EP0932706A1 - Composition amelioree reflechissant l'energie infrarouge et procede de fabrication - Google Patents

Composition amelioree reflechissant l'energie infrarouge et procede de fabrication

Info

Publication number
EP0932706A1
EP0932706A1 EP98922473A EP98922473A EP0932706A1 EP 0932706 A1 EP0932706 A1 EP 0932706A1 EP 98922473 A EP98922473 A EP 98922473A EP 98922473 A EP98922473 A EP 98922473A EP 0932706 A1 EP0932706 A1 EP 0932706A1
Authority
EP
European Patent Office
Prior art keywords
enamel
particles
ground coat
energy
composition
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP98922473A
Other languages
German (de)
English (en)
Inventor
Venkat Subramaniam Venkataramani
Douglas Dwight Geise
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
General Electric Co
Original Assignee
General Electric Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US08/862,886 external-priority patent/US5898180A/en
Priority claimed from US08/862,970 external-priority patent/US5905269A/en
Application filed by General Electric Co filed Critical General Electric Co
Publication of EP0932706A1 publication Critical patent/EP0932706A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F24—HEATING; RANGES; VENTILATING
    • F24C—DOMESTIC STOVES OR RANGES ; DETAILS OF DOMESTIC STOVES OR RANGES, OF GENERAL APPLICATION
    • F24C15/00—Details
    • F24C15/005—Coatings for ovens
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C8/00—Enamels; Glazes; Fusion seal compositions being frit compositions having non-frit additions
    • C03C8/14—Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions
    • C03C8/20—Glass frit mixtures having non-frit additions, e.g. opacifiers, colorants, mill-additions containing titanium compounds; containing zirconium compounds
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23D—ENAMELLING OF, OR APPLYING A VITREOUS LAYER TO, METALS
    • C23D5/00—Coating with enamels or vitreous layers

