is exposed to said electromagnetic radiation.

13. The method according to claim 3, wherein the second material has a crystal lattice with defect sites, wherein the optical polaronic absorption comprises the absorption of the optical energy needed to move an electron between said defect sites.

14. The method according to claim 2, wherein the optical resonance of the optically resonant absorber refers to at least one of the following types of optical resonances: plasmonic resonance, Mie resonance, whispering gallery modes, optical resonance due to electronic transitions of charge carriers from one energy state or band in the electronic structure of the second material to another one upon absorption of photons, or a combination thereof.

15. (canceled)

16. The method according to claim 1, wherein the second material also comprises non-chromic materials, said non-chromic materials being adapted and arranged to enable or enhance the chromic response of the chromic material or chromic materials.

17. (canceled)

18. The method according to claim 1, wherein the second material includes at least one of the following materials: vanadium oxide, tungsten oxide, aluminium doped zinc oxide, tin doped indium oxide, cesium doped tungsten oxide, copper doped tungsten oxide, potassium doped tungsten oxide, sodium doped tungsten oxide, silver doped tungsten oxide, or a combination thereof.

19. The method according to claim 1, wherein both said non-strong optical absorption properties and said substantially non-strong optical absorption properties provide an optical absorption coefficient for the second material, at said specific wavelength, which is less than 1 L.g.sup.1.cm.sup.1, preferably less than 0.5 L.g.sup.1.cm.sup.1 and more preferably less than 0.1 L.g.sup.1.cm.sup.1, and wherein the absorption coefficient provided by the strong optical absorption properties to the same second material at the same specific wavelength, after being stimulated by said stimulus, exhibits an absorption coefficient which is greater than 2 L.g1.cm1, preferably greater than 3 L.g1.cm1, and more preferably greater than 5 L.g1.cm1.

20. The method according to claim 1, comprising producing a 3D object using a layer-by-layer deposition process, by forming a base layer by fusing together said at least those portions of the first material in thermal contact with the second material from the photothermal heat generated thereby, providing at least a further first material supply over the already formed base layer, and then fusing together a region of said further first material supply by applying a further second material supply thereon, applying thereon at least said stimulus and exposing to electromagnetic radiation having said specific wavelength the further second material supply.

21. A system for producing a three-dimensional object, comprising: at least one supplier device for providing: a first material in a non-continuous solid form; and a second material on at least a region to be at least partially fused of said first material, wherein said second material exhibits strong optical absorption properties at a specific wavelength which make the second material be a strong optical absorber; a controllable radiation source for exposing said second material to electromagnetic radiation at said specific wavelength, to be absorbed thereby to photothermally generate heat to fuse at least those portions of the first material in thermal contact with the second material; a supply of said second material, to feed said at least one supplier, in the form of a reversible chromic material which changes its optical properties induced by a stimulus, from non-strong optical absorption properties or substantially non-strong optical absorption properties, at said specific wavelength, to said strong optical absorption properties at said specific wavelength; a stimulus source configured and arranged to apply at least said stimulus to the second material to temporary change its optical properties to said strong optical absorption properties at said specific wavelength; and at least one controller adapted to control said at least one supplier device to provide the first and the second materials, said controllable radiation source to emit said electromagnetic radiation at said specific wavelength to expose the second material thereto, and said stimulus source to apply at least said stimulus to the second material before and/or during at least part of the time during which the second material is exposed to said electromagnetic radiation at said specific wavelength.

22. The system according to claim 21, wherein said strong optical absorption properties are optical resonant properties, said strong optical absorber is an optical resonance absorber that optically resonates when exposed to said electromagnetic radiation to produce said photothermal heat generation, said non-strong optical absorption properties are optical non-resonant properties, and said substantially non-strong optical absorption properties are optical substantially non-resonant properties.

23. The system according to claim 21, wherein said strong optical absorption properties are optical polaronic properties, said strong optical absorber is an optical polaronic absorber, said non-strong optical absorption properties are non-strong optical polaronic properties, and said substantially non-strong optical absorption properties are substantially non-strong optical polaronic properties.

24. The system according to claim 21, wherein said strong optical absorption properties comprise optical resonant properties and optical polaronic properties, said strong optical absorber is both an optical resonance absorber and an optical polaronic absorber, said non-strong optical absorption properties comprise optical non-resonant properties and non-strong optical polaronic properties, and said substantially non-strong optical absorption properties comprise optical substantially non-resonant properties and substantially non-strong optical polaronic properties.

25. A package for producing a three-dimensional object, comprising, enclosed therein: a first material in a non-continuous solid form; and a second material which exhibits strong optical absorption properties which make the second material be a strong optical absorber; wherein said second material is a reversible chromic material which changes its optical properties induced by a stimulus, from non-strong optical absorption properties or substantially non-strong optical absorption properties, at said specific wavelength, to said strong optical absorption properties at said specific wavelength, and wherein the package is configured and arranged to cooperate with at least one supplier device of system for producing a three-dimensional object for providing the first and second materials extracting them from the package, wherein said system for producing a three-dimensional object comprises: at least one supplier device for providing: a first material in a non-continuous solid form; and a second material on at least a region to be at least partially fused of said first material, wherein said second material exhibits strong optical absorption properties at a specific wavelength which make the second material be a strong optical absorber; a controllable radiation source for exposing said second material to electromagnetic radiation at said specific wavelength, to be absorbed thereby to photothermally generate heat to fuse at least those portions of the first material in thermal contact with the second material; a supply of said second material, to feed said at least one supplier, in the form of a reversible chromic material which changes its optical properties induced by a stimulus, from non-strong optical absorption properties or substantially non-strong optical absorption properties, at said specific wavelength, to said strong optical absorption properties at said specific wavelength; a stimulus source configured and arranged to apply at least said stimulus to the second material to temporary change its optical properties to said strong optical absorption properties at said specific wavelength; and at least one controller adapted to control said at least one supplier device to provide the first and the second materials, said controllable radiation source to emit said electromagnetic radiation at said specific wavelength to expose the second material thereto, and said stimulus source to apply at least said stimulus to the second material before and/or during at least part of the time during which the second material is exposed to said electromagnetic radiation at said specific wavelength.

