JP2013063861A - Heat resistant glass for microwave oven and method of manufacturing heat resistant glass for microwave oven - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a heat resistant glass for microwave ovens in which hot spots are less formed than before, and a method of manufacturing the same.SOLUTION: The heat resistant glass for microwave ovens includes a composition of 75-85% of SiO, 0-5% of AlO, 10-20% of BO, 0-5% of LiO, 1-10% NaO, and 0-5% of KO in terms of mass%, and is characterized in that (LiO+NaO+KO)/(SiO+BO+AlO) is 0.045-0.055 and the internal residual stress is 5 MPa or less.

Description

  The present invention relates to a heat-resistant glass for a microwave oven and a method for producing the same, and more particularly relates to a heat-resistant glass for a microwave oven having a predetermined composition, a low thermal expansion coefficient, and less likely to generate hot spots, and a method for producing the same.

  In general, a microwave oven has a built-in microwave oscillator, and a microwave with a frequency of 2.45 GHz is supplied from the oscillator into a cooking chamber to heat and heat the food itself. At this time, the food is placed on a glass turntable installed on the bottom of the cooking chamber. As the glass used for the turntable, borosilicate glass having a low thermal expansion coefficient and excellent thermal shock resistance is used. A turntable made of borosilicate glass is generally manufactured by cooling molten glass to a predetermined viscosity and press-molding it with a mold.

  When the food in the cooking chamber is heated by supplying microwaves in the microwave oven as described above, the turntable, that is, the glass also generates heat. Therefore, when the microwave oven is blown or the like, a phenomenon called a so-called hot spot, in which the turntable generates excessive heat locally and partly melts to form a hole, may occur. Such heat generation of glass is mainly caused by the dielectric loss of glass.

  In view of the above problems, crystallized glass with reduced dielectric loss and a manufacturing method thereof have been developed (for example, Patent Document 1). Although the dielectric loss of glass mainly depends on the glass composition, according to Patent Document 1, in the crystallization process of glass, crystals having low dielectric loss are preferentially precipitated by controlling the crystallization conditions, It is possible to further reduce the dielectric loss of the glass.

JP 61-53131 A

  However, even when the composition is designed so as to reduce the dielectric loss of the glass, the dielectric loss is higher than expected at the time of design, and a hot spot may occur. Moreover, even when the manufacturing method according to Patent Document 1 is used, the dielectric loss of the glass may not be sufficiently reduced. That is, there is still room for improvement in conventional heat-resistant glass for microwave ovens.

  In view of the above problems, an object of the present invention is to provide a heat-resistant glass for microwave ovens and a method for producing the same, in which hot spots are less likely to occur than before.

  As a result of intensive studies, the present inventor has found that the dielectric loss increases due to the residual stress inside the glass generated by press molding. That is, it has been found that the dielectric loss of glass includes dielectric loss due to residual stress and dielectric loss due to composition. Moreover, it discovered that the dielectric loss resulting from a residual stress among these dielectric losses can be reduced by reducing a residual stress. And this inventor discovers that the said subject can be solved by prescribing | regulating a glass composition strictly and making a residual stress below a predetermined value, and proposes as this invention. In addition, as a method for producing such glass, it is proposed to strictly regulate the temperature control range and the temperature decrease rate from the start to the end of temperature decrease of the molded glass.

The heat-resistant glass for microwave ovens according to the present invention is, by mass%, SiO ₂ 75 to 85%, Al ₂ O ₃ 0 to 5%, B ₂ O ₃ 10 to 20%, Li ₂ O 0 to 5%, Na _2. It contains a composition of O 1 to 10%, K ₂ O 0 to 5%, and (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is 0.045 to 0.055 And the internal residual stress is 5 MPa or less.

  Moreover, it is preferable that the dielectric loss in the heat resistant glass for microwave ovens of this invention in frequency 2.45GHz and 25 degreeC is 0.020-0.035. According to such a structure, generation | occurrence | production of the hot spot at the time of cooking and heating within a microwave oven can fully be suppressed.

