Photoluminescent material from hydrogen silsesquioxane resin

Abstract: A photoluminescent material is prepared by either (i) pyrolyzing a H-resin in powder form in a crucible in oxygen, by heating the resin powder to a temperature above 350°C for up to 180 min; or by (ii) preparing a gel from a solution of the resin in a solvent and pyrolyzing the resin gel in oxygen by heating the gel to a temperature above 350°C for up to 180 min.

Description: This invention is directed to the preparation of a photoluminescent material from a hydrogen silisesquioxane resin (H-resin). The heat treatment of H-resin has been previously described, for example, in US patents 4,756,977; 5,145,723; 5,370,903; 5,370,904; 5,372,842; 5,380,567; 5,403,748 and 5,693,565. It is believed, however, that no skilled artisan has devised a process similar to the method described and claimed herein. Moreover, skilled artisans have not determined photoluminescence of a material based on a H-resin in the manner also achieved herein. Furthermore, unlike US Patent 5,635,249. this invention does not require the addition of a phosphor filler to achieve a luminescent effect. This disclosure claims the preparation of a strongly photoluminescent material by the heat treatment of H-resin. The present invention refers to a method of preparing a photoluminescent material comprising the steps of: (i) pyrolyzing a H-resin in powder form in a crucible in oxygen, by heating to a temperature of at least 350°C at a heating rate sufficient to cause the H-resin to melt on its exterior surface before the conversion or cure reaction occurs; and (ii) maintaining the temperature for up to 180 min, alternatively to a method of preparing a photoluminescent material comprising the steps of: (i) preparing a gel from a solution of a H-resin in a solvent; (ii) pyrolyzing said gel in oxygen by heating to a temperature of at least 350°C; and (iii) maintaining the temperature for up to 180 min. The temperature in the pyrolizing step is preferably in the range of 400°C-600°C, more preferred from 400°C-500°C. This temperature is preferably maintained from 5 to 150 min, more preferred from 10 to 100 min. If desired, our method can include an additional step of further pyrolyzing the photoluminescent material produced by the methods above, by heating the photoluminescent material in a different gas, i.e. other than oxygen, to a temperature of at least 400°C for several hours. Reactive gases such as ammonia and nitrous oxide, or an inert gas such as nitrogen, can be used in this step. Photoluminescence of resulting materials are observed in the visible part of the spectrum, predominately the blue region, upon excitation of converted materials with an ultraviolet (UV) light having a wavelength of 337 nanometer (nm). The heating in the first embodiment of the present invention is conducted such that the material melts on its exterior surface before the conversion or cure reaction occurs, i.e., ceramification. Typically, heating is completed within three minutes. Preferably the heating rate is 30°C to 150°C per minute, more preferred 70°C to 120°C per minute and most preferred about 100°C per minute. According to our invention, the material employed is a preceramic silicon-containing resin, more particularly, a hydridosiloxane resin containing units of the formula HSi(OH) x (OR) y O z/2 . R is independently an organic group or a substituted organic group, which when bonded to silicon through the oxygen atom, forms a hydrolyzable substituent. Suitable R groups include alkyl groups such as methyl, ethyl, propyl and butyl; aryl groups such as phenyl; and alkenyl groups such as allyl or vinyl. The value of x is 0-2; y is 0-2; z is 1-3; and the sum of x + y + z is 3. These resins may be (i) fully condensed H-resins generally represented by (HSiO 3/2 ) n ; (ii) resins which are only partially hydrolyzed, i.e., containing some ≡SiOR; and/or (iii) resins which are partially condensed, i.e., containing some ≡SiOH. In addition, the resin may contain less than 10% of silicon atoms having either no hydrogen atoms or two hydrogen atoms or oxygen vacancies, as well as ≡Si-Si≡ bonds, which occur during their formation or handling. H-resins are ladder or cage polymers which generally conform to the following structure: Typically, n has a value of four or more. By way of illustration, when n is four, a bond arrangement for a silsesquioxane cubical octamer is depicted below. As the series is extended, i.e., n being five or more, double-stranded polysiloxanes of indefinitely higher molecular weight are formed, which contain regular and repeated crosslinks in their extended structure. Hydrogen silsesquioxane resins and a general method for their preparation are more completely described in US Patent 3,615,272. According to that patent, nearly fully condensed H-resin containing up to 100-300 parts per million silanol (≡SiOH), are prepared by hydrolyzing trichlorosilane (HSiCl 3 ) in a benzene sulfonic acid hydrate medium, washing