Abstract:
The invention resides in a molecular, inorganic glass and a method of making the glass, the glass being vitreous and resistant to devitrification, that is composed, in substantial part at least, of thermally-stable, zero-dimensional clusters or molecules, composed of four atoms of arsenic and three atoms of sulfur, the glass further containing up to 12 atomic percent of germanium, adjoining clusters being bonded to each other primarily by van der Waals forces, and at least 95% of the glass composition consisting of 42-60% arsenic, 37-48% sulfur plus selenium, the selenium being 0-14%.

Description:
RELATED PATENT APPLICATION 
     The present application claims benefit of priority as a continuation-in-part of U.S. patent application Ser. No. 09/894,587, filed on Jun. 28, 2001, now U.S. Pat. No. 6,503,859, which is incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to molecular, inorganic glasses composed, at least in part, of one species including either thermally stable (i.e., persisting after annealing), zero-dimensional molecules or network-forming groups, composed of arsenic and sulfur atoms and associated with germanium atoms. 
     BACKGROUND OF THE INVENTION 
     The physical structure of a glass may be characterized by its dimensionality, that is, by the number of directions in which its components extend. Thus, vitreous selenium, which is believed to consist of intertwined chains of selenium atoms, is said to have a one-dimensional structure. Likewise, glassy As 2 S 3 , which consists of corrugated sheets of As 2 S 3  pyramids that share corners, is described as having a two-dimensional structure. Considered in these structural terms, most oxide glasses are said to be three-dimensional. 
     A certain class of crystalline, inorganic compounds, known as molecular solids, has a zero-dimensional structure. This structure consists of molecular clusters in a cage-like form. As such, the clusters are repeated periodically in three dimensions, but are only bonded to each adjacent cluster by van der Waals forces. Examples of crystalline, inorganic compounds that have such a structure are known. They include arsenic sulfide, which consists of eight atom clusters designated As 4 S 4 , and phosphorus sulfide, which consists of 14 atom clusters designated as P 4 S 10 . 
     The possibility of an inorganic glass having such a structure has been suggested by R. Zallen. Zallen has suggested that such a zero-dimensional structure may occur temporarily in amorphous arsenic sulfide films which have been fabricated by vapor-phase deposition. Although such structure is found in these films initially upon cooling, it is thermally unstable in that it is lost upon annealing the films. 
     SUMMARY OF THE INVENTION 
     The present invention is based on a discovery that bulk, inorganic glasses can be produced, which consist almost entirely of thermally-stable, zero-dimensional clusters. These novel glasses have some physical properties similar to those observed in organic plastics. The glasses, however, also exhibit other properties that more closely resemble those found in conventional, three-dimensional, inorganic glasses such as complete transparency between the visible and infrared cutoff wavelengths. Unlike plastics, the present glasses are also essentially impermeable to gases and moisture. Hence, for instance, these glasses may be useful as low-temperature, hermetic, sealing materials in various industrial applications. 
     The invention resides, in part, in an inorganic glass that is resistant to devitrification. At least about 95% of the glass composition, in atomic percent, comprises about: 42-60% arsenic (As), up to 48% sulfur (S), 0-14% selenium (Se), or optionally, up to about 12% germanium (Ge); and comprising, in substantial part, at least one species including either thermally stable (i.e., persisting after annealing), zero-dimensional molecules or network-forming groups. Preferably, the glass has 37-48% sulfur, or the content of S+Se is about 37-48%. As used herein, the term “atomic percent” refers to the actual percent of the total number of atoms present in a composition. Thus, the value given for a particular element represents the percent of atoms of that element present in a composition relative to 100 percent for the total number of atoms in the composition. 
     According to an embodiment, at least a portion of the zero-dimensional molecules or clusters has a composition comprising either four atoms of arsenic and three atoms of sulfur (As 4 S 3 ), or four atoms each of arsenic and sulfur (As 4 S 4 ). The zero-dimensional molecular clusters are bonded to each other primarily by van der Waals forces. Alternatively, at least a portion of the network-forming groups comprise a variety of species, expressed in terms of As—(S, As) n  units, wherein n≠zero. Examples may include either AsS 3  (AsS 3/2 , wherein the S atoms are shared with other adjacent As atoms), AsAsS 2 , AsSAs 2 , or AsAs 3 . 