  Moreover, it is preferable that the heat-resistant glass for microwave ovens of this invention has a thermal expansion coefficient in 30-380 degreeC below 37 * 10 <-7> / degreeC. If the thermal expansion coefficient in 30-380 degreeC is 37x10 <-7> / degrees C or less, it can be said that it has sufficient heat resistance and impact resistance as heat resistant glass for microwave ovens.

  Moreover, it is preferable that the heat-resistant glass for microwave ovens of this invention has a disk shape, and is used as a plate of a turntable.

  Further, the heat-resistant glass for microwave oven of the present invention is heated to 10 ° C./min to (annealing point + 30 ° C.), held for 30 minutes, then cooled to the strain point at 3 ° C./min, and further 10 ° C. / It is preferable that the amount of decrease in the dielectric loss in the glass at a frequency of 2.45 GHz and 25 ° C. when the heat treatment for lowering to room temperature in minutes is performed is 0.007 or less.

  Further, the heat-resistant glass for microwave oven of the present invention has a value of (reduction amount of dielectric loss in glass when heat-treated) / (dielectric loss of glass before heat-treatment) at a frequency of 2.45 GHz and 25 ° C. is 0. It is preferable that it is 0.1-0.3.

Method for producing a microwavable heat-resistant glass of the present invention, in _{mass%, SiO 2 75~85%, Al} 2 O 3 0~5%, B 2 O 3 10~20%, Li 2 O 0~5%, It contains 1 to 10% Na ₂ O, 0 to 5% K ₂ O, and (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is 0.045 to 0 The glass raw material to be 0.05 is melted and molded, and the glass obtained by the melting and molding process is gradually cooled so that the internal residual stress is 5 MPa or less.

  Moreover, in the manufacturing method of the heat-resistant glass for microwave ovens of the present invention, in the slow cooling step, the glass obtained in the melting / molding step is changed from (slow cooling point + 50 ° C.) so that the internal residual stress becomes 5 MPa or less. In the temperature range up to the strain point, it is preferable to slowly cool at a temperature drop rate of 0.5 to 3 ° C./min.

  Further, in the method for producing a heat-resistant glass for microwave oven according to the present invention, in the annealing step, the cooling is performed so that a difference in residual stress inside the heat-resistant glass for microwave oven before and after the annealing is 1.5 MPa or more. Preferably it is done. According to such a configuration, a high dielectric loss due to residual stress can be sufficiently suppressed, and a product with stable quality can be manufactured.

  The annealing point is a measure of the temperature at which the residual stress is reduced, and the viscosity of the glass at this temperature is 1013.0 dPa · s. The strain point is a temperature at which the viscous flow of the glass does not occur, and the viscosity of the glass at this temperature is 1014.5 dPa · s.

  According to the heat resistant glass for a microwave oven of the present invention, it is possible to reduce the dielectric loss due to the residual stress by reducing the residual stress. That is, the dielectric loss of the whole glass can be reduced as compared with the conventional case, and the generation of heat and hot spots due to microwaves in the cooking chamber of the microwave oven can be suppressed. Moreover, since it has the composition as described above, the thermal shock resistance of the glass is high and the annealing is easy, so that the residual stress in the glass can be easily reduced. That is, high reliability can be obtained as a product. Moreover, since it has the above composition, it is excellent in meltability, so high temperature melting is not required, and damage to the glass melting furnace and the press mold can be suppressed. Further, since there is no need to perform crystallization treatment as in the prior art, energy, cost, etc. required for production can be suppressed.

Hereinafter, the heat-resistant glass for microwave ovens according to the embodiment of the present invention will be described. Microwave resistant glass according to an embodiment of the present invention has a glass _{composition, SiO 2, Al 2 O 3} , B 2 O 3, Li 2 O, Na 2 O, containing _{K 2} O. In the following description of the content range of each component,% display refers to mass%.