Definitions

  • This invention relates to enhanced reflectance infrared energy reflecting compositions for cooking apparatus.
  • Ovens for cooking food have been known and used for thousands of years.
  • One of the oldest cooking of food resulted when food products were left next to a fire, perhaps on a hot rock, and cooked essentially by a heat transfer method of conduction.
  • an enclosure surrounding the heating element entrapped the heated air, giving rise to cooking by convective heat transfer. This process was the prototype for the modern gas or electric oven.
  • Ovens using infrared energy sources are used for quick heating of food. These quartz halogen lamp ovens can also be used for cooking, and are common in restaurants. In these ovens, most of the heat is infrared energy. This infrared energy is reflected and a majority of the infrared energy is lost into the walls of the oven. The walls of these ovens do not reflect a sufficient amount of infrared energy cooking energy onto the food to be cooked.
  • the metal reflective surfaces provide only a specular reflectance, and do not efficiently disperse and direct the energy to the food to be heated.
  • the specular reflectance by metallic surfaces provides a direct, "angle in equals angle out” type of reflectance.
  • the specular reflectance merely reflects around the oven, without a substantial portion of the energy impinging on the food in the oven.
  • the combination of the deeply penetrating reflected infrared radiation and the intense visible radiation establishes a temperature gradient within the interior of the foodstuff that ensures that the surface temperature of the foodstuff is hotter than the interior, and the products of the cooking, i.e., the water vapor and gases like CO2, are quickly driven to the surface and out of the foodstuff. This process results in a very rapid and efficient cooking of the food.
  • Using combinations of visible and infrared radiation to cook food has significant advantages.
  • the cooking process is very fast.
  • Bakery products for example, can be baked 5 to 10 times faster than conventional convection ovens and conduction cooking processes.
  • the quality of the cooking process is enhanced for many foodstuffs. Vegetables are cooked so fast that they are virtually steamed in their own water vapor, leaving them hot, but with very little loss of any of their nutritive values.
  • the reflectance efficiency of a material composition is dependent on several factors. These factors include the particle size of the reflecting particles and the volume fraction or coverage over the surface of the material composition. An optimum particle size and volume fraction will optimize the reflectance in the desired wavelength.
  • the reflectance efficiency of a material for a certain energy wavelength is dependent on three primary factors. These factors are: (1) a difference in the refractive index of the high index scattering particles and the low index surrounding medium; i.e., the higher the difference between the scattering particles and the medium the better; (2) an optimum particle size (typically about 1/3 to 1/2 the subject wavelength ( ⁇ )); (3) and a volume fraction of the scattering particles to provide a required number of scatterers optimally spaced within the surrounding medium.
  • Enamels typically contain oxide particles, for example, white enamels Q0808A, XT1056-4, T1363 and XT 1032 of the Ferro Co ⁇ oration all contain oxides.
  • enamels comprise a white enamel and further comprise at least one of recrystallized Anatase Ti ⁇ 2 and mill added Anatase Ti ⁇ 2- These enamels, however, are not acceptable infrared reflectors because the size and amount (volume fraction) of Anatase Ti ⁇ 2 particles do not provide a sufficient degree of reflectance.
  • Anatase Ti ⁇ 2 is normally precipitated out of the enamel at a firing condition, for example at about 800°C to about 830°C for 3-10 minutes.
  • Anatase T.O2 has a size equal to or less than about 0.2 ⁇ m, with a reflectivity (defined as a percentage of light reflected over light incident) of about less than 70%.
  • An infrared and visible energy highly efficient reflecting enamel composition comprises a binder and high reflecting non- absorbing metal oxide particles, such as Rutile Ti ⁇ 2 -
  • the reflectivity of the enamel composition is at least about 80% in an infrared and visible energy range between about O. ⁇ m to about 5.0 ⁇ m.
  • the enamel composition also diffusely reflects infrared advisible energies.
  • a method for manufacturing a high reflective infrared and visible energy reflecting enamel composition comprises forming a ground coat of enamel on a substrate; providing a layer of titania powder containing Ti ⁇ 2 particles on the ground coat; and heating the ground coat and the substrate to soften the ground coat, to form a dense layer on the substrate, and to embed Ti ⁇ 2 particles in the ground coat.
  • an oven comprises at least one infrared energy source; at least one internal surface; and a highly reflective infrared and visible energy reflecting enamel composition.
  • the enamel composition comprises a ground coat of enamel provided on the at least one internal surface with Rutile T.O2 particles in the enamel composition.
  • the Rutile T.O 2 particles are embedded in the enamel composition by heating the ground coat and substrate.
  • the enamel composition has an infrared and visible energy reflectivity of at least about 80%.
  • FIG. 1 is an illustration of specular versus diffuse reflectance
  • FIG. 2 is a flow chart illustrating formation of conventional enamels
  • FIG. 3 is a cross-sectional view of a fired enamel on a substrate
  • FIG. 4 is a cross-sectional view of an energy reflecting enamel composition, in accordance with the invention.
  • FIG. 5 is a side cross sectional illustration of an oven provided with reflective enamel composition, in accordance with the invention;
  • Fig. 6 is side cross-sectional view of a substrate covered with a ground coat enamel composition
  • FIG. 7 is a side cross-sectional view of Fig. 6 with a layer of titania powder thereon;
  • FIG. 8 is a side cross-sectional view of Fig. 8 after heat treating.
  • Fig. 9 is side cross-sectional view of Fig. 8 with a layer of enamel after heat treating.
  • the invention is directed to an enamel composition with reflecting particles possessing a particle size and volume fraction to efficiently reflect energy, for example visible and infrared energies.