26. A sensing device comprising a three-dimensional object manufactured according to a method for producing a three-dimensional object, comprising: providing a first material in a non-continuous solid form; providing a second material on at least a region to be at least partially fused of said first material, wherein said second material exhibits strong optical absorption properties at a specific wavelength which make the second material be a strong optical absorber; and exposing said second material to electromagnetic radiation having said specific wavelength, to be absorbed thereby to photothermally generate heat to fuse at least those portions of the first material in thermal contact with the second material, wherein the method further comprises: providing as said second material a reversible chromic material which changes its optical properties induced by a stimulus, from non-strong optical absorption properties or substantially non-strong optical absorption properties, at said specific wavelength, to said strong optical absorption properties at said specific wavelength; and applying at least said stimulus to the second material to temporarily change its optical properties to said strong optical absorption properties at said specific wavelength, wherein at least said stimulus is applied before and/or during at least part of the time during which the second material is exposed to said electromagnetic radiation, wherein in the sensing device the second material is arranged within the three-dimensional object so that its optical properties change from non-strong optical absorption properties or substantially non-strong optical absorption properties, at said specific wavelength, to strong optical absorption properties at said specific wavelength upon detection of a stimulus inducing said change, so that phenomena causing said stimulus can be sensed.

27. The sensing device according to claim 26, further comprising associated measuring components to measure said sensed phenomena.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0160] In the following some preferred embodiments of the invention will be described with reference to the enclosed figures. They are provided only for illustration purposes without however limiting the scope of the invention.

[0161] FIG. 1a summarizes the steps of the method of the first aspect of the present invention, for an embodiment for which the second material, identified as material B, is a photochromic material.

[0162] FIG. 1b summarizes the steps of the method of the first aspect of the present invention, for an embodiment for which the second material, identified as material B, is a thermochromic material.

[0163] FIG. 2 schematically shows the system of the second aspect of the present invention, for an embodiment.

[0164] FIG. 3 shows a 3D dimensional object manufactured according to the present invention, for an embodiment.

[0165] FIG. 4 is a plot showing the variation in the optical properties of nanoparticles of tungsten oxide in an ethanol solution, when submitted to ambient conditions and exposed to UV light, for an experiment for which said nanoparticles are selected as the second material for performing the method of the first aspect of the present invention, for an embodiment.

[0166] FIG. 5 is a plot which shows the time-dependent evolution of plasmon peak after UV treatment of the solution used for FIG. 4, particularly a 1 g/L colloidal solution.

[0167] FIG. 6 is a plot representative of an in-situ observation of photochromism of the 1 g/L colloidal solution under 365 nm UV-pumping.

[0168] FIGS. 7A and 7B graphically show the variation in temperature of disks formed from a dry composite sample of the nanoparticles/ethanol solution used for FIG. 2 mixed with a polymeric powder (first material), before and after being submitted to an UV stimulus, for a laser of 0.5 W/cm.sup.2 (FIG. 7A) and a laser of 3 W/cm.sup.2 (FIG. 7B).

[0169] FIG. 8 shows the evolution of photothermal property of the dry composite sample used for FIGS. 7A and 7B, at room temperature after UV-doping.

[0170] FIG. 9 is a graph showing the results of a photodoping-photothermal study of a 1:1000 weight ratio WO.sub.3/PA12 powder at 125 C. under 3 W/cm.sup.2 808 nm laser and UV-light.

[0171] FIG. 10 is a graph that shows the photothermal degradation of the 1:1000 WO.sub.3/PM12 powder after UV light is turned off, while laser 808 nm 3 W/cm.sup.2 is constantly on. Linear fits are provided.

[0172] FIGS. 11A and 11B show, by means of two graphs, the results of a photodoping-photothermal study of the 1:1000 weight ratio WO.sub.3/PM12 powder at 168 C. under 3 W/cm2 808 nm laser and UV-light, without (FIG. 11A) and with (FIG. 11B) an additional drop of WO.sub.3.

[0173] FIG. 12 is a diagram showing optical absorption waveforms of well and poorly dispersed second material particles, specifically for WO.sub.3 particles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0174] In its generalized form, the method of the first aspect of the invention can be used for printing coloured objects made of composites of at least two materials, a material A (called first material in other parts of the present document) which is the main constituent of the composite and a material B (called second material in other parts of the present document) the amount of which is minute compared to A. Originally the composite is at a non-continuous solid form, such as in a powder form. It is noted that the temperature of the composite can be room temperature or a different temperature such an elevated (compared to room) temperature which however is lower than the sintering temperature of material A. Before or after material B is mixed with material A, material B on its own or the entire A+B composite are irradiated with electromagnetic radiation C or/and are heated at an elevated temperature (compared to room temperature) as to change the optical properties of material B and render it capable of absorbing a second form of electromagnetic radiation called radiation D. If the optical properties of material B can be changed by radiation C then material B is a photochromic material while if the optical properties of B are changed by a change of the temperature, material B is thermochromic material. Material B can be both photochromic and thermochromic (and/or of any of the other chromic types cited above). Radiation D is then applied to the composite and is absorbed mainly or exclusively by material B and causes heating of material B. Part of the heat is passed to material A and results to sintering of the powder into forming a continuous solid object. When subsequently, the sintered solid composite cools to room temperature and is no longer irradiated with radiation C, material B acquires its original optical properties.