SiO ₂ is a main component that forms a glass skeleton structure. The content of SiO _{2 in} the present invention is 75 to 85%, preferably 77 to 82%, more preferably 78 to 81%. In particular, if the content of SiO ₂ is regulated to 78 to 81%, meltability and moldability can be improved without impairing mechanical strength. If the content of SiO ₂ is less than 75%, the mechanical strength of the glass tends to decrease. If the content of SiO ₂ is more than 85%, the high temperature viscosity of the glass becomes high, the meltability and moldability tends to decrease. Therefore, energy required for melting increases and the mold may be damaged during press molding.

Al ₂ O ₃ is a component that increases the chemical durability and mechanical strength of the glass and increases the devitrification resistance by adding an appropriate amount. The content of Al ₂ O ₃ in the present invention is 0 to 5%, preferably 1 to 4%, more preferably 2 to 3%. In particular, if the content of Al ₂ O ₃ is regulated to 2 to 3%, it becomes difficult for crystals to precipitate in the molten glass at the time of molding, and the meltability and moldability can be improved. When the content of Al ₂ O ₃ is more than 5%, the viscosity at high temperature increases, the meltability and the formability tends to decrease. Therefore, energy required for melting increases and the mold may be damaged during press molding.

B ₂ O ₃ is a component that forms a glass skeleton structure and lowers the high-temperature viscosity. In the present invention, the content of B ₂ O ₃ is 10 to 20%, preferably 10 to 15%, more preferably 12 to 15%. In particular, if the content of B ₂ O ₃ is regulated to 12 to 15%, the amount of volatilization from phase separation and molten glass can be suppressed, the energy required for melting can be reduced, and the mold is not easily damaged during press molding. Become. When the content of B ₂ O ₃ is more than 20%, the glass is likely to undergo phase separation. Once phase separation occurs, the thermal expansion coefficient and dielectric loss of the glass (hereinafter referred to as dielectric loss α of the entire glass). In addition to the non-uniformity, the chemical durability tends to decrease. Moreover, the volatilization amount from a molten glass increases, a heterogeneous layer tends to be formed on the molten glass surface, and the homogeneity of glass tends to be lowered. _Further, the content of B 2 _{O 3} is 10% less than the viscosity of the glass becomes too high.

Li ₂ O is a component that lowers the high-temperature viscosity of the glass and increases the meltability. The content of Li ₂ O in the present invention is 0 to 5%, preferably 0 to 3%, more preferably 0 to 1%. When the content of Li ₂ O is more than 5%, the thermal expansion coefficient becomes too high and the dielectric loss α becomes too high. In addition, crystals containing Li are easily precipitated from the molten glass.

Na ₂ O is a component that lowers the high temperature viscosity of the glass and increases the meltability. The content of Na ₂ O in the present invention is 1 to 10%, preferably 2 to 7%, more preferably 4 to 6%. In particular, if the content of Na ₂ O is 4 to 6%, the meltability of the glass can be enhanced without the thermal expansion coefficient and the dielectric loss α being too high. When the content of Na ₂ O is less than 1%, the high-temperature viscosity of the glass increases and the meltability deteriorates. When the content of Na ₂ O is more than 10%, the thermal expansion coefficient and the dielectric loss α are increased, and the reliability of the product is lowered.

K ₂ O is a component that lowers the high temperature viscosity of the glass and increases the meltability, and its content is 0 to 5%, preferably 0 to 3%, more preferably 0 to 1%. When the content of K ₂ O is more than 5%, the thermal expansion coefficient becomes too high and the dielectric loss α becomes too high.

(Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is an index for adjusting the meltability, thermal expansion coefficient, and dielectric loss α of the glass. The range of (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is 0.045 to 0.055, preferably 0.047 to 0.052. In particular, if (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is 0.047 to 0.052, it is possible to suppress an increase in the thermal expansion coefficient and dielectric loss α. The meltability of the glass can also be increased. If (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is less than 0.45, the high-temperature viscosity of the glass increases, the meltability deteriorates and the mold is damaged during press molding. It becomes easy to do. If (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is greater than 0.55, the thermal expansion coefficient and dielectric loss α increase, the thermal shock resistance decreases and hot spots It tends to occur and the reliability of the product decreases.

