WO2009028242A1

WO2009028242A1 - Plasma display panel

Info

Publication number: WO2009028242A1
Application number: PCT/JP2008/059395
Authority: WO
Inventors: Satoru Watanabe; Setsuro Ito; Hiroshi Kajiyama
Original assignee: Asahi Glass Company, Limited
Priority date: 2007-08-24
Filing date: 2008-05-15
Publication date: 2009-03-05
Also published as: JP2009054368A; TW200910405A

Abstract

For a PDP, a panel having favorable discharge properties such as high discharge efficiency and a short discharge delay, being chemically stable and capable of electric power saving, is desired. A protective layer of PDP contains a Mayenite compound wherein at least a part of oxygen constituting the Mayenite compound is substituted by electrons, and a part of the electrons is further substituted by anions of atoms having lowe electron affinity than that of oxygen. The Mayenite compound has an electron density of at least 1 x1015 cm-3, and the secondary electron emission coefficients excited by Ne or Xe ions at an accelerating voltage of 600V, are respectively at least 0.05 at a secondary electron collector voltage at which secondary electrons can be sufficiently captured.

Description

DESCRIPTION

PLASMA DISPLAY PANEL

TECHNICAL FIELD

The present invention relates to a plasma display panel .

BACKGROUND ART A plasma display panel (hereinafter referred to as PDP) has such a structure that one of two glass substrates facing each other with a discharge space in which a discharge gas is sealed, has pairs of display electrodes extending in the lateral direction arranged in the lengthwise direction, and the other has sustaining electrodes extending in the lengthwise direction arranged in the lateral direction, and at intersections of the pairs of display electrodes and the sustaining electrodes in the discharge space, matrix unit luminescence regions (discharge cells) are formed.

The operation principle of a PDP is to utilize a luminescence phenomenon accompanying the gas discharge. As its structure, it has barrier ribs between a transparent front substrate and a back substrate facing each other, and cells (space) are partitioned by the barrier ribs. Into the cells, a Penning gas mixture such as He and Xe or Ne and Xe with small visible luminescence and a high ultraviolet luminous efficiency is sealed to generate plasma discharge in the cells, which makes a phosphor layer on the inner wall of the cells emit light to form an image on the display screen. In the PDP, at a position which faces the unit luminescence regions on a dielectric layer formed to cover the display electrodes and the sustaining electrodes, a magnesium oxide (MgO) film having a function to protect the dielectric layer and a function of secondary electron emission to the unit luminescence regions is formed. As a method of forming such a magnesium oxide film in a PDP production process, a deposition method and a screen printing method of forming a film by coating a dielectric layer with an ink having a magnesium oxide powder mixed therewith have been known (Patent Document 1) .

In a PDP having such a structure, secondary electrons are emitted from the surface of the MgO film by incidence of Penning gas ions into the MgO film. It has been known that in a PDP, a plasma state is formed triggered by the secondary electron current . The problem here is that the MgO film emits no sufficient secondary electrons for plasma formation by the incidence of Xe ions, whereby it emits sufficient secondary electrons by incidence of Ne ions (Non-Patent Document 1) .

Further, MgO is a chemically unstable substance in the air, and accordingly it is difficult to obtain a PDP having favorable properties unless an activating treatment of carrying out heat treatment in vacuum is carried out. To solve this problem, a PDP having a protective layer containing a Mayenite compound wherein a part of oxygen is substituted by electrodes, is proposed (Non-Patent Document 2) . However, since it is necessary to carry out a heat treatment of at least 150⁰C in vacuum in a case of using MgO for the protective layer of PDP to obtain activation, the property of the Mayenite compound may be deteriorated by the heat treatment when the

Mayenite compound is used for a protective layer together with e.g. MgO.