[0175] The method of the first aspect of the present invention is summarized in Figures la and 1 b, for the cases of thermochromic and photochromic material B, respectively. This allows for printing objects the optical properties of which at the time of their use are not restricted by the optical properties required for the formation of the objects. For example, if material B at room temperature and ambient indoors or outdoors lighting conditions is visibly transparent or white but at high temperatures or/and under radiation B becomes coloured and resonant to visible radiation D, then radiation D can be used for printing the object but when printing is finished the object will no longer optically absorb visible radiation D, as long as material A also does not absorb radiation D.

[0176] Material A can actually consist of a set of materials of different chemical compositions such as a set of materials A1, A2, A3, etc., the common characteristic of which is that they do not absorb strongly the electromagnetic radiation D and they do not present a respective photothermal effect as strongly and as efficiently as material B. In addition material A or a set the materials A1, A2, A3, etc. may or may not belong to the same class of materials such as polymers. For example the set of materials A1, A2, A3, . . . , may contain within it the main material to be sintered as well as other additives which may serve additional functionalities such as to provide more efficient or strong binding of the granules upon sintering, or to colour the final object, or to add other functionalities in the final object such to render it electrically conductive or permanently magnetic or paramagnetic or thermally conductive, or to prevent coagulation of the granules of either material A or B or both before binding, or to prevent chemical transformation and chemical reactions of either materials A or B or both, before or after sintering. Nevertheless, materials A1, A2, A3 may also chemically react with each other and with material B before or after sintering. Material A or at least one constituent of the set of materials A1, A2, A3 is the main constituent of the composite of the object that is printed with the method of the present invention. In a specific example, this material is polymer. A non-limiting list of polymer materials that can be used with the method of the invention is: Polylactic Acid (PLA), Acrylonitrile Butadiene Styrene (ABS), PolyAmide (PA), High Impact Polystyrene (HIPS), Thermoplastic Elastomer (TPE). Herein, for clarity of presentation the term material A is used indistinguishably to describe either a single material or a set of materials A1, A2, A3, etc.

[0177] Material B can actually consist of a set of materials of different chemical compositions such as a set of materials B1, B2, B3, etc., which may have the common characteristic that their optical properties change upon absorbing the electromagnetic radiation C or upon being heated, and due to the change in their optical properties they can absorb electromagnetic radiation D and upon this process they present a significant photothermal effect. In addition, a set of materials B1, B2, B3 etc. may include at least one material which exhibits photochromism or thermohromism and the rest of the materials in the set are used to further enable or enhance the photothermal or photochromic response of the aforementioned member of the set, for example by chemically reacting with it, or by forcing it to change its atomic or crystalline structure upon heating, or/and irradiating the material set with radiation D. Material B or a set of materials B1, B2, B3, etc., and any individual member of such set of materials can also have additional functionalities. For example, they may also serve to colour the final object, or to add other functionalities in the final object such as to render it electrically conductive or permanently magnetic or paramagnetic or thermally conductive or to exhibit high electrical capacitance. For clarity of presentation, herein the term material B is used indistinguishably to describe either a single material or a set of materials B1, B2, B3, etc.

[0178] Thermochromic or photochromic materials B can be materials whose optical properties change upon being heated or/and absorbing electromagnetic radiation C due to a number of, known from the scientific literature, different physical effects such as: change of the material's structure, or change of the material's stoichiometry or an increase in the number of free charge carriers. Material B can be organic or inorganic or molecular or in the form of particles of various dimensions such as nanoparticles or larger size particles or thin flakes which may be amorphous or crystalline or polycrystalline. An example of thermochromic materials that can be used with the method of the present invention is vanadium oxide which when heated at temperatures above 50 C. starts absorbing strongly infrared light due to temperature induced change of its crystal structure. An example of photochromic material that can be used with the method of the present invention is tungsten oxide which upon absorbing ultraviolet (UV) electromagnetic radiation changes colour and also develops optical absorption of infrared electromagnetic radiation due to an increase in the number of free charge carriers in the material due to UV-light induced change in the stoichiometry of the material, namely a change in the oxygen to tungsten ratio. In a different example, tungsten oxide is used in combination with water molecules around it or within its structure because water is known to promote the photochromic response of tungsten oxide (Journal of Applied Physics 74, 4527 (1993)). Another example of material that can be used with the method of the present invention is aluminium doped zinc oxide which upon either absorption of ultraviolet light or increase of its temperature may change its visible colour and absorb more strongly infrared light due to an increase in the number of its free charge carriers due to a change in the cation to anion ratio of the material, or/and an increase of the number of electronically activated dopant atoms of the material, or/and due to an increase of the number of charge carriers populating the conduction band of the material.

[0179] A non-limiting list of thermochromic material categories members of which can be used with the method of the present invention is: Spiropyrans and spirooxazines, Diarylethenes, Azobenzenes, Photochromic quinones, inorganic photochromics. A non-limiting list of inorganic photochromics that can be used with our method is: silver chloride, silver halides, zinc halides, yttrium hydride, tungsten oxide and tungsten oxide bronzes, molybdenum oxide and molybdenum oxide bronzes, vanadium oxide and vanadium oxide bronzes, zinc oxide and aluminium doped zinc oxide, indium oxide and tin-doped indium oxide, titanium oxide, and in general binary or ternary or quaternary or more complex oxides which may be further alloyed between them or with other metal oxides or doped with various cations such as metals such as tin, lithium, copper, sodium, potassium, cesium, silver, lead, cadmium, iron, or doped with anions such halide atoms or chalcogen atoms.