The heat-resistant glass for microwave ovens of the present invention may contain other components in addition to the above components. For example, MgO, CaO, SrO, BaO, ZnO, TiO ₂ , ZrO ₂ , SnO ₂ , P ₂ O ₅ , Cr ₂ O ₃ , SB ₂ are used to improve the thermal expansion coefficient, dielectric loss α, high temperature viscosity, and the like. O ₃ , SO ₂ , Cl ₂ , PbO, La ₂ O ₃ , WO ₃ , Co ₃ O ₄ , Nb ₂ O ₅ , Y ₂ O ₃ , CeO _{2 and the} like may be contained. The total content of these components is preferably 3% or less.

Microwave resistant glass of the present _{invention, H 2, CO2, CO,} H 2 O, He, Ne, Ar, may contain minor components such as _{N 2} to 0.1% in total. Moreover, as long as the dielectric loss α is not adversely affected, the total amount of noble metal elements such as Pt and Rh may be contained in the glass up to 500 ppm.

  Moreover, the heat resistant glass for microwave ovens of the present invention has a residual stress inside the glass of 5 MPa or less, preferably 3 MPa or less, more preferably 1 MPa or less. In particular, if the residual stress is 1 MPa or less, the residual stress is sufficiently small, and an increase in the dielectric loss α due to the residual stress can be significantly suppressed. On the other hand, if the residual stress is greater than 5 MPa, hot spots are likely to occur due to dielectric loss caused by the residual stress.

  Furthermore, the heat resistant glass for microwave ovens according to the present invention may have a residual stress inside the glass of 0.1 MPa to 5 MPa, preferably 0.1 MPa to 3 MPa, more preferably 0.1 MPa to 1 MPa. . With such a heat-resistant glass for a microwave oven, the dielectric loss can be made almost as designed, and the heat-resistant glass for a microwave oven of the present invention can be efficiently produced.

  In the heat-resistant glass for microwave oven of the present invention, the dielectric loss α at a frequency of 2.45 GHz and 25 ° C. is preferably 0.025 to 0.035, more preferably 0.025 to 0.033, still more preferably 0.027 to 0.033. When the dielectric loss at a frequency of 2.45 GHz and 25 ° C. is higher than 0.035, even when the dielectric loss due to the residual stress in the glass is low, heat is easily generated by the microwave inside the microwave oven and a hot spot is generated. This reduces the reliability of the product. In order to reduce the dielectric loss α at a frequency of 2.45 GHz and 25 ° C. to less than 0.025, the dielectric loss due to the residual stress in the glass is reduced and the content of alkali metal oxide in the glass is reduced. In this case, the viscosity of the glass becomes high, and it becomes difficult to melt the glass. In particular, if the dielectric loss α at a frequency of 2.45 GHz and 25 ° C. is 0.027 to 0.033, it is difficult for heat to be generated by microwaves inside the microwave oven, and the glass is easily melted.

  In the heat-resistant glass for a microwave oven of the present invention, the thermal expansion coefficient at 30 to 380 ° C. is preferably 37 × 10 −7 / ° C. or less. If the thermal expansion coefficient at 30 to 380 ° C. is not more than 37 × 10 −7 / ° C., sufficient thermal shock resistance of glass can be obtained. On the other hand, when the thermal expansion coefficient at 30 to 380 ° C. is higher than 37 × 10 −7 / ° C., the thermal shock resistance of the glass is deteriorated.

  In the heat resistant glass for a microwave oven of the present invention, the temperature at which the viscosity of the glass is 104 dPa · s is preferably 1230 ° C. or lower. If the temperature at which the viscosity of the glass becomes 104 dPa · s is 1230 ° C. or less, the energy required for melting the glass does not increase excessively, and damage to the mold during press molding can be suppressed. On the other hand, when the temperature at which the viscosity of the glass becomes 104 dPa · s is higher than 1230 ° C., the meltability of the glass decreases, the energy required for melting the glass increases, and the mold is easily damaged during press molding.

  When the heat-resistant glass for a microwave oven of the present invention is used as a turntable, it preferably has a disk shape. The heat-resistant glass for microwave ovens according to the present invention may have any other shape as long as it does not adversely affect production efficiency and dielectric loss α. For example, the heat-resistant glass for a microwave oven according to the present invention may be a rectangular flat plate that can be used as a shelf board.

