Patent Application
20180118604
The present disclosure relates to techniques of forming doped silica glass that includes one or more trace elements.
Advances in analytical instrumentation for measuring trace level concentrations (<0.1 weight percent) of solid materials have revealed the importance of quantifying chemical components at very low mass fractions. In glass materials, components present at trace concentrations can have significant impact on physical or optical properties important for engineering and industrial purposes. In natural solid glassy and crystalline materials, equilibrium thermodynamic and kinetic processes operating at trace concentration levels have been shown to record a wealth of geologic information.
The importance of trace element chemistry on material properties produces a need for more accurate and precise measurements of trace-level components from small regions (<5 micrometers (μm)) of solid materials. To obtain a high degree of confidence with these measurements, well-characterized microanalytical reference materials are needed.
In some examples, the disclosure describes techniques for fabricating high-purity doped silica glass that includes a dopant of a selected element at a specified concentration. In some examples, the dopant may include specified concentrations of titanium such that the doped silica glass includes about 30 μg/g to 3000 μg/g of the titanium dopant.
In some examples, the described techniques may be used to form doped silica glass using a nanoporous silica gel, where the doped silica glass includes a specified mass fraction of a selected metal dopant such as titanium. In some examples, the metal dopant may be substantially uniformly dispersed in the doped silica glass. In some examples, the metal dopant may be uniformly dispersed at the intra-grain, inter-grain, and grain population scales. For example, the concentration of the dopant may be within a range of about ±10 μg/g of a nominal value throughout a volume of the doped silica glass. The doped silica glass may be useful as a reference standard for bulk analysis and microanalysis of sample materials with electron, laser, or ion-beam techniques.
In some examples, the disclosure describes method including adding a metal dopant to a silica gel slurry to form a mixture, wherein the silica gel slurry includes an activated silica gel and a solvent, mixing the mixture of the metal dopant and the silica gel slurry, and removing the solvent from the mixture to form a doped silica gel.
In some examples, the disclosure describes a doped silica glass including at least one layer of silica doped with a metal dopant at a concentration of about 30 μg to about 3000 μg of the metal dopant per gram of silica, wherein the metal dopant is substantially homogeneously dispersed within the at least one layer.
The disclosure describes techniques for fabricating high-purity doped silica glass that includes precise concentrations of a dopant such as titanium. In some examples, the techniques described may be used to form high-purity, amorphous silica that possesses both strong physical absorption and chemical affinity for dopant ions which may render such articles useful for fabricating microanalytical reference materials for trace element analysis. In some examples, the doped silica glass may be used as a reference standard for evaluating natural quartz or other test samples. For example, quartz (SiO2) rarely naturally occurs as a pure substance in nature and often contains numerous elemental impurities at trace (e.g., less than 1000 μg/g) concentration levels. As one example, titanium (Ti) concentrations in naturally-occurring quartz from most geological settings are on order of 1-100 μg/g. Due to the relatively slow diffusion rates of most trace elements in quartz compared with the rates of the crystal growth, trace element distributions in natural quartz are commonly inhomogeneous across small spatial scales. The emergence of trace element in quartz thermometers and barometers has generated interest in accurately detecting low elemental concentrations of impurities (e.g., on the order of 1 μg/g) from relatively small (e.g., <5 micrometers (μm)) regions of quartz materials. Other advances in understanding the solubility of trace elements in minerals has also generated interest in quantifying low elemental concentrations from micrometer-scale regions of natural crystals.
Attempts at in-situ trace element analysis made by pushing the limits of conventional techniques have been met with varied success, in part due to a lack of suitable reference standards available for testing instrument calibrations. In some examples, owing to uncertainties with coupling of the ion-, electron-, or laser-beam with glass matrices, measurement uncertainties at very low mass fractions can arise. Incorporating reference materials into an analysis routine improves the commutability and confidence of measurements and refines the petrological interpretations that rely on them. The accuracy and precision of such analysis depends on the use of an accurate reference standard for comparison.
In some examples, pre-fabricated reference standards may be obtained from the National Institute of Standards and Technology (NIST) for preforming some of these evaluations and calibrations. For example, the NIST SRM 610-617 glass references have become one of the most widely used reference materials for measuring trace elements in quartz. The NIST SRM 610-617 glasses were synthesized in the early 1970s as large (ca. 100 kg) batches of soda-lime glasses spiked with sixty-one trace elements at four different concentration levels. While the NIST SRM 610-617 glasses may be useful in certain applications, the NIST SRM 610-617 glasses were not intended for precise microanalysis and several difficulties and technical challenges have arisen in using these glasses for microanalysis because of inhomogeneities at the mass resolution scale of some analyses. For example, inhomogeneities of some trace elements have been detected in the high concentration NIST SRM 610 and 612 glasses. Additionally, the relatively high number of different trace elements contained in the NIST glasses can generate spectral interferences within the sample. Further, the NIST SRM 610-617 glasses are derived from a finite supply of material and the exact recipe for reproducing the glasses is unknown. As measurement techniques become more refined, the use reference materials that contain a specific concentration or bracket a range of concentrations of dopants that are close to the anticipated levels expected in unknown samples may be needed. Additionally, analytical interferences might be avoided by using materials whose impurity contents can be selectively controlled.
