Patent Application
20170210660
The invention generally relates to the field of glass-ceramic tiles. More specifically, the invention relates to white sintered glass-ceramic tiles with has high reflectivity and low thermal conductivity.
Generally, tiles used in buildings of arid environment are required to have cooling features. The improvement of the thermal efficiency of buildings represents a challenge concerning the reduction of the overall costs for cooling. Although highly reflective coatings prevent heat absorption from the incoming sunlight, the body of conventional ceramic tiles warms up to environmental temperature through heat conduction and convection.
Thus, there is a need for an improved building material with efficient and cost effective cooling features.
The accompanying figures together with the detailed description below forms part of the specification and serves to further illustrate various embodiments and to explain various principles and advantages all in accordance with the invention.
Before describing in detail embodiments that are in accordance with the invention, it should be observed that the embodiments reside primarily in a white sintered glass-ceramic tile and a method for preparing the white sintered glass-ceramic tile.
In this document, terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, or composition that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, or composition. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, or composition that comprises the element.
Generally speaking, pursuant to various embodiments, the invention provides a white sintered glass-ceramic tile and a method for preparing the white sintered glass-ceramic tile. The white sintered glass-ceramic tile includes a glazed glass-ceramic layer and a porous glass-ceramic layer. The white sintered glass-ceramic tile is a single fired tile where each layer is prepared using same starting raw materials.
In an embodiment, the starting raw materials include about 13 weight percentage (wt %) to about 17 wt % of clay, about 8 wt % to about 12 wt % of cullet, about 28 wt % to about 32 wt % of limestone, about 33 wt % to about 37 wt % of silica and about 8 wt % to about 12 wt % pure chemicals. The resulting oxide composition includes about 6 wt % to about 10 wt % of aluminum oxide (Al2O3), about 0 wt % to about 3 wt % of barium oxide (B2O3), about 18 wt % to about 22 wt % of calcium oxide (CaO), about 0 wt % to about 5 wt % of potassium oxide (K2O), about 0 wt % to about 4 wt % Magnesium oxide (MgO), about 0 wt % to about 5 wt % Sodium oxide (Na2O), about 53 wt % to about 60 wt % of Silicon dioxide (SiO2), about 5 wt % to about 9 wt % of Zinc oxide (ZnO).
In another embodiment, the starting raw materials include about 0 wt % to about 4 wt % of alumina, about 8 wt % to about 14 wt % of cullet, about 15 wt % to about 24 wt % of feldspar, about 19 wt % to about 32 wt % of limestone, about 5 wt % to about 14 wt % of magnesia, about 25 wt % to about 31 wt % of silica, and about 3 wt % to about 7 wt % of zinc oxide. The resulting oxide composition includes about 3 wt % to about 10 wt % of Al2O3, about 12 wt % to about 22 wt % of CaO, about 0 wt % to about 4 wt % of K2O, about 6 wt % to about 15 wt % of MgO, about 0 wt % to about 5 wt % of Na2O, about 52 wt % to about 60 wt % of SiO2, and about 4 wt % to 8 wt % of ZnO.
In yet another embodiment, the starting raw materials include about 14.8 weight percentage (wt %) of clay, about 9.9 wt % of cullet, about 29.6 wt % of limestone, about 34.5 wt % of silica and about 11.4 pure chemicals. The resulting oxide composition is about 7.7 wt % of Al2O3, about 1.3 wt % of B2O3, about 0.0 wt % of BaO, about 20.3 wt % of CaO, about 0.4 wt % of Fe2O3, about 2.5 wt % of K2O, about 0.7 wt % of MgO, about 2.5 wt % of Na2O, about 57.4 wt % of SiO2, about 0.7 wt % of TiO2, and about 6.5 wt % of ZnO.
The method for preparing the white sintered glass-ceramic tile includes milling the starting raw materials to obtain a homogenous mixture. The homogenous mixture is melted to obtain a melt which is poured in water to obtain glass frits. The glass frits are milled to obtain homogenized glass frits which are sieved to obtain glass frit powder with particle size of below 100 micron (μm). A mixture of the glass frit powder and one or more foaming agents is cold pressed to form a porous glass-ceramic layer. Thereafter, the glass frit powder is deposited on the porous glass-ceramic layer to obtain a glazed glass-ceramic layer. Finally, both the porous glass-ceramic layer and the glazed glass-ceramic layer are fired together.