     The invention further resides in a method of producing a thermally-stable, As—S glass that is composed, in part at least, of zero-dimensional clusters composed of four atoms of arsenic and three atoms of sulfur, or four atoms each of arsenic and sulfur. The method comprises forming a mixture of 42-60% arsenic, 37-48% S+Se, wherein the Se is 0-14%, with, optionally, up to 12% germanium (Ge), and melting the mixture to form a devitrification-resistant, inorganic glass melt. 
     Additional features and advantages of the present method and array device will be disclosed in the following detailed description. It is understood that both the foregoing summary and the following detailed description and examples are merely representative of the invention, and are intended to provide an overview for understanding the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of an As 4 S 3  molecular cluster in accordance with the invention, and 
     FIG. 2 is a ternary diagram showing the compositional area in accordance with the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention is based in-part on a discovery of bulk glasses that contain either thermally-stable, zero-dimensional, atomic clusters or molecules, or network-forming groups. Although these chalcogenide glasses exhibit properties comparable to those observed in conventional, inorganic glasses having three-dimensional structures, certain embodiments of the unique material also possess some properties similar to those found in ordinary, organic plastics. For instance, the glass is plastically malleable at a temperature of less than or equal to about 125° C. Depending on specific compositions, the temperature at which the glass is plastically malleable can be as low as≦about 50° C. In some embodiments, one can begin to bend plastically the glass at a temperature slightly above normal human body temperature. Therefore, these unique glasses are termed “plastic glasses.” 
     The glasses may be almost entirely composed of the zero-dimensional clusters. A characteristic species of such clusters of atoms in the present glasses are composed of four (4) arsenic atoms and three (3) sulfur atoms. Other species with four As and four S atoms also may co-exist. The species having seven (7) atoms is believed to be arranged in a cage-like manner as shown in FIG. 1 of the accompanying drawings. The structure, as shown, extends in three directions. The clusters exist independently, except as they are bonded together by van der Waals forces. 
     The role of germanium in the present glasses is not clear. It is not known whether or not the germanium atom actually enters the As 4 S 3  cluster as such. In any event, it appears to be closely associated with the As 4 S 3  cluster and is influential in the observed properties of the glasses. 
     Glass formation in the GeAsS system is, of course, well known. Yet, the present compositional area lies outside known glass-forming regions. Intermediate to these regions is a narrow zone in which compositions may be melted, but in which they tend to phase-separate, or spontaneously crystallize, on cooling. 
     The present, inventive, plastic glasses have an electronic absorption edge that lies at a shorter wavelength than that of known GeAsS glasses with comparable sulfur content. This characteristic results in the plastic glasses possessing unexpectedly broad, visible transparency. For example, the absorption edge of known GeAsS glass containing 45% sulfur lies at wavelengths in excess of 700 nm, whereas that of a plastic glass with the same sulfur content occurs at 620 nm. 
     TABLE 1, below, sets forth, in atomic percent, several compositions, which are illustrative of glasses in which the atoms substantially completely occur in clusters, as illustrated in FIG.  1 . Also summarized in the TABLE are the exceptionally low Tg temperatures that characterize these glasses, as well as the softening point (T s ), density, electrical resistivity (log ρ) and coefficient of thermal expansion data (CTE) values measured on the glasses. The latter are exceptionally large, being among the highest values observed in inorganic glasses. 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Element 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (Atomic %) 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
                 Ex. 6 
                 Ex. 7 
                 Ex. 8 
                 Ex. 9 
                 Ex. 10 
                 Ex. 11 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Ge 
                 3 
                 1.3 
                 2.5 
                 1.3 
                 1.3 
                 2.5 
                 3 
                 3 
                 3 
                 5 
                 1.3 
               
