Rawlings, J. Wu, A. ABSTRACT Glass-ceramics are polycrystalline materials of fine microstructure that are produced by the controlled crystallisation devitrification of a glass. Numerous silicate based wastes, such as coal combustion ash, slag from steel production, fly ash and filter dusts from waste incinerators, mud from metal hydrometallurgy, different types of sludge as well as glass cullet or mixtures of them have been considered for the production of glass-ceramics. Developments of glass-. Properties and.
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Glass-Ceramics and Photo-SitallsVIDEO ON THE TOPIC: Posi Joist/Space Joist Manufacture June 2014 (Part 1)
This application is a continuation-in-part of our pending application, Ser. The manufacture of glass-ceramic articles contemplates the carefully controlled crystallization of a glass article in situ. Hence, a glass-forming batch usually containing a nucleating agent is melted, this melt is then simultaneously cooled to a glass and an article of desired dimensions shaped therefrom, and, subsequently, this glass article is exposed to a particular heat treatment schedule which first causes the development of nuclei in the glass that provide points for the growth of crystals thereon as the heat treatment is continued.
Because the crystallization is founded upon the substantially simultaneous growth on essentially innumerable nuclei, the body of a glass-ceramic article is composed of relatively uniformly-sized, fine-grained crystals homogeneously dispersed in a glassy matrix with the crystals comprising the predominant portion of the article.
Since glass-ceramic articles are usually very highly crystalline, the chemical and physical properties thereof are normally quite different from those of the parent glass and more nearly approximate those exhibited by crystalline articles. An extensive study of the theoretical considerations and the practical aspects inherent in the production of glass-ceramic articles along with a discussion of the crystallization mechanism involved can be found in U.
As will be readily understood, the crystal phases developed in glass-ceramic articles are dependent upon the composition of the original glass and the heat treatment to which the glass is subjected. Glass-ceramic articles containing beta-spodumene as the predominant crystal phase are described in the above patent. The diffusion of ions in any medium is a direct function of the structure of the medium itself.
Hence, whereas a crystal has a long range ordered structure of ions, glass has only short range order and has even been deemed to consist of a random network of ions. This basic difference in structure greatly affects the ability of ions to diffuse therein. The structure of glass is characterized by a network or framework composed of polyhedra of oxygen centered by small ions of high polarizing power e. These polyhedra are arranged in a generally random fashion so that only short range order exists.
Thus silica glass is thought to be composed of a random network of SiO 4 tetrahedra, all of whose corners are shared with one another. In silicate glasses containing modifying oxides e. The modifying ions remain in interstitial positions or structural vacancies. In modified aluminosilicate glasses, non-bridging oxygen ions are believed less common because as modifying ions are added to silicate glasses aluminum replaces silicon in the three-dimensional corner shared tetrahedral network and the modifying ions remain in the interstices with the retention of charge balance.
In either case the larger ions of lower valence modifiers are thought to occur geometrically in interstitial positions within the basic silicate or aluminosilicate framework.
They can thus be considered as completely or at least partially surrounded by linked framework silica tetrahedra. In other words, these ions can be considered as present in "structural cages" in the network. Since the glassy network is random, the size of these cages or potential modifier cation positions is variable and the number of cages is large with respect to the number of modifying ions.
Therefore, it is likely that during ion exchange in a molten salt bath a small ion will jump out of a cage and a large ion will jump into another cage, very possibly a larger one.
Even if the exchangeable ion in the glass and the ions in the molten salt are similar in size, it is likely that an ion leaving one cage will be replaced by an ion entering a different and previously vacant cage.
Thus ion exchange phenomena in a glassy network are structurally random and there is no guarantee that certain structural vacancies or positions filled before exchange will be filled after exchange.
The concept of exchanging ions within a crystal structure has been appreciated for many years. These materials generally consist of alternating, parallel, essentially two-dimensional layers which are stacked together with interlayer spaces therebetween.
To maintain electroneutrality between these layers, cations are incorporated into the interlayer spaces. The extent and rate of exchange in these materials is a function not only of the concentrations of the exchanging species but also of the structure of the crystalline phase undergoing exchange.
When these materials are suspended in an aqueous solution which can penetrate between the layers, these cations are freely mobile and can exchange with cations present in the solution. Hence, the cation exchange capacity of these materials arises principally from the replacement of cations at defined positions in the interlayer spaces. These interlayer spaces can be likened to channels and it will be apparent that this type of low temperature ion exchange will occur between the loosely bonded ions in a crystal and those in a solution only if there is a suitable channel within the crystal to allow diffusion to take place.