A typical method for producing PDP is as follows. First of all, on a glass substrate, display electrode pairs each constituted by a transparent electrode such as ITO and a bus electrode such as a Cr-Cu-Cr electrode or a Ag electrode formed on a part of the transparent electrode, are formed. A sputtering method is used for forming the transparent electrode, and a roll coating or a screen printing is used for forming the bus electrode. Further, in order to form a dielectric layer covering the pairs of display electrodes, a glass paste is applied by a screen printing or a roll coating and baked at from 530 to 580⁰C. Thereafter, by using e.g. vacuum vapor deposition, a protective layer is formed on the transparent dielectric layer, to produce a front plate. The front plate and a rear plate produced in advance are sealed together by using a low-melting point glass at a temperature of from about 440 to 500°C to form a panel. The panel thus sealed with the low-melting-point glass is heated at from 250 to 380°C, and a discharge gas such as Ne, a Penning gas mixture of Ne and Xe or a Penning gas mixture of He and Xe is enclosed in the panel while the air in the panel is exhausted so that the pressure of these discharge gas becomes from 100 to 500 Torr. In the method for producing PDP described above, in order to carry out the sealing with low-melting-point glass, a Mayenite compound has been desired, whose property such as secondary electron emission is not significantly deteriorated even it is subjected to a heating treatment of from 250 to 500⁰C in the presence of air, and which is excellent in thermal stability or oxidation resistance. Patent Document 1: JP-A-6-325696

Non-Patent Document 1: Kyoung Sup, Jihwa Lee, and Ki-Woong, J. Appl . Phys, 86, 4049 (1999) Non-Patent Document 2: S. Webster, M. Ono-Kuwahara, S. Ito, K. Tsutsumi, G. Uchida, H. Kajiyama, T. Shinoda, Proceeding of IDW '06, p. 345 (2006)

DISCLOSURE OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

The object of the present invention is to solve the above problems and to provide a PDP for which Ne ions or Xe ions can be used as ions for the excitation, which provides a favorable efficiency of ultraviolet luminescence from the sealed gas, which provides favorable discharge properties such as discharge efficiency and a short discharge delay, and which is chemically stable, excellent in oxidation resistance and is capable of electric power saving.

MEANS FOR SOLVING THE PROBLEM The present invention provides a plasma display panel comprising a front substrate and a rear substrate facing each other via a discharge space, discharge electrodes formed on at least one of the front substrate and the rear substrate, a dielectric layer covering the discharge electrodes, and a protective layer covering the dielectric layer, wherein the protective layer contains a Mayenite compound, at least a part of oxygen constituting the Mayenite compound is substituted by electrons, a part of the electrons is further substituted by anions of atoms having lower electron affinity than that of oxygen, the Mayenite compound has an electron density of at least IxIO¹⁵ cm^"3, and the secondary electron emission coefficients excited by Ne or Xe ions at an accelerating voltage of 600 V, are respectively at least 0.05 at a secondary electron collector voltage at which secondary electrons can be sufficiently captured.

Further, the present invention provides the above plasma display panel, wherein the secondary electron emission coefficient excited by Ne ions is at least 0.05 at a secondary electron collector voltage at which secondary electrons can be sufficiently captured. Further, the present invention provides the above plasma display panel, wherein the secondary electron emission coefficient excited by Xe ions is at least 0.05 at a secondary electron collector voltage at which secondary electrons can be sufficiently captured. Further, the present invention provides the above plasma display panel, wherein the Mayenite compound is 12CaO- 7Al₂O₃ compound, 12SrO- 7Al₂O₃ compound, a mixed crystal compound of these, or an isomorphous compound of any one of these. Further, the present invention provides the above plasma display panel, wherein the Mayenite compound has a part of Al substituted by Si, Ge, B or Ga .

Further, the present invention provides the above plasma display panel, wherein the protective layer has a thin layer having a conductivity of at most l.OxlO^"5 S/cm on the dielectric layer, and on a part of the thin layer, there is disposed the Mayenite compound wherein at least a part of oxygen constituting the Mayenite compound is substituted by electrons, a part of the electrons is further substituted by anions of atoms having lower electron affinity than that of oxygen, and the Mayenite compound has an electron density of at least IxIO¹⁵ cm^"3. Further, the present invention provides the above plasma display panel, wherein the thin layer is a layer containing at least one compound selected from the group consisting of MgO, SrO, CaO, SrCaO and a Mayenite compound.