At step 102, the starting raw materials are milled to obtain a homogenous mixture. For example, the starting raw materials are ball milled in an agate jar for a period of about 30 minutes at a speed of about 300 rotations per minute (rpm) to obtain the homogenous mixture.
At step 104, the homogenous mixture is melted to obtain a melt. In an embodiment, the homogenized mixture is melted in kyanite refractory crucibles at a temperature of about 1400 degree Celsius (° C.) to about 1450° C. for a period of about 90 minutes in static air to obtain the melt.
At step 106, the melt is poured in water to obtain glass frits.
At step 108, the glass frits are milled to obtain homogenized glass frits. The glass frits are dried at a temperature of about 80° C. for overnight and thereafter, ball milled for a period of 30 minutes at a speed of about 400 rpm to obtain the homogenized glass frits.
At step 110, the homogenized glass frits are sieved to obtain glass frit powder with particle size of below 100 μm. The particles with different sizes can also be used. However, particles size less than 100 μm provides an optimum balance between viscous flow sintering of glass and crystallization.
At step 112, a mixture of the glass frit powder and one or more foaming agents are cold pressed to obtain porous glass-ceramic layer. The amount of the one or more foaming agents used is about 0.5 wt % to about 5 wt % of the mixture. The one or more foaming agents are selected from a group including carbon, silicon carbide (SiC), iron [III] oxide (Fe2O3), gypsum (CaSO4.2H2O), silicon nitride (Si3N4) combined with manganese [IV] oxide (MnO2) and cerium [IV] oxide (CeO2). In a preferred embodiment, the foaming agent used is a mixture of Si3N4 and gypsum. The mixture is uniaxially pressed to obtain a porous glass-ceramic layer, which is a well-known process to a person skilled in the art of ceramic processing. For example, the mixture is uniaxially pressed at a pressure of about at 40 MPa to obtain the porous glass-ceramic layer. The porous glass-ceramic layer reduces heat capacity and thermal conductivity of the white sintered glass-ceramic tile.
At step 114, the glass frit powder is deposited on the porous glass-ceramic layer to obtain a glazed glass-ceramic layer. The glass frit powder is lightly cold pressed at a pressure of about 10 megapascal (MPa) to obtain the glazed glass-ceramic layer, which is a well-known process to a person skilled in the art of ceramic processing. The glazed glass-ceramic layer forms a glazed glass-ceramic layer of the white sintered glass-ceramic layer. The glazed glass-ceramic layer is represented by a functional engobe with a high albedo (i.e. with a high diffuse reflectivity or reflecting power). The glazed glass-ceramic layer acts as a reflective coating which reduces the temperature of the surface of the white sintered glass-ceramic tile under hot summer conditions by 4° C. during the day, and by 2° C. during the night. The surface of the white sintered glass-ceramic tile is warmer than the ambient air by only 2° C. during the day and cooler than the ambient air by 5.9° C. on an average during the night.
Finally, at step 116, the porous glass-ceramic layer and the glazed glass-ceramic layer is fired together. In an embodiment, the porous glass-ceramic layer and the glazed glass-ceramic layer is fired in air at a temperature of about 900° C. to 1100° C. for a period of about 10 minutes to about 30 minutes with the heating rate of about 5 degree Celsius per minute (° C./min) to about 120° C./min.
In this working example, Neoparies™ glass-ceramics is used as a reference material. A glass (NP glass) with a chemical composition resembling that of Neoparies is reproduced using natural raw materials from Saudi Arabia and a limited amount of pure chemicals, as reported in Table 1 and Table 2. The pure chemicals are approximately 11 weight percentage (wt %), in the form of about 3 wt % of zinc oxide (ZnO), about 3 wt % of borax and 5.4 wt % of potassium carbonate (K2CO3).
The chemical composition of the starting raw materials is evaluated using X-ray fluorescence. In NP glass, BaO is substituted by increasing CaO. SiO2 is also slightly reduced.