In some examples, the techniques described herein may be used to form reference standards of doped silica glass having an accurate and precise amount of a selected dopant. The doped silica glass described herein may be prepared using nanoporous silica gel, which under certain synthesis conditions, provides an ideal substrate for fabricating trace element doped materials because of the high absorptive capacity of the nanopores and the strong adsorptive capacity of the surface silanol groups of the silica gel. The doped silica glass described herein may be used to improve the confidence levels of petrogenetic reconstructions derived from measurements of trace-level titanium content analysis in quartz test samples. In some examples, the preparation methods outlined below may enable doping precision of about ±5 μg/g or better within a target concentration that is less than about 1000 μg/g. In some examples, due to the commercial availability for standard solutions of various doping materials available in multiple concentrations for most of elements on the periodic table, it is possible that using nanoporous silica gel as the doping substrate as described herein may allow for the fabrication of high-purity glasses with specified dopants at trace-level precision and high accuracy.
In some examples, using silica gel as the doping substrate may provide a high absorptive capacity for metal ions, in part because the pores of silica gel are strongly absorptive due to the nanoporous network. Any suitable type of silica gel may be used with the techniques of
In some examples, silica gel 26 may be synthesized through a sol-gel processing technique involving hydrolysis of silicic alkoxide precursors from which gelaceous silica condenses and is then dried to form a powder. By controlling the pH conditions and the rate of gelation during the sol-gel process, silica gel 26 particles can be synthesized with high levels of purity. In some examples, silica gel 26 may be obtained commercially with specific grain or pore size dimensions. Additionally, or alternatively, the silica gel may be produced or purchased commercially at various purity levels and standards. For example, commercially available silica gel may be acquired in high-purity or ultra high-purity forms with residual impurities ranging from about 0.5 percent by weight (wt. %) to about 0.001 wt. % or less residual impurities. In some examples, the purity level of the silica gel may be reduced via acid washing as described further below. Other techniques may also be available for producing silica gel with high purity levels.
In some examples, silica gel 26 may be essentially free of trace metals (e.g., less than 0.001 wt. % of any specific metal (e.g., less than 0.001 wt. % titanium) such that the background impurities are within the precision of the doping techniques described herein. In other examples, silica gel 26 may include higher levels of initial trace metals/impurities. In such examples, the doping techniques described herein may be applied to such materials as a method of adding a specified amount of the dopant material to the background levels of the initial trace metals/impurities.
As shown in
The technique of
a±1.03 (n = 7).
Following the acid-rinsing, silica gel 26 may washed thoroughly with, for example, de-ionized water (DIW) until no amounts of the acid are detected in the decanted rinsings using, for example, AgNO3 as the indicator. In some examples, the DIW wash cycles may take more than 25 total washings to sufficiently remove the acid. After substantially all acid has been removed, the resultant activated silica gel may be dried using a conventional furnace under a low-temperature (e.g., about 100-120° C.), long duration (e.g., on the order of 100 hours) heat treatment to evacuate residual volatiles from the pores of the activated silica gel without damaging the fragile silica pore network. In some examples the activated silica gel may be stored for an interim period in an air-tight container.
The techniques of
In some examples, the activated silica gel may be added to the solvent in aliquots greater than about 3 μL. In some examples, it may be preferable to use a solvent having shorter chain alkanes (e.g., chain lengths less than the pore size of silica gel) to increase the amount of dopant retained by the activated silica gel. In some examples, the solvent may be ethanol, which is a short-chain, inexpensive, and relatively low-hazard material. In some examples, longer chain alkanes (e.g., heptane, hexadecane, squalane) may result in a decrease in doping efficiency due to physical obstruction of the nanopores in the activated silica gel. In some examples in which the solvent is organic, the solvent may include a carbon chain of 7 or less.