  Moreover, when the heat-resistant glass for microwave ovens forms a disk shape or a flat plate shape as described above, the area of the plate surface (one surface) is preferably 1300 cm 2 or less, more preferably 750 cm 2 or less. The thickness of the heat-resistant glass for microwave ovens of the present invention is preferably 1 to 10 mm, more preferably 3 to 7 mm. If the area is too large or the wall thickness is too thick, the volume of the glass increases, so that it becomes difficult to cool the glass uniformly and reduce the residual stress. On the other hand, if the wall thickness is too thin, it is easily broken when a mechanical impact is applied, and the reliability of strength as a product is lowered.

  The heat-resistant glass for microwave oven of the present invention is heated to 10 ° C./min to (annealing point + 30 ° C.), held for 30 minutes, then cooled to the strain point at 3 ° C./min, and further at 10 ° C./min. The amount of decrease in dielectric loss at a frequency of 2.45 GHz and 25 ° C. at the time of heat treatment for lowering to room temperature (hereinafter simply referred to as “dielectric loss decrease amount γ”) is preferably 0.007 or less, more preferably 0.006 or less. More preferably, it is 0.005 or less. If the reduction amount γ of the dielectric loss is 0.007 or less, the increase in the dielectric loss due to the residual stress in the glass can be suppressed. Therefore, heating by microwaves and generation of hot spots in the cooking chamber of the microwave oven are suppressed, and the reliability as a product is increased. In particular, the smaller the loss γ of the dielectric loss at a frequency of 2.45 GHz and 25 ° C., the less the heat generated by the microwave inside the microwave oven even when the dielectric loss due to the glass composition is high. When the reduction amount γ of the dielectric loss is larger than 0.007, that means that the dielectric loss is significantly larger than the design value due to the residual stress in the glass. Is done.

  In general, the lower the loss γ of the dielectric loss at a frequency of 2.45 GHz and 25 ° C., the better. However, in order to lower it, it is necessary to slow down the temperature-decreasing rate at the time of glass cooling described later. For this reason, when giving priority to the production efficiency of glass, it is preferable to set the reduction amount γ of the dielectric loss when the heat treatment is performed at a frequency of 2.45 GHz and 25 ° C. to 0.001 or more.

  In the heat-resistant glass for microwave oven of the present invention, the value of (dielectric loss decrease γ) / (dielectric loss α of glass before heat treatment) at a frequency of 2.45 GHz and 25 ° C. is preferably 0.1 to 0.3, More preferably, it is 0.1-0.2, More preferably, it is 0.1-0.15. The dielectric loss α of the glass before the heat treatment indicates the dielectric loss of the heat-resistant glass for microwave ovens of the present invention before the heat treatment, that is, the same as the dielectric loss α of the whole glass (hereinafter simply referred to as dielectric loss α Called). In particular, if the value of (dielectric loss decrease γ) / (dielectric loss α) at a frequency of 2.45 GHz and 25 ° C. is 0.1 to 0.15, it results from residual stress with respect to the total dielectric loss of the glass. The influence of dielectric loss is sufficiently reduced, so that an increase in dielectric loss due to residual stress can be suppressed and generation of hot spots can be suppressed. If the value of (dielectric loss reduction amount γ) / (dielectric loss α) is to be less than 0.1, it is necessary to lengthen the slow cooling time of the glass, and there is a concern that the production efficiency is lowered. Or since the dielectric loss of glass is high, there exists a possibility that it will be easily heated with a microwave inside a microwave oven, and a hot spot will occur easily. When the value of (dielectric loss decrease γ) / (dielectric loss α) exceeds 0.3, it means that the dielectric loss is significantly larger than the design value due to residual stress in the glass. There is a concern that the inside of the microwave oven is likely to be heated by microwaves, and hot spots are likely to be generated.

  Subsequently, the manufacturing method of the heat-resistant glass for microwave ovens concerning this invention is demonstrated.