with aqueous sulfuric acid and subsequently washing with distilled water until neutral. The solution is thereafter filtered to remove insoluble material and is then evaporated to dryness, leaving a solid resinous polymer in powder form. US Patent 5,010,159 teaches another method of hydrolyzing hydridosilanes that are dissolved in a hydrocarbon solvent with an aryl sulfonic acid hydrate medium to form the resin. A solid resinous polymer in powder form is conveniently recovered by removing the solvent. The solvent is usually removed by distilling off the solvent at atmospheric pressure to form a concentrate containing 40-80% of the resin and by then removing the remaining solvent under vacuum or mild heat. Other suitable resins are described in US Patent 4,999,397, including those resins produced by hydrolyzing an alkoxy or acyloxy silane in an acidic alcoholic medium according to JP-As 59-178749, 60-86017 and 63-107122. Solutions of resinous polymer are formed by simply dissolving or dispersing the preceramic silicon containing resin in a solvent or in a mixture of solvents. Some suitable solvents which can be employed are aromatic hydrocarbons such as benzene, toluene and xylene; alkanes such as n-heptane, hexane, octane and dodecane; ketones such as methyl ethyl ketone or methyl isobutyl ketone (MIBK); linear polydimethylsiloxanes such as hexamethyldisiloxane and octamethyltrisiloxane; cyclic polydimethylsiloxanes such as octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane; esters such as butyl acetate and isoamyl acetate; or ethers such as diethyl ether and hexyl ether. Generally, enough solvent is used to form the solutions, which typically range from a solids content of 10 to 85 weight percent of the resin. Gels can also be prepared from these solutions by adding a small amount of a solvent that contains an OH group, such as methanol, ethanol or isopropanol. A small quantity of water can also be used. In either case, typically, between 10 -4 to 10 -2 percent by volume of this additive (alcohol or water) are sufficient to cause gelation of the solution. The powder form of the resin and the resin gels referred to hereinafter in the examples, were prepared generally according to these procedures. In a series of laboratory experiments, it was observed that a powder form of hydrogen silsesquioxane resin with a molecular weight distribution of about 2 to 100 kilodalton (kDa) showed a weak blue photoluminescence when excited with UV light at a wavelength of 337 nm obtained from a pulsed nitrogen laser. When a gel of H-resin was prepared from a solution of the H-resin in methyl isobutyl ketone, the gel also showed photoluminescence. The photoluminescence of the gel was somewhat stronger than the photoluminescence of the H-resin powder itself and the photoluminescence of the gel had more color saturation in comparison to the photoluminescence of the H-resin powder itself. However, both the photoluminescence of the gel and the photoluminescence of the H-resin powder were bluish-green to the naked eye. Upon conversion by pyrolysis of H-resin powder and upon conversion by pyrolysis of the H-resin gel, in oxygen at 400°C., a deep blue aquamarine photoluminescence was observed that was stronger than the photoluminescence exhibited by the uncured species. When the pyrolytic conversion of the H-resin gel was performed at 500°C. in oxygen, a material was obtained that exhibited a very strong blue photoluminescence compared to treatment of the H-resin gel at 400°C. When the pyrolytic conversion of the H-resin gel was performed at 500°C. in nitrogen instead of oxygen, the material exhibited a similar, although somewhat less, strong blue photoluminescence compared to treatment of the H-resin gel at 400°C. However, when the H-resin gel was converted at a temperature of 600°C. in oxygen, the material turned yellowish-brown, which is attributed to carbon formation from solvent residue and the photoluminescent intensity decreased, compared to the photoluminescent intensity of the H-resin gel cured at 500°C. The photoluminescent properties of several commercially available silicon containing compounds were also compared with the photoluminescent properties of materials prepared according to this invention and it was found that H-resin gel according to this invention and which had been pyrolyzed at 500 °C, exhibited the strongest emission under identical excitation with UV light at 337 nm. The following Table I provides a qualitative comparison of the emission intensity and coloration, i.e., the color hue, of the observed photoluminescence of the several materials which were examined. Materials prepared according to this invention are italicized for convenience in the comparison. In Table I, Ph indicates the phenyl group -C 6 H 5 and Me indicates the methyl group -CH 3 . Excitation was provided by the use of a pulsed N 2 laser, 337 nm, 2 x 10 14 photons/pulse. Material Emission Intensity Coloration, Hue Dimethylsilane phenylmethylsilane copolymer PSS-120 [Me 2 Si/PhMeSi] n Medium Whitish Green Dimethylsilane phenylmethylsilane copolymer PSS-400 [Me 2 Si/PhMeSi] n Weak Whitish Green Pentaphenylcyclopentasiloxane [PhHSiO] 5 Medium Lucid Whitish Blue Octaphenylcyclotetrasiloxane [Ph 2 SiO] 4 Weak Lucid Whitish Blue H-resin gel in MIBK Weak Lucid Whitish Blue Polymethylsilane [-Me 2 Si-] n Weak Lucid Whitish Blue Pyrolyzed H-resin gel (silica) 400°C., O 2 Strong Aquamarine Blue CarboSil® L-90 fumed silica Weak Aquamarine Blue CarboSil® EH-5 fumed silica Weak Aquamarine Blue Hexaphenyl disilane Ph 6 Si 2 Strong Deep Aquamarine Blue MinSil® 600 fused silica Weak Deep Aquamarine Blue Polydiphenyl silane [-Ph 2 Si-] 5 Weak Deep Aquamarine Blue Hexaphenylcyclo trisiloxane [Ph 2 SiO] 3 Strong Deep Navy Blue Diphenylsilane diol Ph 2 Si[OH] 2 Weak Deep Navy Blue Diphenyl dimethoxy silane Ph 2 Si[OMe] 2 Weak Deep Navy Blue In Table I, CarboSil® is a trademark of the Cabot Corporation, Tuscola, Illinois; and MinSil® is a trademark of Minco Products Incorporated, Minneapolis, Minnesota. It was also determined that the following compounds exhibited no photoluminescence under conditions utilized in examining the compounds in Table I, i.e., phenyldichlorosilane PhHSiCl 2 ; octamethylcyclotetrasiloxane [Me 2 SiO] 4 ; decamethylcyclopentasiloxane [Me 2 SiO] 5 and dodecamethylcyclohexasiloxane [Me 2 SiO] 6 . As an example of the emission characteristics of materials according to this invention, the emission spectrum of a H-resin, prepared from a H-resin gel pyrolyzed at 400°C. for three hours in air according to the above procedure, was measured in the near UV region of the electromagnetic spectrum (i.e., 200-400 nm) and in the visible region of the electromagnetic spectrum (i.e., 400-800 nm), at 77°K under excitation with a 300 nm light. The emission peaked broadly at 430 nm, which is in the blue region of the visible spectrum. Scanning Electron Micrographs (SEM) showed that this photoluminescent material had a highly porous structure with particle size features well below the one micron (micrometer) range. Such solid pyrolyzed H-resins are believed to constitute the only completely inorganic silicon containing compounds capable of providing photoluminescent properties equivalent to the photoluminescent properties of organic aromatic molecules. In addition, these materials emit light in the blue region of the visible spectrum which is not readily accessible by most compounds. Because of these benefits, the materials are useful in flat panel color displays for data processing and computer equipment. This example demonstrates an increase by two orders of magnitude over the phosphorescence that was observed for an uncured H-resin. It also shows the identification of two different emission centers in the oxygen-cured H-resin by means of the position of their phosphorescence maxima and decay time constants. Thus, a quantity of a H-resin was cured by exposing it to a temperature of 420°C. in an oxygen atmosphere for 2.5 hours. The procedure consisted of loosely packing a powder form of H-resin into a quartz crucible and then placing the crucible into a furnace three centimeters above its bottom wall which was at the stated temperature, as the furnace was being purged with oxygen. When the sample was removed from the furnace, its surface was very smooth, as if it had become molten and it had a very white appearance. Beneath its surface, the interior of the H-resin sample was very porous and loosely packed. In its center, it had a fluffy portion which appeared yellowish in color. When the sample was irradiated with 337 nm light, the compacted material exhibited very strong blue photoluminescence which was much stronger than the corresponding uncured H-resin. The fluffy interior portion, however, showed a whitish-yellowish clearly non-blue photoluminescence under the same excitation conditions. The fluffy interior portion of the material did not phosphoresce, whereas the compact portion at the surface of the material clearly showed an extended greenish component which had a 2 to 3 second lifetime at room temperature. A phosphorescence spectrum was obtained by exciting the sample at a wavelength of 320 nm with a flash lamp and measuring the phosphorescence intensity at a delay time of 50 µs (microsecond) after the triggering of the lamp. The phosphorescence showed a strong maximum at 470 nm for the cured material, while the uncured H-resin was essentially devoid of any phosphorescence under similar conditions. Since the converted H-resin exhibited phosphorescence at room temperature with a maximum ranging from 470 nm to 500 nm and the unconverted H-resin exhibited no significant phosphorescence, it is believed that the conversion process produces a new electron state, characterized by a long-lived luminescence. The time constant of this luminescence radiation was determined from the decay of the emitted intensity as a function of time after the excitation light pulse and the value obtained at room temperature was 40 