Patent Application
20170108598
One embodiment is directed generally to thin-film coatings, and in particular to boron based thin-film coatings.
A thin-film coating is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. A thin-film coating is typically provided using a deposition process, which is a controlled synthesis of materials as thin-films. Advances in thin-film deposition techniques have been a significant step in the development and improvements of a wide range of technology, including magnetic recording media, electronic semiconductor devices, LEDs, optical coatings (such as antireflective coatings), hard coatings on cutting tools, energy generation (e.g., thin film solar cells and thin-film batteries) and drug delivery.
One embodiments is an apparatus that includes a first layer of a rare earth element. The apparatus further includes a thin-film coating layer deposited on the first layer, where the thin-film coating layer includes boron.
One embodiment uses thin-film overcoat layers of boron, either in the form of elemental boron (“B”), or boron carbide (“B4C”), or boron nitride (“BN”), as a protective layer over rare earth metals, such as gadolinium (“Gd”), europium (“Eu”), lanthanum (“La”), neodymium (“Nd”), etc. The rare earth metals normally form unstable flaky oxides in moist air that spall off, leading to rapid corrosion and sometimes complete disintegration of the bulk rare earth metal. However, the thin-film overcoat layers in accordance to embodiments of the invention serve to prevent this disintegration.
In general, Gd, like many of the rare earth elements, rapidly forms a “flaky” surface oxide when exposed to moist air. This loosely adhering surface oxide quickly spalls off as a colorless (or white) powder within hours or days, thereby exposing more metal surface to rapid oxidation. This cycle of continuous oxide corrosion can rapidly lead to complete disintegration of the entire Gd metal layer within days, or months, or longer, depending on the layer thickness and the ambient humidity.
However, embodiments remedy this surface oxide flaking problem in ambient air, without significantly affecting the Gd atomic properties, by depositing a low density, non-porous, hard thin-film surface coating with strong adhesion to the underlying Gd layer. Although embodiments are applicable to all elements that exhibit this oxidation problem (e.g., iron (“Fe”)), it is particularly relevant to the rare earth metals, including the Lanthanide and Actinide series, and in particular Gd, Eu, La, and Nd. Further, in one embodiment that uses Gd as a neutron conversion layer, the disclosed thin-film surface coating can itself function as a supplemental neutron conversion layer. In addition to being extremely thin and low density (i.e., highly transparent to emitted Gd conversion electrons), the coating in accordance with embodiments has the benefit of using a low atomic number material that minimizes further backscattering (i.e., beyond that from the bulk Gd itself) and absorption of the emitted Gd conversion electrons.
In one embodiment, “thin-film” refers to an effective thickness generally less than ˜5 μm, and often a thickness of less than 0.5 μm. In other embodiments, the thickness can be less than 0.01 μm. For Gd as a neutron conversion layer, the boron protective overcoat can provide further benefits by using the boron-10 isotope (10B) which in itself is a neutron conversion material and yields a slightly lower density coating than the principal 11B isotope. In this regard, 10B also has a slightly lower density than 10B4C, so for a given mass-areal coating, there is less boron present in B4C than in an equivalent mass-areal coating of pure (i.e., neat) boron. However, for this application the protective overcoat layer can utilize either a thin-film of elemental 10B or 10B4C, because both materials are slightly conductive and the coating should not be an electrical insulator. In other embodiments, natural boron (i.e., 19.9% 10B, and 80.1% 11B) in the form of elemental B or B4C can be used.
Embodiments further involve the use of a thin-film overcoat protective and/or passivation layer of B, or B4C, or BN, over the rare earth metal nitrides (“REN”), which like the rare earth metals, are unstable and reactive in air as they undergo rapid oxidation. The rare-earth nitrides are important because they show great promise in applications ranging from spintronics, to infrared (“IR”) detectors, and even as contacts to III-V compounds.