  The heat-resistant glass for a microwave oven according to the present invention is obtained by melting a glass raw material having the above-described composition and pressing the precursor of the heat-resistant glass for microwave oven press-molded from the molten glass so that the internal residual stress becomes 5 Mpa or less. It can be obtained by slow cooling with a temperature-controllable continuous slow cooling furnace.

  In the production method of the present invention, the rate of cooling in the continuous annealing furnace in the temperature range from (annealing point + 50 ° C.) to the strain point of the heat-resistant glass precursor for microwave oven is preferably 0.5 to 3 ° C./min. More preferably, it is 1-2 degreeC / min. If the rate of temperature decrease from the slow cooling start temperature to the slow cooling end temperature is 0.5 to 3 ° C./minute, the residual stress inside the glass is reduced by slow cooling, and high dielectric loss due to the residual stress can be suppressed. In particular, if the rate of temperature decrease from the slow cooling start temperature to the slow cooling end temperature is 1 to 2 ° C./min, the residual stress inside the glass is greatly reduced by slow cooling, and high production efficiency can be realized. On the other hand, when the rate of temperature decrease from the slow cooling start temperature to the slow cooling end temperature is faster than 3 ° C./minute, the slow cooling time is shortened and it is difficult to reduce the residual stress. In addition, when the temperature lowering rate is lower than 0.5 ° C./min, the residual stress can be reduced, but since the slow cooling time becomes longer, the production efficiency is lowered.

  The slow cooling start temperature and the slow cooling end temperature are not limited to the above temperatures, but if the slow cooling start temperature is too high, the glass tends to soften and deform, and the press-molded shape can be maintained during slow cooling. Disappear. On the other hand, if the annealing start temperature is too low, it is difficult to reduce the residual stress inside the glass, and it becomes impossible to suppress the increase in the dielectric loss due to the residual stress. On the other hand, if the annealing end temperature is too high, residual stress is generated in the glass after the end of annealing and the glass has a high dielectric loss. On the other hand, if the annealing end temperature is too low, the annealing time becomes longer and the production efficiency decreases.

  The difference in dielectric loss between the heat-resistant glass for microwave ovens before and after passing through the continuous annealing furnace at a frequency of 2.45 GHz and 25 ° C. is preferably 0.005 or more, more preferably 0.008 or more. In particular, when the difference in dielectric loss at a frequency of 2.45 GHz and 25 ° C. is 0.008 or more, the ratio of the dielectric loss due to the residual stress in the glass in the dielectric loss α of the entire glass can be sufficiently reduced. That is, an increase in unnecessary dielectric loss can be suppressed, and a product with stable quality can be manufactured.

  Furthermore, in the production method of the present invention, the residual stress difference inside the heat-resistant glass for microwave ovens before and after passing through the continuous annealing furnace (that is, the residual stress difference between the precursor and the heat-resistant glass for microwave ovens) is 1.5 MPa or more. It is preferable to cool slowly. More preferably, the glass is slowly cooled so that the residual stress difference inside the glass before and after passing through the continuous annealing furnace is 2.5 MPa or more. In particular, if the difference in the residual stress inside the glass before and after passing through the continuous annealing furnace is 2.5 MPa or more, a high dielectric loss due to the residual stress can be greatly suppressed, and a product with stable quality can be manufactured. .

  In addition, it is preferable that the area of the precursor of the heat-resistant glass for microwave ovens which passes the inside of a continuous annealing furnace is 1300 cm <2> or less, and thickness is 1-10 mm. If the precursor of the heat-resistant glass for microwave ovens is molded with such dimensions, the residual stress can be further easily reduced by the temperature lowering rate, the high dielectric loss due to the residual stress can be suppressed, and the quality can be stabilized.

  Moreover, although the example which performs slow cooling by a continuous slow cooling furnace was shown above, in the case of a small amount production, unless it brings about the fall of production efficiency, after press molding, in the discontinuous slow cooling furnace which can control the temperature of the rapidly cooled glass ( It may be reheated to (annealing point + 50 ° C.) and then gradually cooled to reduce the residual stress.

  Hereinafter, based on an Example, this invention is demonstrated in detail.