Isomorphous substitution in crystals involves the replacement of the structural cations within the crystal lattice by other cations. This type of substitution may be regarded as a form of ion exchange but the accomplishment thereof requires crystallizing the materials from melts of the appropriate composition.
However, the amount and type of isomorphous substitutions can often be very important in affecting the character of a material which is to be subsequently subjected to the conventional low temperature ion exchange reaction described above. The instant invention contemplates the use of high temperature ion exchange to effect substitutions within the crystalline lattice to thereby produce materials similar to those secured through isomorphous substitution.
However, in contrast to glasses, high temperature ion exchange in crystals is much more restricted. The various ion species are specifically located in defined positions within the lattice. When an ion leaves a crystalline position, the position is generally filled by another ion from an external source of ions.
The geometry of the crystals often restricts the size of the replacing ion. Isomorphous substitution in the crystal can only sometimes be of help in determining which ion pairs are exchangeable under the rigid conditions imposed by the long range repetitive order of crystals. Thus, for example, sodium ions can replace lithium ions in the beta-spodumene crystal structure but this exchange cannot take place in the beta-quartz or beta-eucryptite solid solution structure where the sodium ion appears to be too large for the structure to tolerate and the crystalline structure is destroyed if the exchange is forced to take place.
As opposed to this, the sodium-for-lithium ion exchange can always be carried out in aluminosilicate glasses without any phase change. Hence, in short, crystals, because of their definite geometry, impose stringent limitations upon ion exchange. Glasses, on the other hand, because they are random structures capable of incorporating almost all chemical species in a substantial degree, demonstrate no such basic restrictions. Of course, the ability of a crystalline phase to accept another cation to replace an ion already in its structure through an ion exchange mechanism is not necessarily useful.
Many such exchanges will not lead to compressive stress and consequent strengthening. When strength is the desired goal, it is necessary to tailor the exchange to produce compressive stress in the exchanged layer.
The compressive stress may arise through crowding of the existing structure or through transformation of that structure to one which comes under compression by some other mechanism; e.
Chemical alteration in situ of the crystal phase in a glass-ceramic material by ion exchange is generally disclosed and claimed in an application filed May 5, , Ser.
Voss, entitled "Glass-Ceramic Article and Method" and assigned to a common assignee. In addition to its general disclosure regarding ion exchange in a glass-ceramic material, the Voss application further specifically discloses the strengthening of a glass-ceramic article having a beta-spodumene crystal phase by exchanging the lithium ion of such crystal phase for a sodium ion within a surface layer on the article to develop compressive stress within such surface layer.
We have found that an exchange between potassium and lithium ions in a glass-ceramic material having a beta-spodumene crystal phase does in fact occur. However, we have found that an effective degree of exchange for strengthening purposes can only be attained within a reasonable time i. In turn, however, we have found that a beta-spodumene glass-ceramic article strengthened by a potassium ion exchange provides a distinct advantage with respect to thermal retention, or conversely thermal decay, of the strengthening effect.
The potassium-for-lithium ion exchange is uniquely different from the sodium-for-lithium exchange in this respect. A very satisfactory degree of strengthening is attainable by exchanging sodium for lithium ions in materials characterized by a beta-spodumene crystal phase.
However, it has been found that such strengthening may be lost when the article is exposed to, or operated at, elevated temperatures. Consequently, the use of such strengthening procedure is limited to articles whose operational use will necessarily be within such thermal limitation, or in which the loss of strength can be tolerated. The desirability of a strengthening technique that would extend upward the temperature limit on potential service operations is obvious.
Our invention then resides in a glass-ceramic article having a beta-spodumene crystal phase and being characterized by an integral surface layer wherein the lithium ions in at least a portion of the beta-spodumene crystals is replaced by potassium ions, thereby inducing compressive stresses in the surface layer and increasing the mechanical strength of the article. The term "beta-spodumene" has been used to designate a crystal that is now known to be in the tetragonal system, that has the formula Li 2 O.
Likewise, the term "beta-eucryptite" has been used to designate a crystal in the hexagonal system that has the formula Li 2 O. In lithium-aluminum-silicate glass-ceramic materials, however, the crystal phase does not strictly conform to either of such naturally occurring crystals. Rather, it is more in the nature of a solid solution corresponding generally to the formula Li 2 O.