The starting raw materials are first dried and homogenized by ball milling in an agate jar for 30 minutes at 300 rpm. After ball milling, the starting raw materials are melted in kyanite (Al2SiO5) refractory crucibles at 1400° C. for 90 minutes in static air. The molten glass does not corrode the kyanite refractory crucibles. Thus, the chemical composition of the parent glass is not affected. After complete melting of the starting raw materials, the melt is poured into water to produce glass frits. A drastic quenching provides a number of fragments of the glass frits that are successively dried at temperature of 80° C. overnight. After drying, the glass frits are ball milled and thereafter, sieved to obtain particles with a size below 90 μm. The ball milling of the glass frits is carried out for a time period of 30 minutes at a speed of about 400 rpm. A dilatometric and differential thermal analysis is performed both on powders below 90 μm and coarser particles (above 2-3 mm) to investigate the effect of the particle size on the crystallization behavior.
For the preparation of dense (glazed) samples, the pure glass frits are used whereas for the preparation of porous samples the foaming agents are added in the range 0.5 to 4 wt % with respect to the amount of dry glass frit. The mixtures are cold pressed in a 13 mm steel mold at a pressure of 40 MPa. The investigation of the evolution of the density of samples of white sintered glass-ceramic tiles is performed as a function of the amount of foaming agent added. Samples labeled SN includes only Si3N4 as foaming agent, samples labeled G3SN1 includes mixtures of Si3N4 and gypsum used as oxidizer in a molar ratio gypsum/Si3N4 of 3:1, and sample labeled G6SN1 includes mixtures of Si3N4 and gypsum in a molar ratio gypsum/Si3N4 of 6:1.
The water absorption, WAB, apparent, ρa, bulk, ρb, densities of fired samples is evaluated according to the UNI EN ISO10545 norm, by means of the Archimedes method. The true density, ρt, is evaluated on powdered samples of size below 90 μm by means of helium gas pycnometer.
After selection of optimum compositions, larger samples are realized using 25 grams (g) of the dry glass frit powder uniaxially cold pressed in a steel mold of 50×50 mm2 at a pressure of 40 MPa. To obtain a porous glass-ceramic layer coated with the dense (glazed) glass-ceramic layer, 3 g of the dry glass frit powder is lightly pressed at 10 MPa. After lightly pressing of the dry glass frit powder, 22 g of the glass frits mixed with a selected a foaming agent is deposited on the lightly pressed dry glass frit powder. Thereafter, a layered sample is uniaxially pressed at 40 MPa. Finally, the produced samples are fired in air at 950° C. for 30 minutes by applying the heating rate of 10° C./min.
The Young's modulus of the white sintered glass-ceramic samples is determined using a resonant frequency method in a flexural mode of vibration. Four-point bending tests (40 mm outer span and 20 mm inner span) is performed using an Instron 1121 UTS instrument on at least 15 specimens for each sample type, with dimensions of about 4×2.5×47 mm3. In order to remove surface flaws, all samples are carefully polished to a 6 μm finish before testing, using abrasive papers and diamond paste. The edges of bars are beveled using fine abrasive papers and diamond paste. The cross-head speed is 1 mm/minute until a fracture. The double layered samples are tested by positioning the porous glass-ceramic layer on the compression (upper) side and the dense (glazed) glass-ceramic layer on the tensile (lower) side.
Ground glass-ceramics are investigated by powder X-ray diffraction, employing CuKα radiation (0.15418 nm) and collecting data in the range 2θ=10-70° (0.05° steps and 5 seconds counting time). The identification is performed by means of a semi-automatic software package, supported by data from PDF-2 database. A selected dense (glazed) and polished sample is investigated using Vickers indentation, and the microhardness (HV) is assessed by averaging 20 indentations produced at low load (5N).
The polished surfaces of the samples are characterized by a scanning electron microscopy (SEM). The cell size distribution is evaluated from acquired SEM images using image processing software. The area of the cells, evaluated by means of the image processing software, is converted into the area of a circle of an equivalent surface. The values obtained by image analysis are converted to 3-Dimensional (3-D) values using a stereological equation φsphere=φcircle/0.785, to determine the actual cell size.