  • a material's reflectance, such as enamel is dependent on several factors, including: a size of reflecting particles, for example oxide particles; a volume fraction of the reflecting particles over the surface of the material; and a difference in the refractive index of the particles and the surrounding medium. The larger the size and volume fraction of oxide particles in the material, the greater the reflectance for the material.
  • Conventional enamel compositions normally have a white coloration and comprise titania, T.O2, that in part provides an enamel with its white color and appearance.
  • Ti ⁇ 2 in conventional enamel compositions is normally Anatase Ti ⁇ 2 that is precipitated out at a firing condition, by conventional firing for example in a range between about 800°C to about 830°C for a time period of about 3 to about 10 minutes.
  • Anatase Ti ⁇ 2 possess a particle size less than or equal to about 0.2 ⁇ m, and with reflectance values for conventional enamel compositions are about 70%. It has been determined that a diffuse reflectance value of at least about 80% is desirable for applications in cooking ovens relying on intense visible and infrared radiation for reduced cooking time.
  • an size and volume fraction of reflecting oxide particles in the material will result in an acceptable reflectance of energy in a desired wavelength ( ⁇ ).
  • ⁇ a desired wavelength
  • an optimum size for reflecting particles is in a range between about 1/3 wavelength ( ⁇ ) to about 1/2( ⁇ ) of a desired radiation, and an optimum volume fraction is in a range between about 15% to about 40% of the enamel.
  • Reflecting oxide particles are added to an enamel composition to enhance the reflectivity.
  • These particles comprise metal oxide particles, such as Na2 ⁇ , K2O, S_2 ⁇ , Ti ⁇ 2, ZnO, Z1O2, Sb2 ⁇ 3 and compounds thereof, all of which are effective energy reflectors.
  • the above metal oxides and their compounds (metal oxide) are especially effective reflectors for energy with a wavelength range between about 0.04 ⁇ m to about 5.0 ⁇ m. These wavelengths are emitted by visible and infrared energy sources commonly used in rapid cooking ovens (including, but are not limited to, lamps, such as quartz halogen lamps).
  • metal oxides diffusely reflect and disperse visible and infrared energy, so as to efficiently and effectively heat food in an oven with a reflectivity above at least about an 80% reflectance value.
  • reflecting oxide particles also diffuse and disperse the energy, when compared to the direct "angle in equals angle out" specular reflectance of known materials. Diffused energy reflectance causes more energy to impinge on the food, compared to a specular reflection. The difference in specular versus diffused reflection is illustrated in Fig. 1.
  • Rutile Ti ⁇ 2 particles provide effective reflecting particles within the scope of the invention.
  • Rutile Ti0 possesses a high reflective index and a large particle size, greater than about 0.2 ⁇ m.
  • Rutile Ti ⁇ 2 has an index of refraction in a range between about 2.65 to about 2.75. Accordingly, Rutile Ti ⁇ 2 will exhibit a higher reflectance of infrared energy and will act as a suitable reflective particle for enamel compositions, when compared to conventional enamels containing Anatase Ti ⁇ 2.
  • Enamel compositions comprise at least a frit (glass) that includes a binder and oxides; mill additives, such as clay; and other materials for other properties, such as thickness, color, and appearance.
  • Oxides in the frit may include Na2 ⁇ , K2O, S.2O, T.O2, ZnO, Zr ⁇ 2, Sb2 ⁇ 3, oxides and compounds thereof that occur naturally in the frit. Altematively, non-naturally occurring oxides can be added to the frit during preparation.
  • the metal oxide particles in an enamel at least in part, provide reflectance of visible and infrared energies.
  • Fig. 2 is a flow chart illustrating the formation of an enamel-coated substrate.
  • step S1 the individual components, are provided.
  • step S2 the components provided in step S1 are mixed to an enamel slurry.
  • the enamel slurry is coated on an appropriate substrate in step S3.
  • step S4 the coated enamel substrate is fired to densify, harden, and glassify the enamel composition.
  • An enamel composition 100 on a substrate 101 is illustrated in Fig. 3.
  • a ground coat 102 is provided on the substrate 101. Firing of the enamel composition 100, ground coat 101 (if provided), and substrate 101 results in a smooth glassified hardened enamel surface with the enamel composition 100 softening and flowing over the substrate 101.
  • An enamel composition that is fired (heated) for a long time and at a high temperature changes Anatase Ti ⁇ 2 in it into Rutile Ti ⁇ 2-
  • the long heating process recrystallizes the Anatase Ti ⁇ 2 to Rutile Ti ⁇ 2 at temperatures greater than conventional firing temperatures for periods longer than convention firing time periods.
  • the heating temperature can be increased higher than conventional firing temperatures, with the heating time period unchanged. In other words, with an increase of one of the heating time and temperature, the other of the heating time and temperature need not be increased.
  • the Rutile phase of Ti0 2 has a smaller weight percentage compared to the Anatase phase. While at first glance this may appear to be contrary to Rutile Ti ⁇ 2 having a higher infrared reflectance, Rutile Ti ⁇ 2 has a larger particle size than Anatase Ti ⁇ 2.
  • An enamel composition comprising Rutile T.O2 possess a higher reflectivity than Anatase Ti ⁇ 2 because of the respective particle sizes. Therefore, the particle size of a Rutile Ti0 2 , according to the invention, increases the visible to infrared reflectance of the enamel composition by increasing one or both of volume fraction and particle size.
  • An enamel composition with a Ti ⁇ 2 metal oxide according to the invention comprises a layer of Rutile Ti0 2 .
  • the reflectivity of the enamel is enhanced because of a high volume fraction of particles and by the size of the Rutile Ti ⁇ 2 particles.
  • Separated recrystallized and precipitated Rutile Ti ⁇ 2 particles are blended into a binder system of an enamel composition, for example in its slurry form, to enhance the reflectivity of the enamel composition.
  • the blended enamel composition with Rutile Ti ⁇ 2 is provided on an appropriate substrate, for example, by known processes, such as dry or wet processes.