  Table 1 shows examples (samples No. 1 and 2) and comparative examples (samples No. 3 and 4) of the present invention.

First, various glass raw materials such as natural raw materials and chemical raw materials were weighed and mixed so as to have the glass composition shown in Table 1 to prepare a glass batch. Next, this glass batch was melted at 1600 ° C. for 8 hours in a platinum crucible, and the obtained glass melt was press-molded to obtain a disk-shaped glass sample. Further, this glass sample was passed through a temperature-controllable continuous annealing furnace having thermocouples installed in various places.

  Moreover, the temperature-fall rate of the glass sample was computed based on the display temperature of the thermocouple of each part of a continuous slow cooling furnace, and the feed rate of the conveyor in a continuous slow cooling furnace. In addition, the conventionally known arbitrary method may be used for the calculation method of the temperature decrease rate.

  For each sample obtained as described above, the residual stress, the residual stress difference before and after passing through the slow cooling furnace, the thermal expansion coefficient at 30 to 380 ° C., the temperature at which the viscosity of the glass becomes 104 dPa · s, the frequency 2.45 GHz, 25 ° C. The relative dielectric constant and dielectric loss tangent at were measured.

  In addition, the residual stress of the glass sample was calculated by the Senarmon method for each sample before passing through the slow cooling furnace using a polarizing microscope. Similarly, the residual stress after passing through the slow cooling furnace was determined, and the difference value of the residual stress before and after passing through the slow cooling furnace was calculated as the residual stress difference inside the glass before and after slow cooling.

  Moreover, the thermal expansion coefficient measured the average linear thermal expansion coefficient in 30-380 degreeC using the thermal expansion coefficient measuring apparatus by MAC SCIENCE.

  The temperature at which the viscosity of the glass was 104 dPa · s was measured by a platinum ball pulling method.

  The relative dielectric constant and dielectric loss tangent at a frequency of 2.45 GHz and 25 ° C. were measured from changes in the resonance frequency and load using a cavity resonator. The dielectric loss was calculated by multiplying the dielectric constant and the dielectric loss tangent.

  Furthermore, the dielectric loss α of each sample glass was calculated by multiplying the relative dielectric constant and the dielectric loss tangent measured as described above. Also, each sample was heated to 10 ° C./min (slow cooling point + 30 ° C.) in a small annealing furnace, held for 30 minutes, then cooled to the strain point at 3 ° C./min, and further to room temperature at 10 ° C./min. The dielectric loss β of the glass after reducing the residual stress by performing a heat treatment to lower the temperature was calculated. Further, the difference between the dielectric loss α of the glass sample after passing through the continuous annealing furnace and the dielectric loss β after the heat treatment was calculated as a decrease amount γ of the dielectric loss upon the heat treatment.

  The dielectric loss α includes a dielectric loss due to the glass composition and a dielectric loss due to the residual stress of the glass. And the residual stress in a glass sample is reduced by the above-mentioned heat processing. Therefore, it is considered that the reduction amount γ of the dielectric loss corresponds to the dielectric loss due to the residual stress of the glass. Therefore, the smaller the decrease amount γ of the dielectric loss, the more the increase in unnecessary dielectric loss is suppressed, and the hot spot is less likely to occur.

  As is apparent from Table 1, sample No. which is an example of the present invention. In Nos. 1 and 2, the rate of temperature decrease from (slow cooling point + 50 ° C.) to the strain point was in the range of 1 to 2 ° C./min, and the residual stress was as small as 2 MPa or less. Therefore, the dielectric loss was as low as 0.031, and the decrease amount γ of the dielectric loss was 0.007 or less. The value of (dielectric loss decrease γ) / (dielectric loss α) was in the range of 0.1 to 0.3. That is, a heat-resistant glass for microwave ovens that hardly generates hot spots was obtained. Sample No. Since 1 and 2 have a preferable glass composition in the present invention, the coefficient of thermal expansion at 30 to 380 ° C. is 37 × 10 −7 / ° C. or less, and the temperature at which the glass viscosity is 104 dPa · s is 1230 ° C. or less. there were. That is, sample no. Nos. 1 and 2 are heat-resistant glass for microwave ovens having high heat resistance, impact resistance and excellent moldability.