[0180] A non-limiting list of thermochromic materials that can be used with the method of the present invention is: leuco dyes such as spirolactones, fluorans, spiropyrans, and fulgides, Cuprous mercury iodide (Cu.sub.2Hgl.sub.4), Silver mercury iodide (Ag.sub.2Hgl.sub.4), Mercury(II) iodide, Bis(dimethylammonium) tetrachloronickelate, Bis(diethylammonium) tetrarchlorocuprate, Chromium(III) oxide:aluminium(III) oxide, Cd.sub.xZn.sub.1xS.sub.ySe.sub.1y (x=0.5-1, y=0.5-1), Zn.sub.xCd.sub.yHg.sub.1xyO.sub.aS.sub.bSe.sub.cTe.sub.1abc (x=0-0.5, y=0.5-1, a=0-0.5, b=0.5-1, c=0- 0.5), Hg.sub.xCd.sub.yZn.sub.1xyS.sub.bSe.sub.1b (x=0-1, y=0-1, b=0.5-1), molybdenum oxide and molybdenum oxide bronzes, vanadium oxide and vanadium oxide bronzes, zinc oxide and aluminium doped zinc oxide, indium oxide and tin-doped indium oxide, titanium oxide, and in general binary or ternary or quaternary oxides which may be alloyed between them or with other metal oxides or doped with various cations such as metals such as tin, lithium, copper, sodium, potassium, cesium, silver, lead, cadmium, iron, or doped with anions such as halide atoms or chalcogen atoms.

[0181] Electromagnetic radiation C can be in the form of microwaves or visible light or UV or NIR, or mid-IR of far-IR electromagnetic waves or a combination of these, which can be produced through a variety of radiation sources. A non-limited list of such radiation sources is: laser, UV lamps, LED, halogen lamps, IR-lamps. The wavelength and intensity of the electromagnetic irradiation is chosen as to preferentially or completely be absorbed by material B and change its optical properties so as to absorb efficiently radiation D and consequently be heated due to the photothermal effect. Electromagnetic radiation C can contain electromagnetic waves of several different wavelengths all of some of which can be absorbed by material B and change its optical properties in the aforementioned manner.

[0182] Electromagnetic radiation D can be in the form of microwaves or visible light or UV or NIR, or mid-IR of far-IR electromagnetic waves or a combination of these, which can be produced through a variety of radiation sources. A non-limited list of such radiation sources is: laser, UV lamps, LED, halogen lamps, IR-lamps. Electromagnetic radiation D can contain electromagnetic waves of several different wavelengths all or some of which can be absorbed by material B and cause it to heat up.

[0183] In a preferred example of the method of the present invention material B is in the form of inorganic nanoparticles or micron-size particles since such small size particles can exhibit enhanced photochromic or/and thermochromic properties compared to larger size particles of the same material. This happens because small particles have an increased surface-to-volume ratio compared to larger particles and may require less energy for changing their stoichiometry, or structure, or surface chemistry, or free charge carrier density compared to larger size particles of the same material. A more specific example of such material, is tungsten oxide nanoparticles which are smaller than 100 nm. The surface of the nanoparticles or microparticles can be covered with other organic or inorganic materials such as atoms, molecules or other crystalline or non-crystalline materials which may be used for several purposes such as to enhance the photochromic properties of the particles, or to prevent their aggregation or to chemically stabilize the particles or to prevent sintering between the particles of material B or to prevent their interaction with material A or B or other materials such as gases such as atmospheric oxygen, or to control their optical, photothermal and other physical properties. For example, the surface of nanoparticles can be covered with molecules which act as anti-agglomeration agents and may belong to the following non-limiting list of chemical compounds and compound categories: Cetyltrimethyl ammonium bromide (CTAB) or others alkyl trimethyl ammonium halides (Lauryltrimethylammonium bromide (DTAB), Myristyltrimethylammonium bromide (MTAB), etc.), Polyethylene glycol (PEG) and derivatives, polyvinylpyrrolidone (PVP), Polycationic polymers such as polyvinylpyrrolidone, polyethylene imine, polyallyl amine, polylysine and co-polymers, polymers containing Sulphur or thiol groups such as polystyrene sulfonates, polysulfides, polysulfones and co-polymers, Silica, oleic acid, myristic acid, octanoic acid, steraic acid, any other organic cabroxylic acid, oleylamine, butylamine, any other organic amine, trioctylphosphine oxide, any other organic phosphine oxide, 1-octadecanethiol, dodecanethiol, any other organic thiol, 3-mercapopropionic acid, any other functionalized organic thiol, hydroxide, acetate ion, iodine ion, bromine ion, chlorium ion, sulfur ion, trioctylphosphine, any other organic phosphine, trioctylamine, Triphenylphosphine, any analogues of the above and any combination of all of the above.