Also, there is evidence to indicate that, when other ions such as magnesium are present in the parent glass, they may appear in the crystal phase to some extent without altering the basic eucryptite or spodumene crystal structure. Thus, the characteristic geometric pattern of the crystal, as shown by X-ray diffraction pattern analysis, invariably falls in either the hexagonal or tetragonal system.
Accordingly, it has become customary, in identifying glass-ceramics, to essentially disregard stoichiometry and to term those lithium-aluminum-silicate crystal phases that are classifiable in the hexagonal system as beta-eucryptite crystal phases and those that are classifiable in the tetragonal system as beta-spodumene crystal phases.
That practice is followed here. Where the oxide stoichiometry in the crystal in such that the coefficient "n" is less than about 3. With larger proportions of silica "n" from about 3. In general, lithium-aluminum-silicate glass-ceramic materials do not lend themselves to strengthening when the crystal phase is of the metastable beta-eucryptite form, but readily strengthen when the crystal phase is of the beta-spodumene type, that is classifiable in the tetragonal system. X-ray diffraction pattern analysis indicates that substitution of a potassium ion for lithium in a beta-spodumene crystal leads to obvious distortion of the spodumene structure, as one would expect from considerations of the difference in size between the potassium and lithium ions.
Whereas the sodium for lithium substitution leads only to subtle changes in the X-ray diffraction pattern, the potassium for lithium substitution shows striking changes in the positions and relative intensities of the characteristic peaks.
The latter exchange also results in formation of a new minor phase. There is an apparent shift in the beta-spodumene diffraction lines to larger d-spacings indicating some expansion of the lattice.
In addition, the low density polymorph of silica, cristobalite, is formed. The crystalline phase assemblage present after ion exchange, normally being lower in density or higher in specific volume than the original material, would be expected to cause surface compression. The invention may be practiced with any glass-ceramic article having a beta-spodumene type crystal phase and is not otherwise limited with respect to the composition of, or the manner of initially producing, the glass-ceramic article.
In general, glass-ceramic articles having a beta-spodumene crystal phase may be produced by initially forming a corresponding glass article from a glass composed essentially of the oxides of Li 2 O, Al 2 O 3 , and SiO 2 , these oxides being so proportioned in the glass composition as to permit subsequent formation of a beta-spodumene crystal phase from the glass. Glass compositions corresponding to the formula Li 2 O. Beta-spodumene is isostructural with and is a stuffed derivative of the silica polymorph, keatite.
Beta-spodumene is derived through the replacement of some of the silicon atoms with aluminum atoms, the electrical neutrality being maintained by the insertion of lithium ions into interstices in the cell framework. The amount of aluminum-for-silicon substitution can vary widely and the crystal structure is capable of a broad range of other substituent cations such as magnesium and zinc ions.
As described by B. Skinner and H. Evans, Jr. Eight of these tetrahedra form spiral chains about fourfold screw axes in the centers of the lateral faces of the unit cell, while the remaining four tetrahedra lie on a horizontal diagonal twofold rotary axes, cross-linking the spiral chains.
The aluminum atoms are thought to substitute for silicon atoms at random positions within the structure whereas the lithium ions can go into the structure either in an eightfold general position or a fourfold special position on the rotary axes. A careful study of the beta-spodumene structure shows that these cation positions are connected by channels which extend continuously thoughout the structure. These channels provide means for movement of the lithium ions under the influence of a chemical or physical potential such as a bath of molten potassium salt.
The selected glass is melted and articles formed therefrom in accordance with conventional practice for such an aluminosilicate glass. Customarily, a nucleating agent such as titania is included in an amount of several percent in the glass composition, and the glass is thermally treated within a temperature range in a controlled manner to effect nucleation, that is to provide a mechanism for development of a fine-grained crystal phase throughout the glass body.
As indicated earlier, a metastable beta-eucryptite crystal phase may separate initially and then convert to the beta-spodumene phase with additional heat treatment. The beta-spodumene glass-ceramic article thus produced contains within its crystal phase lithium ions which have been found to be exchangeable with certain ions of larger ionic radius. In accordance with the present invention, a portion of the lithium ions of the beta-spodumene solid solution in a surface layer on the glass-ceramic article is replaced by potassium ions.
This chemical change in the crystal composition with the accompanying changes in the surface phase assemblage described above results in the development of compressive stresses in the modified surface layer with consequent increase in the mechanical strength of the article. The replacement of the small-diameter lithium ions with larger-diameter potassium ions is on a one-for-one basis such that the total concentration of alkali metal ions molarwise is the same before and after the ion exchange.