Further, the present invention provides the above plasma display panel, wherein the content of the Mayenite compound is at least 5 vol% to the total volume of the materials forming the protective layer. Further, the present invention provides a process for producing a plasma display panel comprising a front substrate and a rear substrate facing each other via a discharge space, discharge electrodes formed on at least one of the front substrate and the rear substrate, a dielectric layer covering the discharge electrodes, and a protective layer covering the dielectric layer; which comprises a step of disposing a Mayenite compound on the dielectric layer, wherein at least a part of oxygen constituting the Mayenite compound is substituted by electrons, a part of the electrons is further substituted by anions of atoms having lower electron affinity than that of oxygen, and the Mayenite compound has an electron density of at least IxIO¹⁵ cm^"3.

Still further, the present invention provides the above process for producing a plasma display panel, wherein the thin layer is a layer containing at least one compound selected from the group consisting of MgO, SrO, CaO, SrCaO and a Mayenite compound.

EFFECTS OF THE INVENTION

The PDP comprising a protective layer containing a Mayenite compound of the present invention has favorable discharge properties such as a high ultraviolet luminous efficiency, a high discharge efficiency and a short discharge delay, the PDP is chemically stable, and is excellent in oxidation resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1: A cross section schematically illustrating a first embodiment of the present invention in which Mayenite particles are disposed on a protective layer of a PDP.

Fig. 2: A cross section schematically illustrating a second embodiment of the present invention in which Mayenite particles are contained in a protective layer of a PDP. Fig. 3: A schematic view of a secondary electron emission coefficient measurement apparatus.

Fig. 4: A graph showing the relation between a secondary electron emission coefficient ( y ) of sample B before heating treatment and a collector voltage, when Ne⁺ is used as ions for excitation.

Fig. 5: A graph showing the relation between a secondary electron emission coefficient ( γ ) of sample C before heating treatment and a collector voltage, when Ne⁺ is used as ions for excitation.

Fig. 6: A graph showing the relation between a secondary electron emission coefficient ( γ ) of sample B before heating treatment and a collector voltage, when Xe⁺ is used as ions for excitation.

Fig. 7: A graph showing the relation between a secondary electron emission coefficient ( y ) of sample C before heating treatment and a collector voltage, when Xe⁺ is used as ions for excitation.

Fig. 8: A graph showing an optical-absorption spectrum of sample D obtained by transforming a diffused reflection spectrum by Kubelka-Munk method.

Fig. 9: A graph showing an ESR signal of sample D. Fig. 10: A graph showing the relation between a secondary electron emission coefficient ( γ ) of sample D before heating treatment and a collector voltage, when Ne⁺ or Xe⁺ is used as ions for excitation.

MEANINGS OF SYMBOLS 12 : Thin layer

14 : Mayenite compound particles wherein a part of oxygen constituting the Mayenite compound is substituted by electrons and anions 20: Protective layer 22: Base material 24 : Mayenite compound particles wherein a part of oxygen constituting the Mayenite compound is substituted by electrons and anions

BEST MODE FOR CARRYING OUT THE INVENTION A PDP usually has a front substrate and a rear substrate facing each other via a discharge space, discharge electrodes formed on at least one of the front substrate and the rear substrate, a dielectric layer covering the discharge electrodes, and a protective layer in the form of a thin film covering the dielectric layer. In a conventional PDP, a MgO film is mainly used for the protective layer. In a PDP using a MgO film for the protective layer, MgO is irradiated with Ne ions as excitation ions to emit secondary electrons, which then forms a plasma state, and from excited Xe atoms or Xe molecules present in the plasma, vacuum ultraviolet- rays are emitted. Further, in the plasma, a Penning gas is present as ionized.

In the present invention, by the protective layer containing a Mayenite compound, not only Ne ions but also Xe ions can be used as excitation ions, and also in a case where Xe ions are used, a high secondary electron emission coefficient is obtained, and the efficiency of ultraviolet luminescence from a PDP will improve. Here, the secondary electron emission coefficient is measured by irradiating a target (a sample to be measured) disposed in a vacuum container with Ne ions or Xe ions by an ion gun, and collecting secondary electrons using a secondary electron collector disposed near the target .