For REN, BN might be the favored thin-film boron protective overcoat. Ostensibly, BN has the potential advantage of being able to form a strong bonded transition layer to the REN material by forming interstitial bonds to both the B and N atoms of the BN layer. As a protective coating, BN is extremely hard and chemically stable, even at high temperatures. As an insulator, a thin-film BN protective overcoat/passivation layer could be very thin (e.g., less than 0.01 μm).
PPS 10 is based on surface-discharge, 4-electrode configuration in which the front conductive layer 22 can serve as a front electrode or drift electrode which can also be a thin metal coating. In another embodiment, the front conductive layer can also be a conversion layer or thin sheet such as gadolinium (Gd) foil that can capture a neutral ionizing particle such as a thermal neutron and then emit a fast conversion electron (e.g., 72 keV) into the discharge gas 16. For many applications the PPS front conductive layer 22 can be combined with the front substrate 12 by making the front substrate a metal plate or metal foil. For detector 10, the gas gap is also known as the “drift region” for the discharge gas that fills the region between the front substrate 12 and the back substrate 14.
PPS 10 in one embodiment is a highly integrated array with roughly 10 to 106 micro-detection cells per cm2, each of which can act as an independent, position-sensitive, radiation sensor. PPS embodiments, in general, efficiently collect free-electrons and ions created in a gas by the passage of an ionizing particle and then, via the drift field, “channel” the electrons and ions into the higher field region where an avalanche develops leading to breakdown.
A PPS in accordance with one embodiment uses a discharge gas that fills the discharge-gap which defines an orthogonal ion-pair creation drift region of the PPS pixel array 10 of
In
In considering a suitable protective surface treatment overcoat for the application of Gd as a thin-film conversion layer for Gd-based neutron detectors, such as for front conductive layer 22 of
In one embodiment, the overcoat material has a low sputtering yield with respect to ion bombardment. Three electrically-conductive elements can be used in one embodiment to provide this feature: Be, B and C, as well as the compound B4C. Although Be has the lowest density, it has a somewhat higher sputtering yield than either B or C, but more importantly it is toxic and thus requires special handling and so is more difficult to work with. Carbon in the form of either graphite or poly-diamond films is physically acceptable, but diamond, which is the more stable form of carbon, in addition to having a significantly higher density than B, is also orders-of-magnitude less conductive than B which could prove problematic and such films are more difficult to fabricate. Graphite is a good conductor, but is also difficult to fabricate as a film in its pure form without also getting some amount of amorphous carbon which is not stable with respect to physical migration in a plasma discharge environment.
Carbon as graphene can be used if it could be fabricated thick enough and uniformly over the Gd coated “vertical” cathode side walls 255 of a microcavity-PPS based neutron detector of
With regard to carbon, there are two known carbides of Gd—GdC2 and Gd2C3. An interstitial Gd—C bonding layer (i.e., interface layer) between either graphite or graphene to Gd is used to provide adhesion in one embodiment. The carbon in B4C also provides some adhesive benefit in bonding B4C to Gd. However, the boron atoms themselves are more likely than the carbon atoms in B4C to form a strong interstitial Gd bond (i.e., Gd—B) as the rare earth borides are well known commercially including their use as cathodes for various applications.
More specifically, all of the rare earth elements form “stable” hexaborides, so by choosing boron in one embodiment a highly adherent hexaboride (GdB6) interface layer forms that bonds the boron to the gadolinium. Alternatively, a lower polyboride composition adhesion/interface layer is formed, such as the tetraboride (GdB4) or diboride (GdB2). In either embodiment, both 10B and/or 10B4C have strong adhesion to Gd via interstitial atoms of boron forming strong covalent Gd—B bonds between the adjacent Gd and B layers.