[0184] The method of the first aspect of the present invention can be used for printing 3D projects and as mentioned earlier, this can happen through a layer-by-layer deposition fabrication methodology. For example, the method can be applied for 3D printing using the system of the second aspect of the present invention, some components of which are schematically shown in FIG. 2 for an embodiment. The system operates as follows: a thin layer of powder of material A or A+B mixture is applied on the printing region of the system which is located in the printing region system called P. The layer is formed using a layer forming system called L. System L may take powder from a feedstock system of powder, this system is called F. The powder may consists of particulates/granules) the size of which may be 0.001-1000microns, and preferably be 0.001-100microns. The thickness of the powder layer may be 0.00001-10 centimetres, and preferably be 0.001-0.1 centimetres and most preferably be 0.001 to 0.01 centimetres. The temperature of the powder and the temperature of the powder layer in systems P, L, F may be controlled through the application of a heating or/and cooling system called system T. For example, a heating system may contain thermometers and temperature monitoring systems and electrical resistive heaters and/or IR heating lamps which heat the F, L, P and everything within them. When the powder layer is deposited, then material B may be deposited on the powder via a material deposition system I. If powder layer produced by L consists of pre-mixed A+B then I may not be necessary. If the photochromic or thermochromic properties of B may be enhanced when B is in contact with other material such as B2, B3 etc., then block I may be used to deposit B2, B3, etc. on the A+B powder layer. I may contain several tanks which may contain B, B2, B3 separately or combined. Deposition system I may contain a system of tanks called IT tanks in which colour pigments dissolved in liquids are deposited on the powder layer as to colour it. Such colour pigments can optionally exist in the same tanks with either B or B1, B2 etc. Deposition system I may contain a subsystem called ID for dropping droplets of any liquid contained in the tanks of the system as to deposit B, B2, B3 etc. or/and colours on the powder layer. For example, the subsystem ID may consist of an inkjet head or a multiple of inkjet heads each of which may be fed with liquid from one or more tanks. The inkjet heads may be physically attached to the tanks of IT or may connected to the tanks via a hydraulic system called IH. The deposition system ID or parts of it such inkjet heads may move across the x and y directions as to form deposition patterns on top of the powder layer. A subsystem called IS can be used for controlling the motion of any of I and any subsystem of it across the x, y, z axes. When system I deposits on the powder layer materials which may be dispersed in a liquid then the constituents of the liquids are selected and engineered as to achieve the following: i) the materials should be able to disperse within the liquid, ii) the liquid and its contents should be able to disperse across the cross section of the powder, and iii) the liquid will evaporate after being deposited. In the case that material B is a photochromic material, then before, during and after deposition of materials on the powder layer the powder layer or the materials may be irradiated with radiation C via a radiation C system called CS. CS may irradiate systems L, F P, I and preferably irradiates P or parts of it such as the upper surface of the powder layer. CS contains at least one source of radiation C and may contain a subsystem called CSI for controlling the intensity and spectral profile of radiation C. CS may also contain an optical subsystem called CSO which may serve various tasks such as focusing radiation C on the powder layer, or directing/allowing radiation C to illuminate only certain parts of the powder layer, such as areas of various geometrical shapes and patterns. In a non-limiting example of CSO, if the radiation C source is a laser, then CSO may include a set of galvo-mirrors that scan the laser beam across the x-y directions on the powder layer. CS may also contain a mechanical system which moves the radiation source across any of the x,y,z directions. In another example, CS may also contain sources of radiation C that are located inside or close to the tanks of IT or inkjet heads of ID or other parts of system I and subsystem ID as to irradiate material B before it has been deposited on the powder layer. If material B is a thermochromic material then system CS is optional.

[0185] After materials have been deposited on the powder layer and radiation C has altered the optical properties of material B then the powder layer is irradiated with radiation D which is provided by a system called DS. DS may illuminate systems L, F P, I and preferably illuminates P or parts of it such as the upper surface of the powder layer. DS contains at least one source of radiation D and may contain a subsystem called DSI for controlling the intensity and spectral profile of radiation D. DS may also contain an optical subsystem called DSO which may serve various tasks such as focusing radiation D on the powder layer, or directing/allowing radiation D to illuminate only certain parts of the powder layer, such as areas of various geometrical shapes and patterns. In a non-limiting example of DSO, if the radiation D source is a laser then DSO may include a set of galvo-mirrors which scan the laser beam across the x-y directions on the powder layer. DS may also contain a mechanical system which moves the radiation source across the x,y,z directions.

[0186] If material B is a photochromic material, then when the powder layer is irradiated with radiation D, the parts of the powder layer that contain material B, and have been irradiated with radiation C, and have been irradiated with radiation D, are sintered, because in those parts material B absorbs radiation D and is thus heated to temperatures above the sintering point of material A. If the material B is only a thermochromic material, then when the powder layer is irradiated with radiation D, the parts of the powder layer that contain material B, and have been heated to the temperatures required for changing the optical properties of material B as to be able to absorb radiation D, and have been irradiated with radiation D, are sintered.

[0187] The sintered solid parts of the powder layer consist of a composite material that contains at least materials A and B. The morphology of the final composite layer will depend on the morphology of the initial powder layer, the volume and deposition area of the ink which was deposited by the system I (if that was necessary) and the morphological changes that will be induced by the sintering process of both the polymer and the ink. Generally, the planar morphology of the initial powder layer is preserved and translated to a planar morphology of the sintered parts of the powder layer. The dimensions and shape of the final sintered parts may depend on: 1) the size and shape of the areas on which system I may have deposited material, 2) the size and shape of the areas which were irradiated by CS, 3) the size and shape of the areas which were irradiated by DS, 4) a combination of any of the above 1-3.

[0188] 3D printing can occur though repetition of the method of the present invention, meaning that after parts of the powder layer have been sintered, then a new powder layer is deposited on top and the aforementioned processes are repeated. Every time the process is repeated, the sintered parts may also be sintered with -meaning strongly adhered to- the parts which were sintered at the previous powder layer, and the size and shape of the sintered parts of the powder layer can be different compared to the ones of the previous layer. This way, 3D dimensional objects of desired shapes and size are formed a shown in FIG. 3. The objects can be easily mechanically removed, e.g. by hand, from the un-sintered powder that surrounds them. The method of the present invention and any obvious variations of it can be combined, in a successive manner, with other additive manufacturing methods for making complex objects that contain, only partially, components which are made with our method.