  On the other hand, sample No. which is a comparative example. No. 3, the glass composition is within the preferred range in the present invention, but the rate of temperature drop during slow cooling is fast, so the residual stress is larger than in the examples of the present invention, so the dielectric loss α of the glass is 0.039. The dielectric loss reduction amount γ was as high as 0.012. Moreover, the value of (dielectric loss decrease γ) / (dielectric loss α) at a frequency of 2.45 GHz and 25 ° C. was 0.31, which was outside the range of 0.1 to 0.3. That is, sample no. No. 3 is a glass that easily generates hot spots.

  In addition, sample No. No. 4 has a temperature-decreasing rate during slow cooling within the range of the present invention (1-2 ° C./min), but the glass composition is outside the range of the present invention. It was higher than That is, sample no. No. 4 is a glass that easily generates hot spots and has low thermal shock resistance.

  The heat-resistant glass for a microwave oven and the method for producing the same according to the present invention are useful as a heat-resistant glass for a microwave oven and a method for producing the same.

Claims (9)

By _{mass%, SiO 2 75~85%, Al} 2 O 3 0~5%, B 2 O 3 10~20%, Li 2 O 0~5%, Na 2 O 1~10%, K 2 O 0~ 5% composition and mass ratio (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) is 0.045 to 0.055,
A heat-resistant glass for microwave ovens, wherein the internal residual stress is 5 MPa or less.   2. The heat-resistant glass for microwave oven according to claim 1, wherein a dielectric loss at a frequency of 2.45 GHz and 25 ° C. is 0.020 to 0.035.   The heat-resistant glass for microwave oven according to claim 1, wherein a thermal expansion coefficient at 30 to 380 ° C. is 37 × 10 −7 / ° C. or less.   The heat-resistant glass for a microwave oven according to any one of claims 1 to 3, wherein the heat-resistant glass has a disk shape and is used as a plate of a turntable.   The temperature was raised to 10 ° C./min (slow cooling point + 30 ° C.), held for 30 minutes, then lowered to the strain point at 3 ° C./min, and further subjected to heat treatment to lower the temperature to 10 ° C./min to room temperature. The heat-resistant glass for microwave oven according to any one of claims 1 to 4, wherein a decrease amount of dielectric loss in the glass at a frequency of 2.45 GHz and 25 ° C is 0.007 or less.   The value of (reduction amount of dielectric loss in the glass when the heat treatment is performed) / (dielectric loss of the glass before the heat treatment) at a frequency of 2.45 GHz and 25 ° C. is 0.1 to 0.3. The heat-resistant glass for microwave ovens according to claim 5. (1) in _{mass%, SiO 2 75~85%, Al} 2 O 3 0~5%, B 2 O 3 10~20%, Li 2 O 0~5%, Na 2 O 1~10%, K 2 Melting and molding a glass raw material containing a composition of O 0 to 5% and (Li ₂ O + Na ₂ O + K ₂ O) / (SiO ₂ + B ₂ O ₃ + Al ₂ O ₃ ) being 0.045 to 0.055 Melting and molding process,
(2) A method for producing a heat-resistant glass for a microwave oven, comprising a slow cooling step of slowly cooling the glass obtained in the melting and molding step so that the internal residual stress is 5 MPa or less.   In the slow cooling step, the glass obtained in the melting / molding step is from (slow cooling point + 50 ° C.) to the strain point of the heat-resistant glass for microwave oven before slow cooling so that the internal residual stress is 5 MPa or less. The method for producing heat-resistant glass for microwave ovens according to claim 7, wherein the glass is slowly cooled at a temperature-decreasing rate of 0.5 to 3 ° C./min in the temperature range.   9. The slow cooling process according to claim 7 or 8, wherein in the slow cooling step, slow cooling is performed so that a difference in residual stress inside the heat resistant glass for microwave oven before and after slow cooling becomes 1.5 MPa or more. Manufacturing method of heat-resistant glass for microwave ovens.