The secondary electron collector voltage at which secondary electrons can be sufficiently captured in the present invention is not particularly limited so long as it is a voltage at which secondary electrons can be sufficiently captured and varies depending upon the material of the target . The number of secondary electrons which can be captured increases as the collector voltage increases, and the number of secondary electrons which can be captured is saturated by degrees along with the increase of the voltage. The secondary electron collector voltage at which secondary electrons can be sufficiently captured means a voltage at which the number of secondary electrons which can be captured is saturated. For example, in the case of an electrically conductive Mayenite compound, the secondary electron emission coefficient Y is substantially saturated at 70 V, and accordingly a value at 70 V may be regarded as the Y value .

In the present invention, a Mayenite compound means 12CaO- 7Al₂O₃ (hereinafter sometimes referred to as C12A7) crystals and an analogue having a crystal structure similar to the C12A7 crystals. A Mayenite compound has a cage structure and includes oxygen ions in the cage. An oxygen ion included in the cage is usually called as a free oxygen ion. The Mayenite compound in the present invention includes an isomorphous compound having a part of or all cations or anions in the framework or the cage substituted, so long as the framework of the C12A7 crystal lattice and the cage structure formed by the framework are maintained.

Specifically, the following compounds (1) to (4) may be mentioned as examples of the Mayenite compound, but the Mayenite compound is not limited thereto. (1) Strontium aluminate Sr_I2Al₁₄O₃₃ having a part of or all cations in the framework of the C12A7 compound substituted, and calcium strontium aluminate Ca_I2-XSr_xAIi₄O₃₃ which is mixed crystals having a mixture ratio of Ca and Sr optionally changed. (2) Ca_I2AIi₀Si₄O₃₅ which is a silicon-substituted Mayenite .

(3) One having free oxygen in the cage substituted by an anion such as H^", H₂ ^", H^2", 0^", O₂ ^", OH^", F^", Cl^", Br^", S^2" or Au^", such as Ca_i2Al_I4O₃₂ : 2OH^" or Ca₁₂AIi₄O₃₂ : 2F^" . (4) One having both cation and anion substituted, such as Wadalite Ca₁₂AIi₀Si₄O₃₂ : 6Cl^" .

The included electrons are loosely bound in the cage and can freely move in crystals thereby to impart electrical conductivity to the Mayenite compound. A C12A7 compound having all free oxygen substituted by electrons may sometimes be represented as [Ca₂₄Al₂₈O₆₄] ⁴⁺(4e^") . In the Mayenite compound of the present invention, a part or all of oxygen in the cage is substituted by- electrons, and a part of the electrons is substituted by anions of atoms having lower electron affinity than that of oxygen. As a result, in the Mayenite compound of the present invention, cages including electrons and cages including the anions are present .

The electron density of the Mayenite compound of the present invention is at least IxIO¹⁵ cm^"3. The conductivity of the electrically conductive Mayenite compound is preferably at least 1. OxICT⁴ S/cm, more preferably at least 1.0 S/cm, furthermore preferably at least 100 S/cm.

Further, the density of the anions in the Mayenite compound in which a part of the electrons is substituted by anions, is preferably at least IxIO¹⁵ cm^"3. When the density of anions is at least IxIO¹⁵ cm^"3, the electron emission property improves such that the secondary electron emission coefficient increases. In the present invention, a part of the electrons is substituted by anions of atoms having smaller electron affinity than oxygen. Here, the electron affinity is an energy required to produce an anion from an atom, which is equal to a work required to separate an electron from an anion. In general, an enthalpy change in a process of producing an anion from an atom of an optional atom M, that is, an enthalpy change of M→M^", is defined as an electron affinity. In the present invention, the electron affinity of oxygen is defined as a value in the change of O→O^2" . Further, the electron affinity of an atom in the present invention includes an enthalpy change in a process of M→M^2" besides that of M→M^" .

Comparison of the magnitudes of electron affinities can be more accurate by using the values in the cage of the Mayenite compound. However, since there is existing data of such a value in vacuum is available, and since it is considered that there is no significant difference between a value in the cage and a value in vacuum, such a value in vacuum can be used. In terms of the value of electron affinity in vacuum, anions of atoms having smaller electron affinity than oxygen may, for example, be H^', H₂ ^", H^2", 0^", O₂ ^", F^", Cl^", Br^", I^" and S^2". These anions can more easily supply an electron into the cage as compared with an oxygen ion, and thus, an electron emission property improves such that the secondary electron emission coefficient increases. For example, when a part of electrons is substituted by H^" being an anion of an atom having smaller electron affinity than oxygen, an electron is introduced into the cage in a process shown in formula (1) , and as a result, an electron emission property improves such that the secondary electron emission coefficient increases, such being preferred.