  • the reflectance by oxide particles in an enamel composition is enhanced by placing separated and recrystallized Rutile Ti ⁇ 2 oxide particles into an enamel composition 1 , as illustrated in Fig. 4.
  • the enamel composition 1 comprises Rutile Ti ⁇ 2 particles 2 formed by recrystallizing and precipitating the particles. The particles are ground out into separate Ti ⁇ 2 particles and sorted. The sorted Rutile Ti ⁇ 2 particles 2 are placed in the enamel composition as a mill addition to a binder phase (binder solution) so the enamel is saturated with T.O2 particles. While a binder phase 3 may originally contain some T.O2 in solution, the amount of the T.O2 in solution is normally at a minimum solubility.
  • the binder phase 3 comprises at least one of mixed alkali borosilicate, suitable glass compositions, including precursors of the enamel oxides, such as at least one of nitrates, hydroxides, chlorides, alkoxides, and carboxylates.
  • the saturated enamel composition causes precipitation of Ti ⁇ 2 as Rutile Ti ⁇ 2 particles.
  • the preparation of the enamel for optimization of Rutile Ti ⁇ 2 is also achieved by at least one of removing and lowering a phosphate composition in the enamel for example in a phosphate-stabilized (4% P2O5) enamel. Reflectance may be further enhanced by the addition of additional enamel constituents that promote the recrystallization and growth of Rutile Ti ⁇ 2.
  • the additional enamel constituents that are added to the enamel comprise further amounts of constituents already within the enamel composition, and alternatively comprise mill additions not normally present in an enamel.
  • an enamel composition with enhanced reflectance comprises a Ti ⁇ 2 particle, either Anatase or Rutile, coated with a glassy coating.
  • the coating comprises at least one of binder solutions, and a mixture of at least one of alkali/alkaline earth siiicate/borates, phosphates, and fluorine, and altematively comprises nucleation and growth aids.
  • Nucleation and growth aids include, for example, ZnO, Ce ⁇ 2 and others, as known in the art.
  • Fig. 5 is a front cross section of an oven provided with an enamel coating prepared as in the invention.
  • the oven includes an outer enclosure 100 which includes an inner wall 120 coupled to the outer enclosure 100.
  • An insulating layer 140 is formed between the outer enclosure 100 and the inner wall 120, where the insulating layer 140 may be a layer of air.
  • the energy for cooking is supplied by a heating lamp 16 and a lamp 18.
  • These lamps are generally quartz body, tungsten- halogen lamps such as 1.5 KW 208 V quartz-halogen lamp, and the oven includes any number of lamps.
  • Surfaces 121 of the inner wall 120 are provided with an enamel composition 122, according to the invention, to form a reflecting surface 121 , which will disperse the infrared energy to the food to be heated.
  • a control circuit 34 shown as a circuit block, controls the operation of lamps 16 and 18.
  • the lamps 16 and 18 produce very high intensity radiation, such as visible and infrared radiations.
  • the use of high intensity visible and infrared radiations provides a very rapid method of high-quality cooking and baking.
  • the radiant energy from the lamps 16 and 18 radiates in all directions. A portion of the energy radiates directly onto the food item 32 and the remainder will reflect until it impinges onto the food item 32.
  • an enamel composition is formed by placing a ground coat of enamel 1002 on a substrate 1001 , for example as an electrocoated powder coating by an electrostatic process.
  • the substrate 1001 may be formed from any appropriate material usable as an infrared heating oven internal surface, such as, but not limited to, one of aluminum, steel, and alloys thereof.
  • a layer of titania powder 1030 is then placed on the ground coat 1002.
  • the substrate 1001 , ground coat 1002, and titanium powder layer 1030, are heated for a first heat cycle at a first temperature. This first heat cycle softens the ground coat 1002 and causes the ground coat 1002 flow and spread over the substrate 1001.
  • the first heat cycle ends with cooling of the elements.
  • the ground coat 1002 upon cooling, forms a densified, hardened layer.
  • the first heat cycle causes a majority of the Ti ⁇ 2 particles 1003 of the titanium powder layer 1030 to sink into and embed in the ground coat 1002.
  • the embedding occurs at a top surface, while the ground coat 1002 is soft and flows.
  • the first heat cycle duration can be extended to cause a majority of the Ti ⁇ 2 particles to sink into and become embedded in the ground coat 1002 below the top surface 1008 of the ground coat 1002. This process also form heat treated infrared reflecting enamel composition 1005.
  • a suitable white enamel 1006 which comprises a glass enamel composition, is then layered on the heat treated infrared reflecting enamel composition 1005, as illustrated in Fig. 9.
  • This enamel 1006 provides for enhanced cleaning without harm of enamel.
  • the enamel 1006 is formed with a suitable thickness, for example in a range between about 0.00127 m to about 0.00762 m.
  • the enamel 1006 and the heat treated infrared reflecting enamel composition 1005 forms an enamel assembly 1010.
  • the enamel assembly 1010 is heat treated, for a second heat cycle, at a second temperature, for example, above a range between about 830°C to about 855°C.
  • the second heat cycle occurs for an appropriate time period, for example, longer than at least 3 minutes, which softens the enamel assembly 1010.
  • the enamel 1006 flows to provide a hard, glazed, and recrystallized surface.
  • the high reflective infrared reflecting enamel composition now comprises high reflective infrared and visible light reflecting Rutile Ti ⁇ 2-
  • the reflectivity value of the resultant enamel assembly 1010 is preferably at least in a range between about 80% to about 90%, with Rutile Ti ⁇ 2 particles possessing a size in a range between about 0.2 ⁇ m and 2.0 ⁇ m, in a volume fraction up to about 15 wt%.
  • the Rutile Ti ⁇ 2 particles scatter and reflect energy incident on the ground coat.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • General Engineering & Computer Science (AREA)
  • Combustion & Propulsion (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • General Chemical & Material Sciences (AREA)
  • Geochemistry & Mineralogy (AREA)
  • Glass Compositions (AREA)