After formation of silica gel slurry (14), a dopant material may be added to the silica gel slurry (16). In some examples, the dopant may include any suitable dopant including, but not limited to, aluminum (Al), antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), bismuth (Bi), boron (B), cadmium (Cd), calcium (Ca), carbon (C), cerium (Ce), cesium (Cs), chromium (Cr), cobalt (Co), copper (Cu), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), gallium (Ga), germanium (Ge), gold (Au), hafnium (Hf), holmium (Ho), indium (In), iridium (Ir), iron (Fe), lanthanum (La), lead (Pb), lithium (Li), lutetium (Lu), magnesium (Mg), manganese (Mn), mercury (Hg), molybdenum (Mo), neodymium (Nd), nickel (Ni), niobium (Nb), osmium (Os), palladium (Pd), phosphorous (P), platinum (Pt), potassium (K), praseodymium (Pr), rhenium (Re), rhodium (Rh), rubidium (Rb), ruthenium (Ru), samarium (Sm), scandium (Sc), selenium (Se), silicon (Si), silver (Ag), sodium (Na), strontium (Sr), sulfur (S), tantalum (Ta), tellurium (Te), terbium (Tb), thallium (Th), thulium (Tm), tin (Sn), titanium (Ti), tungsten (W), vanadium (V), ytterbium (Yb), yttrium (Y), zinc (Zn), zirconium (Zr), or other elements of interest. In some examples, the dopant may be a metal such as a transition metal (e.g., titanium). In some examples, the dopant may be provided as a standard solution (e.g., metal plasma standard solution). For example, in the examples of a titanium dopant, the titanium dopant may be provided as a plasma standard solution, for example, Ti in 5 g/100 g HNO3. Such standards may be commercially-available for most metal dopants in different concentrations. For example, standards of titanium may be purchased in concentrations of 10 μg/mL (e.g., Alfa Aesar, no. 45267) or 1000 μg/mL (Alfa Aesar, no. 35768).
In some examples, the dopant (e.g., titanium) may be added to silica slurry gel using a micropipette to add precise amounts of the metal standard solution (e.g., Ti in 5 g/100 g HNO3). In some examples, the use of a micropipette may provide a 1-3 μL precision which may translate to a minimum measurement bias in doping precision of 3-8 μg/g depending on the concentration of the standard solutions. In some examples, the resulting precision of the dopant added may be about ±5 μg/g based on the amount of silica (e.g.,
After adding the dopant to the silica gel slurry, the pH of the mixture optionally may be adjusted (18) to values between about 7 to about 10 using titration. The pH of the mixture may influence the silanization of the silica surface as well as the stability of the dopant (e.g., titanium) species in the plasma standard solution (in examples in which a plasma standard solution is used). In some examples, the pH of the mixture may be adjusted to about a pH of 8.
For example, the resultant mixture may be titrated using 3 mol/L NH4OH and 0.3 mol/L NH4OH as base buffers and 0.2 mol/L HNO3 as an acid buffer to stabilize the pH. Optionally, in some examples, once the pH of the mixture is adjusted (18) at the desired level, the vessel containing the activated silica gel, dopant, and solvents may be placed into an oscillating apparatus (e.g., for about 3 hours) to ensure sufficient mixing (20). After being thoroughly mixed, the contents of the mixture may be filtered (22) using, for example, vacuum filtration to remove the solvents. The resultant material may be placed in a conventional furnace (e.g., at 100° C. for 24 hour then at 120° C. for 120 hour) to produce a doped-silica gel. The doped silica gel may be stored in an air-tight container prior to further processing.
The technique of
In some examples, doped silica gel may be hot-pressed as multiple glass layers (e.g., 3-7 layers of doped silica glass), each having its own specified concentration of dopant. For example, collections of doped silica gel each at different concentrations may be hot-pressed for short duration to compact and densify (without crystallizing) the silica glass layers. In some such examples, crystallization of the silica during hot-pressing should be avoided as the solubility of the dopant in the growing crystals may impact the desired dopant concentration in resultant silica layers. In some such examples, the glass layers may be hot-pressed at 100 MPa over a temperature ramp to a target of about 1100° C. over a duration of about 1.5 hour, followed by a pressure increase to about 300 MPa to form the layered aggregates without inducing crystallization of the silica gel. In some examples, each layer may define a thickness of about 0.5 mm to about 3 mm with each layer including a selected dopant concentration of between about 30-3000 μg/g of the dopant relative to the mass of silica (e.g., about 30-3000 μg dopant/g silica) with the dopant substantially homogeneously dispersed throughout the respective layer. In some examples, the respective layers may consist essentially of silica and the dopant such that any other impurity is less than or equal to the level of doping precision. For example, the respective layers may consist of silica, the dopant, and less than about 10 μg/g of any other impurity (e.g., trace metals). Additionally, or alternatively, the respective layers may include silica, the dopant, and background impurities. In such examples, each layer may include a specified and incremental change in dopant concentration (e.g., step changes of 30 μg/g or more) in addition to any background impurities. In any of the above examples, the dopant may be substantially homogeneously dispersed within a given layer. In some examples, the glass layers may also include a blank/reference layer where the dopant material has not been added.