H^" → H⁰ + e^" (1) In the present invention, since a part of electrons in the Mayenite compound is substituted by anions of atoms having smaller electron affinity than oxygen, oxidation resistance is significantly improved. Detail of the mechanism of this function is not clear, but the mechanism is presumed to be as follows.

Since a Mayenite compound is a compound having high reduction property, at a high temperature, electrons are more easily released from cages of the compound as compared with from oxygen ions. Accordingly, when a

Mayenite compound only including electrons is subjected to a high temperature such as 500⁰C in the air, electrons supplied from the cages alter oxygen in the air to oxygen ions, which facilitates adoption of oxygen ions into the cages. This is because a framework structure forming the cage structure of a Mayenite compound has a positive electric charge, and accordingly, oxygen ions having negative electron charges are captured into the cages but oxygen molecules or oxygen atoms that are electrically neutral are hardly taken into the cages.

For the reason described above, as compared with a Mayenite compound including only electrons, a Mayenite compound having less capability of supplying electrons to oxygen in the air, can suppress a reaction of forming oxygen ions from oxygen molecules and slow the speed of capturing oxygen ion into the cage .

In the present invention, it is presumed that since a part of electrons in the Mayenite compound is substituted by anions of atoms having smaller electron affinity than oxygen, the Mayenite compound has less capability of supplying electrons as described above, and accordingly, the oxidation resistance is significantly improved .

In the Mayenite compound of the present invention, a part of Al contained in the Mayenite compound may be substituted by Si, Ge, Ga or B . Further, the Mayenite compound may contain at least one type selected from the group consisting of Si, Ge, Ga and B; at least one type selected from the group consisting of Li, Na and K; at least one type selected from the group consisting of Mg and Ba; at least one type of rare earth element selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho₇ Er, Tm and Yb; or at least one type of transition metal element of typical metal element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni and Cu. In general, a compound having a low work function has high secondary electron emission performance. For example, the bulk of an electrically conductive Mayenite compound is cleft or ground in vacuum to obtain a clean surface, and the work function on that occasion is about 2 eV. The clean surface means no attachment of impurities such as a degenerated layer or an organic substance on the surface. Further, such a clean surface can be obtained also by holding a Mayenite compound in ultra-high vacuum at a temperature of approximately 65O⁰C or higher. Further, when a part of electrons in the cage on the outermost layer disappear by applying appropriate treatment to the surface of an electrically conductive Mayenite compound, the effective work function can be lowered to 1 eV or lower. The thickness of the surface modified layer is preferably at most 1 nm. If the thickness exceeds 1 nm, no effect of lowering the work function may be obtained. In a case where an electrically conductive Mayenite is used in the present invention, the surface state of the Mayenite compound may be the clean surface, but preferred is the above- described surface modified layer, whereby an increase of the secondary electron emission properties can be expected since the work function is low. To impart the above-described surface modified layer to the electrically conductive Mayenite compound, for example, electrons in the cage may be substituted by an anion such as O^2", H^', O₂ ^", 0^', S^2", F^", OH^" or Cl^". For example, in a case where they are substituted by O^2", heat treatment under an oxygen partial pressure P₀₂ by the Pa unit higher than the oxygen partial pressure represented by the mathematical formula 2, where T is the temperature: P_O2=10⁵Xexp [{-7. 9X10⁴/ (T+273) } +14. 4]

On the surface of the Mayenite compound used in the present invention, preferably no impurities such as an organic substance are attached, so as not to decrease the secondary electron emission properties.

The secondary electron emission coefficient γ of the protective layer containing the Mayenite compound of the present invention is, when Ne or Xe is used as excitation ions at an accelerating voltage of 600 V, is at least 0.05, preferably at least 0.1. This is because by secondary electrons, Xe atoms become Xe ions, which emit ultraviolet rays, whereby the efficiency of ultraviolet luminescence from Xe will improve. The secondary electron emission coefficient Y is more preferably at least 0.2. This is because the efficiency of ultraviolet luminescence from Xe will further improve, whereby a PDP having favorable discharge properties such as a high discharge efficiency and a small discharge delay will be obtained.