ms (millisecond). A second luminescing center was identified in the cured H-resin material when it was excited at 360 nm instead of at 320 nm. Under these conditions, the emission maximum was found at a wavelength of 560 nm and the lifetime of this center was 15 ms. The Table II below summarizes these results. Phosphorescence of Cured H-resin (delay time 50 microsecond, measured at room temperature) excitation wavelength λ exec [nm] 320 360 excitation energy hv [eV] 3.9 3.4 emission maximum λ e,mx [nm] 495 560 decay time constant λ ph [ms] 40 15 This example demonstrates the existence and quantification of long time constants of visible phosphorescence for an oxygen cured H-resin. Thus, measurements were performed to determine the phosphorescence decay time constants for an oxygen cured H-resin prepared from a H-resin gel, with the goal of identifying the number and spectral characteristics of the emission centers. A full emission spectrum was recorded after excitation at λ = 280 nm at increasing delay times after the excitation flash. It established that the spectral distribution did not change between 0.1 ms and 2 s; and that the emission maximum occured at a wavelength of λ = 438 nm. Otherwise, the spectral distribution consisted of a wide featureless band tailing off towards 650 nm. A semilogarithmic plot of the phosphorescence intensity at 438 nm versus time at long delay times after excitation revealed that the emission process was characterized by more than one time constant and that the decay mechanism therefore could not be described by a single electronic state. Rather, a short time constant of τ 1 = 90.7 µs and a long time constant of τ 2 = 3.44 s describe the experimental data more accurately, as seen in the semilogarithmic plot of phosphorescence intensity at the emission maximum (λ = 438 nm) versus delay time after excitation at λ = 280 nm. The plot revealed that at long times, i.e., t > 500 ms, the data points fell on a straight line indicating exponential decay. For shorter times, a different decay process dominated. The data were fit to a double exponential without offset over the full temporal range with a correlation factor of r = 0.9995. The large value of τ 2 = 3.44 s confirmed visual observations of a phosphorescence that continued for several seconds after the end of excitation with a 337 nm pulsed nitrogen laser.

Claims: A method of preparing a photoluminescent material comprising the steps of: (i) pyrolyzing a H-resin in powder form in a crucible in oxygen, by heating to a temperature of at least 350°C at a heating rate sufficient to cause the H-resin to melt on its exterior surface before the conversion or cure reaction occurs; and (ii) maintaining the temperature for up to 180 min. A method of preparing a photoluminescent material comprising the steps of: (i) preparing a gel from a solution of a H-resin in a solvent; (ii) pyrolyzing said gel in oxygen by heating to a temperature of at least 350°C; and (iii) maintaining the temperature for up to 180 min. The method of claim 2 in which the solvent is selected from the group consisting of aromatic hydrocarbons, alkanes, ketones, cyclic polydimethylsiloxanes, esters, ethers and mixtures thereof. The method according to claim 3 in which the gel is prepared by dissolving or dispersing the resin in the solvent or mixture of solvents to form the solution; and by adding to the solution, water, methanol, ethanol or isopropanol. The method according to any of the preceding claims in which the resin is a hydridosiloxane containing units of the formula HSi(OH) x (OR) y O z/2 wherein R is independently an organic group or a substituted organic group which when bonded to silicon through the oxygen atom forms a hydrolyzable substituent; x is 0-2; y is 0-2; z is 1-3; and the sum of x+y+z is 3. The method of claim 5 in which R is an alkyl group, an aryl group or an alkenyl group. The method according to any of the preceding claims in which the H-resin has a molecular weight distribution of 2 to 100 kDa. The method according to any of the preceding claims including the additional step of further pyrolyzing the photoluminescent material by heating the photoluminescent material in a gas different from oxygen to a temperature of at least 400°C for up to 2 hours. The method according to claim 8 in which the gas is ammonia or nitrous oxide. A photoluminescent material obtainable by the method according to any of the preceding claims. A photoluminescent material according to claim 10 having a smooth compact surface portion surrounding a porous and loosely packed interior portion, the smooth compact surface portion exhibiting blue photoluminescence when irradiated with 337nm light, and the porous and loosely packed interior portion exhibiting photoluminescence predominately at longer wavelengths when irradiated under the same conditions.

Inventors: PERNISZ UDO C, PERNISZ, UDO C.
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1999
Main classification: C09K11
IPC codes: C09K11, C09K9