The thermal diffusivity is evaluated on disks with 10 millimeter (mm) diameter and thickness of about 2 mm by a laser flash using a neodymium-glass laser operating at 1.053 The laser delivers a standard pulse of 30 Joule (J) in 450 micro seconds (μs) and is used to heat up the front face of a cylindrical sample. The absorbed heat diffuses throughout the sample, and a liquid-nitrogen-cooled infrared detector (Hg—Cd—Te) is used to monitor the evolution of the back face temperature. The samples are coated with a thin graphite layer to increase the emissivity of receiving and emitting faces of the samples. The thermal diffusivity is calculated by using a Degiovanni's expression, which takes into account the heat losses from the samples during the experiment compared with a simpler Parker expression which only considers adiabatic conditions. The thermal diffusivity (a) is measured perpendicular to the disk faces, and the thermal conductivity is then calculated by using the relation:
λ=αρc
where ρ is the bulk density and c is the specific heat of the material.
The reflectance is evaluated by means of a UV-V is a spectrophotometer equipped with integrating sphere (ISN-470). The white standard employed is a Barium sulfate (BaSO4) plate.
Results:
The formation of specific crystal phases is beneficial for improved wear and chemical durability for materials that needs to be applied both as cladding and floor tiles. Neoparies are well-known for featuring such optimum phase assemblage, combined with a white color, for formulations that do not contain any pigments. In addition, Neoparies are developed by viscous flow sintering with concurrent crystallization (“sinter-crystallization”) of glass powders, i.e. by a process in which the introduction of additives (foaming agents, reinforcing phases) is particularly simple.
For porous white sintered glass-ceramic samples, a variety of foaming agents are available and well-studied. However, only a few, such as CeO2 and Si3N4, are suitable for generating porosity without affecting the final color. CeO2 is already an effective foaming agent in porcelain stoneware, due to the reduction to CeO and oxygen gas evolution at a high temperature. However, the sintering temperature of Neoparies-like compositions is in the 950-1100° C. range, well below the temperatures suitable for using CeO2. Although Si3N4 are more expensive than CeO2, Si3N4 can lead to gas generation in a desired temperature range. In particular, Si3N4 releases nitrogen gas and leaves silica residue when fired in an oxidizing environment even at temperatures as low as 800-850° C. The oxidation of Si3N4 is mainly attributed to the reaction with the environmental oxygen diffusing through the porosity available inside the pressed powders. During sintering, the permeability decreases and the environmental oxygen becomes insufficient to fully oxidize Si3N4. Therefore, gypsum is added to enhance the oxidation of Si3N4.
Considering the relatively large gap between the dilatometric softening point and the crystallization temperature (TSC-TD>200° C.), the NP glass is expected to lead to dense (glazed) sinter-crystallized glass-ceramics when fired at TSC (rounded at 950° C.). The verification of the expectation is carried out with samples from pure NP glass. The NP glass exhibits a bulk density of 2.56±0.04 g/cm3 after heat treatment at 950° C. The bulk density of the NP glass is close to the true density of 2.68±0.02 g/cm3, indicating that a residual porosity not exceeding 4.5 volume percentage (vol %) is present in the sample. The true density, ρt, is not affected by the presence of the foaming agents and is 2.68±0.02 g/cm3 for the set of samples analyzed.
An enhancement of foaming is achieved by the addition of gypsum along with Si3N4. Gypsum completely dehydrates in the interval 120-180° C. transforming into anhydrite [III] (CaSO4). CaSO4 decomposes into calcium oxide (CaO) and sulfur trioxide (SO3) at a temperature above 1180° C. When coupled with a reducing agent, CaSO4 may turn to calcium sulphite and sulphide, with release of oxygen. Such redox coupling is actually exploited for improving the oxidation of silicon carbide (SiC) in commercial glass foams. In analogy, Si3N4 is expected to be oxidized not only by reaction with atmospheric oxygen, but also by reaction with oxygen released from gypsum.