In this regard GdB6 is much more stable than Gd metal, as evidenced by the melting point of GdB6 being 2510° C. versus that of Gd being 1313° C. Further, GdB6 is stable in moist air, while Gd disintegrates via oxidation as previously discussed. Therefore, in one embodiment, boron is chosen over carbon, with 10B chosen over 10B4C, and a layer of pure 10B will bond more strongly to Gd than would 10B4C. Finally a 10B thin-film coating is easier to fabricate by electron beam (E-beam) deposition than 10B4C.
To demonstrate the viability of embodiments of the invention for application with Gd based neutron detectors, in one embodiment for gaseous micropattern type detectors a thin-film of Gd with a 10B overcoat was successfully patterned by E-beam deposition (i.e., the 10B and Gd were deposited within the cavity walls) on several standard microcavity-PPS glass-ceramic (i.e., Macor®) substrates that had been previously pattern coated with thin-film Pt using a Cr adhesion layer. An adhesion layer of ˜0.1 μm of Cr was used for the Gd/10B deposition. The resulting Cr/Gd/10B thin-film deposition run on the Pt coated microcavity substrates also included both alumina-ceramic and glass substrate witness slides that were coated at the same time, and which were subsequently used for the test/analysis results summarized below.
Unlike the microcavity glass-ceramic substrates which had been previously coated with Cr/Pt, the ceramic and glass witness slides had no previous metallization and so the Cr adhesion layer was coated directly on the “bare” witness slide substrates. Measurement of the deposited thin-film on the ceramic witness slide by scanning electron microscope (“SEM”) determined that the Gd layer thickness was ˜2.8 μm, with the 10B overcoat layer thickness ˜0.9 μm. The microcavity substrate size was 56 mm×56 mm and 1.5 mm thick, with each cavity having rectangular dimensions of 1.0 mm×2.0 mm, and being 1.0 mm deep, as shown in
The initial coating quality observations were made two days after the deposition and reconfirmed more than a year later with the witness slides left open to the ambient atmosphere. Observations indicated that the Cr/Gd/10B thin-film coating stuck very well to the ceramic substrates and only came off in a few areas on the glass substrates. The patterned areas on the glass substrate held very well. The deposited film on both types of substrates did not show any sign of degradation in the open atmosphere. After several more days of sitting in the open environment, no flaking or degradation of the film could be discerned, and in trying to scrape the coating with a gloved finger, nothing came off. Even after 10 months from the Cr/Gd/10B deposition, during which the ceramic witness slide had been left in an open Petri dish, continuously exposed to the ambient air/humidity atmosphere, absolutely no flaking or degradation of the thin-film Cr/Gd/10B coating was observed.
The film has also been observed under a microscope, and rubbed, and lightly adhesion tested using Scotch® tape, and still no flaking or degradation had been observed. However, under a more aggressive Scotch® tape adhesion pull test (i.e. using Scotch® Magic™ Tape No. 810) the entire Cr/Gd/10B coating did pull off cleanly from some sections of the ceramic witness slide, leaving the substrate “bare” in these sections after 10 months. This adhesion failure to the ceramic substrate could be viewed positively in that it demonstrated that the 10B to Gd adhesion is, and has remained, very strong, since the tape only made contact with the 10B top surface layer, and the 10B pulled with it both the Gd and Cr coatings underneath. Thus not only was the 10B to Gd adhesion very strong, but also the Gd to Cr adhesion. However the Cr to ceramic substrate adhesion has obviously deteriorated with time and exposure to the ambient atmosphere. It is noted that the cleaning procedure for the glass and ceramic substrates prior to the thin-film deposition was quite minimal and a more aggressive substrate surface cleaning process prior to deposition would help.
The neutron detection efficiency of the Gd/10B coated microcavity-PPS neutron detector with its Gd/10B coating over the microcavity cathode walls shown in
Further, in one embodiment, to maximize the geometric fill-factor for maximum efficiency, the wall thickness between adjacent cavities needs to be minimized.