Abstract

L'invention concerne une composition d'émail réfléchissant l'énergie infrarouge, qui comporte une couche primaire (2, 1002) d'émail sur un substrat (1, 1001) et des particules (1, 1003) de TiO2 incluses dans la couche primaire d'émail. La composition d'émail réfléchissant l'infrarouge possède un pouvoir réfléchissant d'énergie infrarouge égal à au moins 80 %. Un procédé de fabrication de la composition est également décrit, qui prévoit l'inclusion de TiO2 dans l'émail. L'émail peut également comporter des particules d'oxyde métallique qui permettent d'améliorer le pouvoir réfléchissant d'énergie infrarouge de l'émail. De plus, les particules d'oxyde métallique dans l'émail peuvent être formées de plusieurs manières afin d'améliorer le pouvoir réfléchissant.
EP98922473A 1997-05-23 1998-05-19 Composition amelioree reflechissant l'energie infrarouge et procede de fabrication Withdrawn EP0932706A1 (fr)

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
US08/862,886 US5898180A (en) 1997-05-23 1997-05-23 Infrared energy reflecting composition and method of manufacture
US862886 1997-05-23
US08/862,970 US5905269A (en) 1997-05-23 1997-05-23 Enhanced infrared energy reflecting composition and method of manufacture
US862970 1997-05-23
PCT/US1998/010457 WO1998053118A1 (fr) 1997-05-23 1998-05-19 Composition amelioree reflechissant l'energie infrarouge et procede de fabrication

Publications (1)

Publication Number Publication Date
EP0932706A1 true EP0932706A1 (fr) 1999-08-04

Family

ID=27127729

Family Applications (1)

Application Number Title Priority Date Filing Date
EP98922473A Withdrawn EP0932706A1 (fr) 1997-05-23 1998-05-19 Composition amelioree reflechissant l'energie infrarouge et procede de fabrication

Country Status (2)

Country Link
EP (1) EP0932706A1 (fr)
WO (1) WO1998053118A1 (fr)

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102004008849B4 (de) * 2003-02-20 2008-01-17 Miele & Cie. Kg Verfahren zur Herstellung einer wärmestrahlungsreflektierenden Emailschicht und Verwendung des Verfahrens
KR102609736B1 (ko) * 2018-06-15 2023-12-07 필립모리스 프로덕츠 에스.에이. 에어로졸 발생 장치용 먼지-기피성, 열-반사성 코팅

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE2211135C3 (de) * 1972-03-08 1981-06-19 Bayer Ag, 5090 Leverkusen Verfahren zur Herstellung von selbstreinigenden Emailschichten für Koch- und Backgeräte unter Verwendung eines Emailschlickers mit Eisen- und/oder Titanoxiden als Katalysatoren
DE2646513C2 (de) * 1976-10-15 1984-10-04 Bfg Glassgroup, Paris Verfahren zur Herstellung einer wärmereflektierenden Natron-Kalk-Silikatglasscheibe
JPS557586A (en) * 1978-08-26 1980-01-19 Toomei Kogyo Kk Low temperature boiling kettle
JPS59173272A (ja) * 1983-05-13 1984-10-01 Toomei Kogyo Kk 特に遠赤外線発生素子を調合した釉薬を用いた琺瑯の製造方法
JPS63117928A (ja) * 1986-11-07 1988-05-21 Masao Yamamoto 遠赤外線輻射琺瑯の製造方法
DE4126790C2 (de) * 1991-08-14 2002-06-06 Miele & Cie Verfahren zur Herstellung eines Backofens mit einer emaillierten Backmuffel

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO9853118A1 *

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Publication number Publication date
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