[0189] The system that can be used for our implementing the method may contain additional components to the ones that were described above. A non-limiting list of such components are: a power system W which powers all other systems and subsystems of the system, an electronic system E which controls all other systems and subsystems of the system, a mechanical system M which contains all or some systems of the system, and a computer system O that controls system E.

[0190] By means of the present invention a solution is provided to the above mentioned problems associated to the prior art methods, i.e. to the fact that metallic plasmonic particles do tend to absorb in the visible and this problem can be further increased by the structural transformations of the particles which are induced by the high temperatures at which 3D printing occurs.

[0191] Herein, experimental evidence is provide of the method of the first aspect of the invention, using photochromic nanomaterials.

[0192] For obtaining said experimental evidence, a specific example of photochromic material B was developed by the present inventors for use with the method of the present invention, particularly tungsten oxide nanoparticles. The present inventors synthesized the nanoparticles by adopting a colloidal method from the scientific literature (Solid State Sciences, 69, 50-55, 2017) for fabricating good colloidal suspensions of this material at quantities which are high enough for use in 3D printing. The method can be scaled up for industrial production. A particularly nice aspect of the synthetic method is that it is relatively green since no highly toxic chemicals are used and the main solvents used are: water, ethanol, glacial acid (which is a weak acid).

[0193] Specifically, the above referred colloidal method comprises the following steps: in a flask with a condenser attached to it, the present inventors mixed 0.2 g of tungsten chloride and 100 ml of ethanol. Then the flask was heated under stirring to reflux of its contents. The liquid in the flask became clear yellow due to dissolution of the tungsten chloride. Then a mixture of 6 ml water and 2 ml acetic acid was quickly injected in the flask, using a 12 ml syringe. The colour of the liquid of the flask became instantly deep blue due to formation of tungsten oxide nanoparticles with a stoichiometry of WO.sub.x where x<3. The solution was kept to reflux for 1 hour and was then cooled down.

[0194] The nanoparticles of the solution were then isolated from the liquid by centrifuging the solution to which 30 ml of hexane had been added. After centrifugation, the nanoparticles had formed a blue solid precipitate, which was further washed with ethanol and ultimately was re-dispersed in ethanol. The WO.sub.3 nanoparticles are ligand free, and immediately after synthesis they exhibit a deep blue colour. This implies that they are non-stoichiometric and, as stated above, their chemical formula is WO.sub.x where x<3. Due to this stoichiometric imbalance, the nanoparticles behave as heavily charged semiconductors and for this reason they exhibit a broad, but rather distinctive, plasmon peak centred at 1000-1100nm.

[0195] When this solution is left in ambient environmental conditions for 1 day its colour changes from blue to faintly yellow-bronze colour and it also turns from non-transparent to transparent due to oxidation of the nanoparticles. This results to loss of the plasmon peak of the particles. When the solution of the nanoparticles is exposed to UV light coming from a UV-lamp, its colour turns again back to the original one, which is blue and non-transparent, because UV-light induces a partial loss of oxygen from the nanoparticles structure. This phenomenon is a rather impressive manifestation of the photochromic effect which is well known for WO.sub.3. If the solution is again left to ambient conditions it becomes transparent and in general the aforementioned steps can be repeated several times for changing the optical properties of the nanoparticles from one form to the other, as shown in FIG. 4. Then, the nanoparticles can be sequentially coloured, decoloured, coloured and so on.

[0196] Trying to understand in detail the photochromic behaviour of this nanomaterial and its potential application for 3D printing, the present inventors performed a series of optical and photothermal characterization of WO.sub.3 colloidal solutions, and polymer-WO.sub.3 NP composites, which results are discussed below.

[0197] 1. Colour Cycling of colloidal solutions: As stated above, FIG. 4 shows the colour of the WO.sub.3 suspension in ethanol before UV treatment and after UV treatment. The macroscopically observed transient photochromic behaviour of the material, is further confirmed by the absorption spectra of 1 g/L solutions. The spectra were taken over the period of 3 days before and after UV treatment. The following main observations can be made: the plasmon peak is fully re-covered, strengthened and blue shifted after UV-treatment, then fully lost upon 1 day aging.

[0198] 2. Transient character of plasmon peak: The transient character of the plasmon peak of the material was studied via monitoring the evolution of the plasmon peak after UV treatment. The spectra are shown in FIG. 5 and the main observation is that half of the plasmon peak is lost in the first 20 minutes after UV radiation, while subsequent degradation of the plasmon peak happens at a slower rate.

[0199] 3. In-situ observation of photochromism: The photochromic effect was observed in-situ by adding a 365 nm portable UV-lamp inside the sample chamber of the spectrophotometer. Prior to the measurements, the sample was already partially photodoped. Additional photodoping was observed as a function of UV-exposure time. The results are shown in FIG. 6. The main conclusions are that photodoping is cumulative and that for best utilization of the photodoping (photochromic) effect the nanoparticles must be illuminated with UV while photochromism is being utilized. In other words, photochromism can be best used for printing, via using a printer in which nanoparticles are irradiated with UV light (for photodoping) before and while being sintered with IR light.