               
                 As 
                 52 
                 55 
                 55 
                 58.7 
                 53.7 
                 51.3 
                 52 
                 52 
                 52 
                 50 
                 56.2 
               
               
                 S 
                 45 
                 43.7 
                 42.5 
                 40 
                 45 
                 46.2 
                 40.5 
                 31.5 
                 36 
                 45 
                 42.5 
               
               
                 Se 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 4.5 
                 13.5 
                 9 
                 — 
                 — 
               
               
                 Tg ° C. 
                 39 
                 19 
                 42 
                 21 
                 14 
                 34 
                 43 
                 54 
                 45 
                 51 
                 29 
               
               
                 Density 
                 3.42 
                 3.43 
                 3.49 
                 3.44 
                 3.40 
                 3.39 
                 — 
                 — 
                   
                 3.42 
                 3.47 
               
               
                 CTE × 10 −7 /° C. 
                 958 
               
               
                 Ts 
                 68 
                   
                 85 
                   
                   
                 49 
                 78 
                   
                 79 
                   
                 49 
               
               
                 log ρ 
                 13.3 
               
               
                   
               
             
          
         
       
     
     These glasses have very low Tg values that are below 100° C., and generally no greater than 50° C. Since some of the values are below an ambient temperature of about 25° C., the corresponding glass can be deformed plastically at normal room temperatures. In general, these glasses are very durable. For example, a sample of glass 1 in TABLE 1 was soaked in water at room temperature for six months without showing any weight loss. The glass is also resistant to acid attack, and to devitrification. 
     These glasses are composed essentially of 46-60% arsenic (As), 39-48% sulfur (S) plus selenium (Se), the selenium being 0-14%, and, optionally, up to 6% germanium (Ge). A particularly preferred range of compositions, which includes preferred compositions 1 and 6, consists essentially of 48-54% As, 44.5-47.5% S and 1.5-4.5 Ge. 
     A 36 gram batch was mixed in accordance with the proportions of Example 1 in TABLE 1. The batch was sealed within an evacuated, fused silica ampoule and melted at 800° C. for 40 hours. The melt was initially quenched by plunging the ampoule into water. The resultant glass was cooled slowly from 200° C. to room temperature (25° C.) with no sign of devitrification. The glass rod thus produced was then reheated in air to 100° C., about 30° C. above the glass softening point, and cooled. Again, no sign of crystallization was observed. 
     The plastic glass of Example 1 was completely transparent in the near infrared (IR), which may be useful for optical telecommunication transmissions. In contrast to organic polymers, it showed no discrete absorption peaks, other than impurities, between its visible absorption edge near 600 nm and the IR edge at about 12 μm. In further contrast to organic polymers, this glass tends to be quite impermeable. 
     In addition to the plastic glasses just described, wherein the As 4 S 3  clusters essentially constitute the glass, an adjacent compositional area has been identified in which the As 4 S 3  structure is prominent. 
     Glasses in this adjacent compositional area, having higher Tg values over 100° C., are composed essentially of 42-60% arsenic (As), 37-48% sulfur (S) plus selenium (Sc), the selenium being 0-14%, and 5-12% germanium (Ge). 
     These glasses, as might be expected, have substantially larger Tg values, generally over 100° C. In contrast, glasses, where the As 4 S 3  cluster structure essentially constitutes the entire glass, have a Tg under 100° C., and generally below 50° C. 
     TABLE 2 shows examples of these transitional, or modified, glass compositions. Also, the Tg, density and CTE values are presented. As becomes apparent, the Tg values are significantly higher, while the CTE values are lower. These glasses are batched and melted in the same manner as those in TABLE 1 above. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Element 
                   
                   
                   
                   
               
               
                   
                 (Atomic %) 
                 Ex. 12 
                 Ex. 13 
                 Ex. 14 
                 Ex. 15 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Ge 
                 7.5 
                 5 
                 5 
                 10 
               
               
                   
                 As 
                 52.5 
                 55 
                 52.5 
                 45 
               
               
                   
                 S 
                 40 
                 40 
                 42.5 
                 45 
               
               
                   
                 Tg ° C. 
                 202 
                 192 
                   
                 220 
               
               
                   
                 Density 
                 3.52 
                   
                 3.43 
               
               
                   
                 CTE × 10 −7 /° C. 
                 405 
               
               
                   