Both mixtures G3SN1 and G6SN1 formed a limited amount of porosity when the foaming agent is added in a small percentage (below 1 wt %). Thus, the introduction of Si3N4 needs to be sufficiently high to enable the generation of porosity. On performing some preliminary investigation, 0.5 wt % to 2 wt % gypsum additions did not decrease the final density. Furthermore, in the G3SN1 and G6SN1 samples the role of gypsum is to oxidize Si3N4 particles. When the total amount of foaming agent is used in percentages of 0.25 or 0.5 wt %, only traces of Si3N4 are introduced. When 1 wt % and above of Si3N4 is used, the addition of gypsum in both formulations (samples G6SN1 and G3SN1) contributes to the expansion. The expansion leads to a total porosity higher than 40 vol %. However, the main effect of adding gypsum is to increase the amount of open porosity. The presence of the open porosity leads to high values for the water absorption (in some cases >20 wt %), but the detrimental effect of the presence of the open porosity can be limited by application of a dense glaze on the tile surface.
After the preliminary tests, further studies are performed on larger samples selecting an amount of foaming agent of 0.5 wt % (SN sample) and 2 wt % (G3SN1 sample), respectively. The first sample is manufactured without applying a glaze, because of low water absorption of the first sample. The second sample possesses the highest amount of porosity. A mixture with 2 wt % of the foaming agent is preferred, in order to limit the amount of additive, as a sufficiently low density value is already obtained for that amount of the foaming agent. Due to high water absorption of the second sample, a glaze with the same composition of the body is applied to the second sample.
Table 1 provides water absorption (WAB), bulk density (ρb) and apparent density (ρb) values for different mixtures where the samples are directly inserted in the furnace at 950° C. and heated for 3 hours.
In fact, the samples reported in Table 3 are produced by directly inserting the samples in a kiln, whereas the samples reported in Table 4 are obtained by applying a heating rate of 10° C./min. The heating rate affected both density and water absorption, in fact differently to samples G3SN1. Sample SN had a water absorption value higher than 2 wt %. Thus, the water absorption values indicate that an application of a glaze is considered for both the mixtures to protect the porous layer.
Although the bending strength of bars cut from the tiles of the dense (glazed) samples is considerably higher, the porous glass ceramics possessed mechanical properties similar to that of dense (glazed) commercial sinter-crystallized glass ceramics of similar composition. In particular, the porous samples had a high strength associated with a density decreased by about 25% to 28% with respect to the dense (glazed) samples. Thus, the glazed side prevents the water absorption considering the negligible absorption displayed by the dense (glazed) sample. For the dense (glazed) samples, the total residual porosity, evaluated by gas pycnometry, is of 4.5 vol %.
Table 4 provides bulk density (ρb), elastic modulus (), bending strength (σ) and hardness (HV) of dense (glazed) and porous glass ceramic tiles where the samples are heated at the rate of 10° C./min to 950° C. and for a time period of about 3 hours at 950° C.
Various embodiments of the invention provide a white colored glass-ceramic tile with assured high reflectance in the visible region. The high density of scattering centers provided by the crystal phases limits the heat absorption from the incoming visible sun light producing a greater reflectance and a higher opacity. The high reflectance coupled to the low thermal conductivity indicates that the white sintered porous glass ceramic tiles are suitable to be employed as an advanced barrier to heat. Thus, an efficient thermal insulation is provided for buildings constructed using the white sintered porous glass ceramic tiles. Thus, the absorbance of thermal radiations is reduced and the demand on cooling power is reduced. The white sintered glass-ceramic with low thermal conductivity also mitigates summer urban heat islands, and in turn lower the citywide ambient air temperature and increase human comfort. The application of the glaze further improves the reflectance of the material, prevents the incorporation of dust and water. Thus, a longer durability is favored due to application of the glaze. Furthermore, the porous tiles can be applied as cladding materials for improved building insulation due to the lightweight and the high albedo, whereas the lightweight materials can also be used for paving due to the high strength. The while sintered glass-ceramic tiles with a low thermal conductivity can also compete with wood for paving, providing an improved thermal insulation between the ground or different floors in a building and even for a higher comfort when walking barefoot. The higher thermal conductivity of the dense (glazed) tiles limits the benefits for thermal insulation applications. However, the dense (glazed) tiles can be used for paving when strength requirements are more important than the thermal insulation properties.
Those skilled in the art will realize that the above recognized advantages and other advantages described herein are merely exemplary and are not meant to be a complete rendering of all of the advantages of the various embodiments of the invention.
In the foregoing specification, specific embodiments of the invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification is to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