[0200] 4. Photothermal effect of solid samples at room temperature: The photothermal effect at room temperature of WO.sub.3 drop casted on polymer PA12 powder was monitored before and after UV-illumination. Results are shown in FIGS. 7A and 7B and it is obvious that UV-induced photochromism is required for acquiring a technologically relevant photothermic response from the nanoparticles. Specifically, the present inventors mixed the nanoparticle/ethanol solution with powder of PA12 (which is a polymer also known as nylon 12) which can be considered as an example of material A in the present invention. The mixture was left until the ethanol from the nanoparticle solution evaporated and the nanoparticle/PA12 composite was dry. The WO.sub.x/PA12 weight ratio of the composite was 1:1000. The present inventors then formed thin disks of this powder. Then the powder disks were exposed to 808 nm laser light which is an example of electromagnetic radiation D in the present invention. The temperature of the powder was monitored using a thermal camera and found that the powder's temperature did not increase. Then the laser was switched off, and the powder was irradiated with UV light produced by a UV lamp which was located close to the powder. The UV light in this case is an example of radiation C in the present invention. Then the powder was irradiated again with the laser and found that this time the powder's temperature increased by more than 30 C., as shown in FIGS. 7A and 7B. This happened because of the photochromic properties the WO, nanoparticles which upon exposure to UV light became blue and developed a plasmon absorption peak in the infrared part of the spectrum. As result of that, the UV-illuminated nanoparticles of the powder could subsequently absorb the laser beam and be heated thus also heating the PA12 content of the powder. A similar experiment was performed but at elevated baseline temperature of the powder, namely at 168 C. which is below the sintering point of the PA12 powder which can be sintered at temperatures >180 C. It is noted that the powder was heated to 168 C. via thermal convection due to positioning the powder on the hot surface of a hotplate and by additionally illuminating the surface of the powder with IR light coming from a set of IR heating lamps. Then, the powder was first illuminated with UV light and then irradiated with the 808 nm laser beam of a 3 W/cm.sup.2 power density, the temperature of the powder was increased to >180 C. resulting to sintering and/or melting of the polymer. When the laser was turned off, the sintered area of the powder cooled down and now appeared as solid object. When the powder is not illuminated with UV light prior to being irradiated with the laser, then it cannot be heated to the temperatures required for sintering of the powder.

[0201] 5. Transient character of photothermal property of solid samples: The photothermal properties of the aforementioned samples, was further studied as a function of time elapsed since UV-exposure. The results are shown in FIG. 8. The main conclusion is that at room temperature the photothermal property of photodoped samples is partially preserved for several minutes after UV-exposure.

[0202] Specifically, it can be stated that when the UV light is turned off the coloration of the WO.sub.x nanoparticles does not change instantly, instead a few seconds or minutes or hours are required depending on various parameters such as the atmospheric conditions, such as the humidity and oxygen content of the atmosphere, the morphology and exact stoichiometry of the nanoparticles, the presence of any materials around the nanoparticles that may hinder or accelerate the changes of the optical properties and the temperature of the nanoparticles and the medium they are in. For example, it was found that a solid object of WO.sub.x/PA12 is originally white and has the same colour as a solid object which is made only of PA12. When the WO.sub.x/PA12 and PA12 objects are irradiated with UV light, the PA12 object remains white while the WO.sub.x/PA12 object it becomes blue due to the photochromic effect of the WO.sub.x nanoparticles. If this blue object is left at room temperature it becomes white again over a period of 1 day. Instead, if the blue object is heated to 150 C., it becomes white within 30 seconds due to an accelerated oxidation of the WO.sub.x particles at elevated conditions and under ambient atmosphere. Similarly, it was found that the time required for the WO.sub.x nanoparticles to develop a plasmon absorption peak in the IR, and the intensity and centre of this peak depend on the temperature of the nanoparticles or the nanoparticles/polymer mixture and the intensity of the UV light. Therefore, in the method of the present invention the temperature of materials A, B or A+B mixture before, during and after the sintering process may be controlled as to control the photochromic or photothermic response and optical properties of material B or of the A+B mixture. In addition, the atmospheric conditions of materials A, B or A+B mixture before, during and after the sintering process may be controlled as to control the photochromic or thermochromic responses of material B or the A+B mixture. In addition the time of duration of exposure of material B or material A+B to radiation C and the intensity and spectral profile of radiation C before, during and after the sintering process, may be controlled as to control the photochromic or thermochromic responses of material B or the A+B mixture.

[0203] 6. Photochromism and photothermal behaviour at 125 C. with in situ UV: 3D printing using SLS typically happens at elevated baseline temperatures in the 140-170 C. range. For this reason, it is imperative to study photochromism and related photothermal properties at high temperatures. Hence, the present inventors prepared WO.sub.3/PA12 powder mixtures of 1:1000 weight ratio and positioned them on a hotplate. The temperature of the hotplate was set to 170 C., but the temperature of the powder surface was 125 C. The photothermal effect with and without UV irradiation was recorded, and depicted in the graph of FIG. 9. It is important to note that in this case UV-irradiation was provided in-situ from a UV lamp positioned at the side of the hotplate. That allowed to monitor the photothermal response as a function of the irradiation time. The main conclusions are that photochromism does happen at 125 C. but the overall effects are less pronounced compared to when the materials are at room temperature. Possible explanations for this difference is that high temperatures favour oxidation of the nanoparticles which acts against photochromism. In addition, at high temperatures the sample is dryer and many believe that the presence of water is associated with photochromism of WO.sub.3 nanoparticles. Another conclusion from this experiment is that the photochromic (photodoping) effect is cumulative and does not wear off upon heating the particles with the laser. In addition, as already stated above, the effect is preserved for at least few seconds-minutes after UV is off. Lastly, for the particular UV lamp used here, the photodoping effect saturates after about 5 minutes.