In experimental results for one embodiment, the resistivity of the 10B coating after 10 months of ambient atmosphere exposure was evaluated using pointed probes placed from ˜0.1 to 1 cm apart, and yielded values ranging from hundreds of kΩ to ˜1 MΩ. However, when the probes were pressed down very hard, thereby penetrating the 10B surface layer to the Gd/Cr base layer beneath, the resistivity dropped by approximately four (4) orders-of-magnitude to less than 50Ω. The pressure required to penetrate the surface layer was considerable, attesting to the hardness of the ˜0.9 μm thin-film 10B coating. This experiment further demonstrated that although 10B is a poor conductor, it is not an insulator, and is more than adequate as a cathode if coated as a thin-film over a conductive metal such as Gd, which has ˜80× higher resistivity than Cu. In addition, this experiment revealed a new application for boron coatings in that a thin-film coating of B and/or B4C, and possibly other non-metals and semiconductors such as nitrides, oxides, silicon, etc., on an insulator or semiconductor substrate surface such as silicon, glass, ceramic, or polymer, can be used for high-resistivity thin-film vertical or conventionally oriented high-resistivity thin-film planar resistors. In the case of B, it was thus demonstrated that ˜1 μm of B will result in a physically stable resistive layer with very high resistivity compared to conventional resistor materials such as nichrome.
Embodiments can be used with thin-film coatings of B and B4C over other types of Gd or Gd2O3 based neutron detectors, including other types of Gd coated gas detectors such as gas electron multipliers (“GEM”), Gd coated vacuum detectors such as multichannel plate (“MCP”) detectors, and Gd coated semiconductor detectors.
Embodiments can be used for other additional applications. For example, one potential application is to improve the physical properties of rare earth magnets, which are known to be extremely brittle and vulnerable to corrosion. Such magnets are usually plated or coated to protect them from breaking, chipping, or crumbling into powder. In particular, neodymium (Nd) magnets are generally considered the strongest and most affordable type of rare earth magnet, and are made of either a sintered or bonded ahoy of neodymium-iron-boron (e.g., Nd2Fe14B), abbreviated as NIB.
Rare earth magnets are used in numerous applications requiring strong, compact permanent magnets, such as electric motors for cordless tools, hard drives, magnetic hold downs, jewelry clasps, etc. They have a number of excellent magnetic properties, but are more vulnerable to oxidation than samarium-cobalt magnets. Corrosion can cause unprotected NIB magnets to spall off a surface layer, or to crumble into a powder. The use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection, but even with such coatings these magnets are still brittle and lack mechanical strength. If however the Nd or NIB particles were B, or B4C, or BN coated, and even better if they were hot-pressed (i.e., sintered) after being B, or B4C, or BN coated, then the physical deficiencies of these magnets can be significantly alleviated, including their brittleness and loss of strength upon continuous exposure to humid air.
Further, in connection with stability, for the three protective boron based overcoats disclosed above, BN has the highest melting point of 2967° C. and in its cubic form is almost as hard as diamond and is generally considered to be chemically more stable than diamond. In comparison, B4C has a melting point of 2350° C., and elemental crystalline B has a melting point of 2077° C. Both B4C and elemental B are very hard, but neither is as hard as BN.
As disclosed, embodiments use boron based thin-film coatings for rare earth metals and rare earth nitrides to enhance their physical and/or chemical stability, and/or the performance or functionality of such devices based on these materials for a number of different applications. Examples of such applications include gadolinium (Gd) based neutron detectors, rare earth based magnets, infrared detectors, and a variety of spintronic devices such as for memory storage, magnetic sensors, and for quantum computing. Further, the coatings in accordance to embodiments can also be used for achieving high resistivity in thin-film based devices including both vertical and planar resistors.
Several embodiments are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the disclosed embodiments are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.
This application claims priority to U.S. Provisional Patent Application Ser. No. 62/284,938 filed Oct. 14, 2015, the specification of which is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62284938 | Oct 2015 | US |