The secondary electron emission coefficient γ when Ne is used as excitation ions is at least 0.05, more preferably at least 0.2. Further, the secondary electron emission coefficient Y when Xe is used as excitation ions is at least 0.05, more preferably at least 0.07.

The Mayenite compound to be used for the PDP of the present invention can be prepared, for example, as follows. However, another preparation method may be employed, or preparation conditions may be changed.

CaO or SrO and Al₂O₃ in a molar ratio of CaO or SrO to Al₂O₃ of from 11.8:7.2 to 12.2:6.8 are blended or mixed, and the resulting material is heated to 1,200 to

1,350⁰C in the air to prepare a Mayenite compound by solid phase reaction.

A powder or a sintered product of the Mayenite compound prepared in the above manner, is subjected to a heat treatment (hereinafter referred to as hydrogenation treatment) at a temperature of from 800 to 1,415⁰C in an atmosphere containing hydrogen and having at most 100 ppm of water vapor, whereby a Mayenite compound in which a part of free oxygen ion is substituted by H^" is obtained. The partial pressure of oxygen gas in the atmosphere of the hydrogenation treatment is preferably from 1x1O⁴ to l.lxlO⁵ Pa in order to obtain sufficient H^" concentration. For example, by supplying a mixed gas of hydrogen gas and nitrogen gas into an electric furnace having an alumina furnace tube so that the hydrogen gas concentration becomes 20 vol%, the above-mentioned hydrogen gas partial pressure can be obtained.

By irradiating the Mayenite compound in which a part of free oxygen ions is substituted by H^" with ultraviolet rays of from 140 to 380 nm, it is possible to introduce electrons separated from H^" into the cage, and to thereby obtain an electrically conductive Mayenite compound containing cages including free oxygen ions or H^" and cages including electrons.

Further, by irradiating with an electron beam the Mayenite compound in which a part of free oxygen ions is substituted by H^", it is possible to introduce electrons into cages and to thereby obtain an electrically conductive Mayenite compound containing cages including free oxygen ions or H^" and cages including electrons. Further, by placing in plasma a Mayenite compound that has been subjected to a hydrogenation treatment, it is also possible to obtain an electrically conductive Mayenite compound containing cages including free oxygen ions and H^" and cages containing electrons . An electrically conductive Mayenite compound containing cages including free oxygen ions or H^" and cages containing electrons produced in the above manner, have optical-absorptions at 2.8 eV and 0.4 eV. By measuring the optical-absorption coefficient, the electron density can be calculated. When the sample is a powder, the electron density can be easily obtained by using a diffused reflection method. Further, since an electron in a cage is spin active, it is also possible to measure the electron density in the cages by using an electron spin resonance (ESR) .

The hydrogen ion H^" concentration in the above electrically conductive Mayenite compound can be quantified by using a secondary ion mass spectrometer (SIMS) . Further, at this time, in order to distinguish H^" from OH^", it is preferred to quantify the concentration of OH^" by e.g. measuring the infrared absorption spectrum (IR) . By subtracting the concentration of OH^" quantified by IR from the total amount of H^" concentration quantified by SIMS, it is possible to accurately quantify the concentration of only H^". Further, as another method of quantifying H^" concentration, a method of quantifying hydrogen atoms H⁰ produced by separating an electron from H^" (H^" → H⁰ + e^") by using an ESR is also possible. In a case of carrying out this measurement, it is desirable to make the sample temperature to be at most 200 K.

Further, in order to obtain a Mayenite compound in which the anions substituting free oxygen ions is Cl^" or F^", the following method may, for example, be mentioned. A raw material prepared by blending CaCO₃ and Al₂O₃ so that the molar ratio becomes 11:7, is subjected to a heating treatment in the air at from 1,100 to 1,150⁰C to obtain a sintered product (hereinafter referred to as C11A7) , and the C11A7 and CaCl₂ are blended so that the molar ratio between Ca and Al becomes 12:7 to obtain a blended product C11A7 'CaCl₂, and the blended product is left in the air at a temperature of from 900 to 1,300⁰C, to obtain a Mayenite compound Ca₁₂Al₁₄O₃₂ : 2Cl^" in which Cl^" is introduced into the cages. Further, when the C11A7 and CaF₂ are blended so that the molar ratio between Ca and Al becomes 12:7 to obtain a blended product C11A7- CaF₂, and the blended product is left in the air at a temperature of from 900 to 1,300⁰C to obtain a Mayenite compound

in which F^" is introduced in the cages .