Therefore, it can be appreciated that the concentration of potassium ions in the surface layer will be much greater than in the interior portion with the opposite situation holding with respect to the lithium ion concentrations. These differences in the potassium and lithium ion concentrations produce the desired compressive stresses.
Glass-Ceramics Their Production
Glass-ceramics have an amorphous phase and one or more crystalline phases and are produced by a so-called "controlled crystallization" in contrast to a spontaneous crystallization, which is usually not wanted in glass manufacturing. Glass-ceramics have the fabrication advantage of glass, as well as special properties of ceramics. In manufacturing, glass-ceramics are valued for having the strength of ceramic but the hermetic sealing properties of glass. Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass-manufacturing process. The glass is cooled down and is then reheated in a second step. In this heat treatment the glass partly crystallizes. In most cases nucleation agents are added to the base composition of the glass-ceramic.
US4074993A - Potassium ion-exchange on surface of beta-spodumene - Google Patents
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Фонтейн долго молчал. Потом, тяжело вздохнув, скомандовал: - Хорошо.
У них там прямо-таки дискотека! - пролопотал Бринкерхофф. Фонтейн смотрел в окно, пытаясь понять, что происходит. За несколько лет работы ТРАНСТЕКСТА ничего подобного не случалось. Перегрелся, подумал. Интересно, почему Стратмор его до сих пор не отключил. Ему понадобилось всего несколько мгновений, чтобы принять решение. Фонтейн схватил со стола заседаний трубку внутреннего телефона и набрал номер шифровалки. В трубке послышались короткие гудки. В сердцах он швырнул трубку на рычаг.
- Черт! - Фонтейн снова схватил трубку и набрал номер мобильника Стратмора.
К несчастью для того, кто это придумал, коммандер Стратмор не нашел в этой выходке ничего забавного. Два часа спустя был издан ставший знаковым приказ: СОТРУДНИК КАРЛ ОСТИН УВОЛЕН ЗА НЕДОСТОЙНЫЙ ПОСТУПОК С этого дня никто больше не доставлял ей неприятностей; всем стало ясно, что Сьюзан Флетчер - любимица коммандера Стратмора.
Но не только молодые криптографы научились уважать Стратмора; еще в начале своей карьеры он был замечен начальством как человек, разработавший целый ряд неортодоксальных и в высшей степени успешных разведывательных операций.
Ну что, вы решили. Я ее убиваю. Стратмор мгновенно взвесил все варианты.
В руке Хейл сжимал беретту. Вскрикнув, она оторвала взгляд от неестественно выгнутой руки и посмотрела ему в лицо. То, что она увидела, казалось неправдоподобным. Половина лица Хейла была залита кровью, на ковре расплылось темное пятно. Сьюзан отпрянула.
О Боже. Значит, она слышала звук выстрела Хейла, а не коммандера. Как в тумане она приблизилась к бездыханному телу.
Остановка поисков ключа Цифровой крепости высвободила бы достаточно энергии для срабатывания дверных замков. - Успокойся, Сьюзан, - сказал Стратмор, положив руку ей на плечо. Это умиротворяющее прикосновение вывело Сьюзан из оцепенения.
Внезапно она вспомнила, зачем искала Стратмора, и повернулась к. - Коммандер.
Шестьдесят четыре буквы. Юлий Цезарь всегда с нами. Мидж развела руками. - О чем. - Квадрат Цезаря, - просияла Сьюзан. - Читается сверху. Танкадо прислал нам письмо. ГЛАВА 122 - Шесть минут! - крикнул техник. Сьюзан отдала приказ: - Перепечатайте сверху. Нужно читать по вертикали, а не по горизонтали.
Сильный палец нажал на плунжер, вытолкнув синеватую жидкость в старческую вену. Клушар проснулся лишь на несколько секунд. Он успел бы вскрикнуть от боли, если бы сильная рука не зажала ему рот. Старик не мог даже пошевелиться.
Три часа. Стратмор поднял брови. - Целых три часа.
Сьюзан нахмурилась, почувствовав себя слегка оскорбленной. Ее основная работа в последние три года заключалась в тонкой настройке самого секретного компьютера в мире: большая часть программ, обеспечивавших феноменальное быстродействие ТРАНСТЕКСТА, была ее творением.
Электричество. Окрыленная надеждой, Сьюзан нажала на кнопку. И опять за дверью что-то как будто включилось.