                 Ts 
                 329 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 2 is a ternary diagram showing the total compositional area for glasses in accordance with the present invention. In the diagram, the apex, labeled S, represents 100% S; the right end of the base line, labeled Ge, represents 100% Ge; the left end of the base line, labeled As, represents 100% As. Glass compositions in accordance with the present invention are included within the area labeled ABCDEF. As noted earlier, the plotted values are in terms of atomic percent, that is the actual percent of the total number of atoms present represented by each element. 
     In terms of composition, glasses in accordance with the present invention consist essentially of 42-60% As, 37-48% S and up to 12% Ge. Up to about 30% of the sulfur content may be replaced by selenium, that is an amount up to about 14% Se. Attempts to substitute tellurium for sulfur, tin for Ge, and phosphorus or antimony for arsenic have been largely unsuccessful. The melts have tended to result in crystallized materials if such alternative additives exceed about 5% in total. Individually, these optional ingredients must not exceed 5% tellurium, 2% tin, 5% antimony, and 2.5% phosphorus, the total collectively not exceeding 5%. 
     Table 3 provides additional compositional examples 16-26, according to the present invention, and compares them to the glasses in examples 1-15. 
     The unusual properties of the inventive glasses, particularly their low Tgs and softening points, render them good candidates for a variety of potential uses. These applications may range from optical elements to durable sealing glasses to a host material for unstable, inorganic dyes. An optical element may include a moldable lens or microlens, an array of lenses or microlenses, a surface relief diffuser, a grating, or other diffractive element. The lens, microlens, or array of lenses or microlenses, may have either a spherical, aspherical, or aspheric-diffractive surface. 
     The present invention has been described in general and in detail by way of examples. Persons of skill in the art understand that the invention is not limited necessarily to the embodiments specifically disclosed, but that modifications and variations may be made without departing from the scope of the invention as defined by the following claims or their equivalents, including other equivalent components presently known, or to be developed, which may be used within the scope of the present invention. Therefore, unless changes otherwise depart from the scope of the invention, the changes should be construed as being included herein. 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 EXAMPLE 
                 Ge 
                 As2S3 
                 S 
                 Absorption Edge (nm) 
                 T† 
                 Soft. Pt. (° C.) 
                 CTE (ppm/° C.) 
                 Density at 20° C. 
                 Molar volume at 20° C. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 3 
                 52 
                 45 
                 619.2 
                 39.1 
                 68 
                 95.75 
                 3.4156 
                 16.268 
               
               
                 2 
                 1.25 
                 55 
                 43.75 
                   
                 18.8 
                   
                   
                 3.4321 
                 16.357 
               
               
                 3 
                 2.5 
                 55 
                 42.5 
                 671.3 
                 42 
                 85 
                   
                 3.4889 
                 16.236 
               
               
                 4 
                 1.25 
                 58.75 
                 40 
                 635 
                 21.1 
                   
                   
                 3.4445 
                 16.765 
               
               
                 5 
                 1.25 
                 53.75 
                 45 
                   
                 14.2 
                   
                   
                 3.4004 
                 16.352 
               
               
                 6 
                 2.5 
                 51.25 
                 46.25 
                 595.4 
                 33.6 
                 49 
                   
                 3.3908 
                 16.232 
               
               
                 10 
                 5 
                 50 
                 45 
                   
                 50.7 
                   
                   
                 3.4243 
                 16.213 
               
               
                 11 
                 1.25 
                 56.25 
                 42.5 
                 650.1 
                 29.5 
                 49 
                   
                 3.4725 
                 16.321 
               
               
                 12 
                 7.5 
                 52.5 
                 40 
                 683.2 
                 202 
                 329 
                 40.54 
                 3.5175 
                 16.376 
               
               
                 13 
                 5 
                 55 
                 40 
                   
                 191.9 
               
               
                 14 
                 5 
                 52.5 
                 42.5 
                   
                   
                   
                   
                 3.4325 
                 16.486 
               
               
                 15 
                 10 
                 45 
                 45 
                 690.5 
                   
                 341 
                 40.49 
                 3.388 
                 16.352 
               
               
                 16 
                 10 
                 50 
                 40 
                 723 
                 267.3 
                 383 
                 28.86 
                 3.5333 
                 16.286 
               
               
                 17 
                 15 
                 45 
                 40 
                 728.2 
                 320.7 
                 435 
                 17.84 
                 3.559 
                 16.136 
               
               
                 18 
                 2.5 
                 57.5 
                 40 
                   
                 40.2 
               
               
                 19 
                 3 
                 51 
                 46 
                 609 
                   
                   
                   
                 3.4066 
                 16.185 
               
               
                 20 
                 2 
                 52 
                 46 
                 591.6 
                   
                   
                   
                 3.3915 
                 16.264 
               
               
                 21 
                 3 
                 50 
                 47 
                 604.7 
                   
                   
                   
                 3.3845 
                 16.164 
               
               
                 22 
                 2 
                 51 
                 47 
                   
                   
                   
                   
                 3.3813 
                 16.186 
               
               
                 23 
                 2.5 
                 51.5 
                 46 
                 611.6 
                   
                   
                   
                 3.4012 
                 16.214 
               
               
                 24 
                 3 
                 50.5 
                 46.5 
                 600 
                   
                   
                   
                 3.3949 
                 16.178 
               
               
                 25 
                 2.5 
                 51 
                 46.5 
                 588.7 
                   
                   
                   
                 3.3946 
                 16.182 
               
               
                 26 
                 2 
                 51.5 
                 46.5 
                 585.7 
                   
                   
                   
                 3.388 
                 16.217