[0204] 7. Kinetics of degradation of photodoping-photothermal effect after UV-irradiation at 125 C.: Since photochromism and the associated photothermal behaviour of WO.sub.3 nanoparticles are transient effects, it is important to study their evolution after UV light is off. It is further meaningful to do this at elevated temperatures since these are usually used for 3D printing. Thus, it was measured how the IR laser induced temperature rise of a 1:1000 WO.sub.3/PA12 powder drops after UV light is off. The results are shown in FIG. 10 and from there it is quite clear that two main time-regions can be distinguished. One where photothermal degradation is fast and one where degradation is slow. At first approximation, these regions can be described as linear.

[0205] 8. Photochromism and photothermal behaviour at 168 C. with in situ UV: The apparatus including a hot plate, used in items 6 and 7 above, was further modified by adding an IR heating lamp array for raising the surface temperature of the powder bed to 170 C. which is the one typically used in SLS of carbon black-nylon. The photothermal behaviour of the 1:1000 WO.sub.3/PA12 is shown in FIG. 11A. It is obvious that the temperature rise is very small which further confirms our previous observation on the decrease of photothermal response with increasing temperature. For this reason, the concentration of the nanoparticles was further increased by dropping on the powder bed a drop of 1 g/L WO.sub.3/ethanol. For this modified sample, as shown in FIG. 11B, temperature rises of 20 C. under 3 W/cm.sup.2 808nm laser were observed, and UV-induced photochromism is required for achieving a good photothermal response.

[0206] 9. Transient Photochromism on objects: A likely implication of photochromism is that the colour of printed objects will be improving with time due to disappearance of the colour of the WO.sub.3 nanoparticles after the printing process is complete. To demonstrate that, some WO.sub.3 and PA12 (1:1000 weight ratio) were melted together, and its colour was observed before UV light, after UV light, and after 1 day of aging at ambient (indoors) lighting conditions. The obtained results confirmed the above mentioned implication.

[0207] Finally, FIG. 12 is a diagram showing optical absorption waveforms of well and poorly dispersed second material particles, specifically for WO.sub.3 particles (small particles with an average diameter around 5 nm), used herein to complement the definition given in a previous section in this document regarding the importance of the term well dispersed for the definition of the terms strong and non-strong optical absorption properties.

[0208] As stated in that previous section, it is important to note the term well dispersed for the above given definition of strong and non-strong optical absorption properties.

[0209] A solution of small nanoparticles (<25 nm average diameter) that is well dispersed should appear clear or coloured but still transparent at certain wavelengths, i.e. it should not appear cloudy, and one should be able to clearly see objects on the far side. That is, assuming the solution itself is transparent. One could potentially do these experiments for particles in a polymer matrix, but many polymers absorb slightly and so will not appear transparent anyway.

[0210] For larger nanoparticles, (>25 nm average diameter), there may be some scattering present due to the size of the particles. A broader definition of well dispersed that also covers this case is Particles that maintain a large enough distance from each other so as to not significantly effect each other's optical properties in terms of scattering and/or absorption.

[0211] The reason this is important is that in many experiments, if one has a highly scattering sample, and does not try to account for this, then one will measure the scattering as absorption and so poorly dispersed nanoparticles can appear to be better absorbers than well dispersed particles, even if this is not the case. The present inventors have found that for some samples of the second material that are not well dispersed, one gets agglomerations, leading to large scattering. So if one is not careful it can look like the absorption is very high for the non-strong sample.

[0212] FIG. 12 is a good example of this. The WO.sub.3 nanoparticles are about 5 nm in diameter, and if well dispersed, in their non-strong absorption state (waveform with circle marks) they absorb almost nothing up until their bandgap below 400 nm. As there is virtually no absorption above 400 nm for these particles, it can be stated that the scattering is negligible. On UV excitation (used as an example of stimulus for inducing the chromic effect), the extinction increases is suddenly more than 100 times (waveform with square marks).

[0213] In the poorly dispersed case (waveform with triangle marks), the extinction (absorption+scattering) is quite high up to about 1400 nm, due to large amounts of scattering due to the large size of the agglomerated particles. Therefore if one measures the increase in extinction after UV illumination (waveform with rhombus marks) at the peak wavelength of an NIR lamp at 1100 nm, then one might only see an increase by a factor of 2 or 3.

[0214] This is due to the high scattering one starts with in this case. If one were to use a more specialised apparatus like an integrating sphere then one would see that the absorption in the weak absorption case is very low. If one takes these particles and re-disperse them, then one returns to something like the waveform with circle marks. Therefore defining the above numbers (i.e. those given for defining the strong and non-strong optical absorption properties) in terms of being well dispersed is crucial. This is also the case for larger particles ((>25 nm average diameter).

[0215] A person skilled in the art could introduce changes and modifications in the embodiments described without departing from the scope of the invention as it is defined in the attached claims. For example, a method or system as the ones of the present invention which comprises the use of any type of radiation (such as ultrasound, thermal, electric, electrostatic, magnetic, or ionizing radiation), electromagnetic or of another kind, to expose the strong optical absorbent particles, which are well known in the art for exciting such kind of particles causing them to generate heat, is to be considered equivalent to the one of the present invention.

REFERENCES

[0216] [1] Salje & B Guttler, Anderson transition and intermediate polaron formation in WO3-x Transport properties and optical absorption, Philosophical Magazine B, (1984).

[0217] [2] Li et al, A plasmonic non-stoichiometric WO3-x homojunction with stabilizing surface plasmonic resonance for selective photochromic modulation, Chemistry Communications (2018), DOI: 10.1039/C8CCO2211A

[0218] [3] Zheng et al, The preparation of a high performance near-infrared shielding CsxWO3/SiO2 composite resin coating and research on its optical stability under ultraviolet illumination, Journal of Materials Chemistry, (2015).