The technique of
Activation of silica gel 26. Approximately 30 g of silica gel was placed into a pristine 250 ml Erlenmeyer flask using plastic disposable spatula. Approximately 100 mL of 6 mol/L HCl was added to the to the flask using a graduated cylinder along with a 1.25 inch PTFE magnetic stir bar. The flask containing the mixture was place onto a stirring plate and stirred on low power under a fume hood with a piece of parafilm adhesive paper placed over the flask to reduce evaporation. Stirring of the mixture commenced for durations of 3-168 hours. After stirring, the flask was removed from the stirring plate and approximately 100 mL DIW was added. The mixture was returned to the stir plate and stirred on low power for 1 min before the stir bar was removed. The flask was then placed into a sonicator for 5 min to settle before being decanted. Approximately 100 ml of DIW was added and swirled vigorously in the flask for 30 seconds, and the flask was returned to the sonicator for an additional 5 min. The mixture was decanted and tested for chloride in rinsings using 0.100 mol/L AgNO3. The rinsing procedures were repeated with DIW until no chloride appeared in the rinsing (>25× total rinsings). Once obtained, the mixture was decanted to remove as much liquid as possible before placing the flask into conventional oven at 100° C. for 24 hours. The resultant powder was transferred from the flask into a glass petri dish, placed into a conventional oven at 120° C. for 120 hours. The activated silica was then weighed and stored in an air-tight container.
Example titanium dopant procedure for activated silica gel. Approximately 1.500 g of activated silica gel with 40 mL ethanol solvent was added into a 60 mL Nalgene plastic jar with screw lid to create a silica gel slurry. A titanium plasma standard solution was added to the jar at a desired concentration. The amount of titanium plasma standard solution added was calculated based on the following examples, to dope 1.500 g silica gel at a level of 802 μg/g, 1200 μL of a 1000 μg/mL Ti plasma standard solution and 372 μL of a 10 μg/mL Ti plasma standard solution were added. To dope 1.500 g silica at a level of 2916 μg/g, 4370 μL of the 1000 μg/mL plasma standard solution and 681 μL of the 10 μg/mL plasma standard solution were added. The cap was screwed onto the jar and shaken vigorously for several seconds. The solutions were titrated to the desired pH (e.g., pH of 8) with ammonium hydroxide (3 mol/L and 0.3 mol/L NH4OH) and nitric acid (0.2 mol/L HNO3). As an example for the titration, to reach a pH of 8 at the 802 μg/g doping concentration level, the pH was buffered with about 355 μL of 0.3 mol/L NH4OH; at the 2916 μg/g doping concentration level, the pH was buffered with about 920 μL of 0.3 mol/L NH4OH. After pH of 8 stabilized, the cap was screwed onto jar and placed into an oscillating apparatus for 3 hours. The jar was removed from oscillating apparatus and the contents filtered using vacuum filtration assembly equipped with, e.g., 0.45 μm nylon filter paper. The contents were rinsed with DIW and the rinse was included in the filtration apparatus. About 1 L of DIW was used to rinse a 1.5 g sample. Once completed, the filter was extracted from the filtration apparatus and the filter and powder were placed in a glass petri dish and dried in conventional oven at approximately 100° C. for 24 hours. The filter paper was removed, and the petri dish of power material was placed into a conventional oven at 120° C. for about 120 hours. The titanium doped silica gel was extracted to obtain the final mass before being stored in air-tight container.
Table 2 below shows the results of coupled plasma-optical emission spectrometry (ICP-OES) performed on 100 mg samples of doped silica glass prepared using the techniques of
The concentration levels of titanium dopant were measured in doped silica glass using an electron probe microanalysis (EPMA) with a Cameca SX-100 electron microprobe equipped with enlarged diffracting crystals (LPET) and LaB6 electron source. Titanium K-α X-rays were collected for 120 seconds on peak and 60 seconds on high/low background from four spectrometers and aggregated. Si K-β was measured on the final spectrometer (TAP). Analysis of unknown samples was performed at 15 kV and 200 nA with a 10 μm spot size. Standardization for titanium was performed on rutile at low current (15 nA) to help prevent peak shifts in the pulse-height analyzer at high count rates. Calculated detection limits for titanium in quartz following this routine demonstrated a detection precision of about 7 μg/g.
A comparative study was performed to compare the effectiveness of silica gel 26 as the doping substrate versus crystalline quartz as the doping substrate. Pure quartz separates from Black Hills Quartzite (BHQ) at 20-50 μm grain size were activated in HCl and doped with 300 and 3000 μg/g following the same techniques of
Doped silica gel was tested for use as reference materials by fabricating multi-layered aggregates doped with different concentrations.
Various examples have been described. These and other examples are within the scope of the following claims.
This application claims the benefit of U.S. Provisional Patent Application No. 62/417,051, filed Nov. 3, 2016, the entire content of which being incorporated herein by reference.
This invention was made with government support under EAR-0911497 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62417051 | Nov 2016 | US |