Further, by a hydration reaction, it is possible to make H₂O and a Mayenite compound react to thereby introduce OH^" into the Mayenite compound. For example, by putting a powder of Mayenite compound in a solvent such as a pure ethanol or an ethanol containing water, and leaving them at a room temperature for 1 hour as they are stirred, it is possible to carry out the hydration reaction. As an alternative, by using water vapor, it is possible to carry out a hydration reaction in a gas phase .

From a Mayenite compound in which free oxygen ions are substituted by Cl^" or F^" thus obtained, it is possible to obtain an electrically conductive Mayenite compound containing cages including free oxygen ions, Cl^" or F^" and cages including electrons, by irradiating the above Mayenite compound with ultraviolet rays or an electron beam, or by leaving the above Mayenite compound in a plasma.

The electrically conductive Mayenite compound of the present invention containing cages including H^" and cages including electrons, is excellent in thermal stability or oxidation resistance such that even if it is subjected to a heat treatment of from 250 to 380⁰C in the presence of air after the sealing step with low-melting point glass in the process of producing PDP, less deterioration in properties such as secondary electron emission occur as compared with a Mayenite compound in which a part of oxygen constituting the Mayenite compound in the cages is substituted only by electrons. Further, in the above-mentioned production process of PDP, a front plate and a rear plate prepared in advance are sealed together by using a low-melting point glass at from about 440 to 500⁰C. In this step, the electrically conductive Mayenite compound containing cages including H^" and electrons, is excellent in thermal stability or oxidation resistance, and thus, even if it is subjected to a temperature of e.g. 460⁰C, less deterioration in the property such as secondary electron emission, occurs. Further, the Mayenite compound of the present invention is employed for forming a protection layer of PDP, and by carrying out a heat treatment or an aging by sustain discharge in the PDP, the electron emission property of the Mayenite compound can be further improved.

The method for such a treatment is not particularly limited, and it may, for example, be a heat treatment in vacuum at at least 300⁰C, irradiation of ultraviolet rays, irradiation of an electron beam, irradiation of ions, irradiation of an X-ray, irradiation of γ-line, exposure to plasma, chemical etching or a combination of these. From now, with respect to a case of the Mayenite compound in which a part or all of free oxygen is substituted by H^', a method of treating it in a PDP is described. In a case of forming a protection layer containing the Mayenite compound, by irradiation a protection layer formed by the above method with ultraviolet rays or an electron beam in the PDP for at least 1 second, electrons separated from H^" are present in the cages together with H^", and as a result, it becomes possible to produce a PDP having a protection layer excellent in electron emission property such that the secondary electron emission coefficient caused by Xe excitation is high.

Further, the above treatment is preferably carried out after a front plate and a rear plate are joined. The reason is as follows. In a case of joining the front plate and the rear plate by sealing them by using a glass frit and carrying out a heat treatment in the air, since a hydrogen-containing Mayenite compound has high oxidation resistance, it is possible to prevent deterioration of the protection layer. The method for the treatment in this case may, for example, be a method of sealing a discharge gas in a discharge space and carrying out a plasma discharge. When a protection layer containing a hydrogen- containing Mayenite compound is thus exposed to a plasma, the hydrogen-containing Mayenite compound is irradiated with ultraviolet rays, an electron beam or ions, which triggers an effect that electrons are separated from H^" and introduced into cases, and cages including H^" and cages including an electron are produced, and as a result, it is possible to produce a PDP having a protection layer excellent in electron emission property such that the secondary electron emission coefficient caused by Xe excitation is high. The composition of the discharge gas employed at this time may, for example, be a mixed gas of Ne and Xe. Further, the panel is preferably heated while the inside of the panel is maintained to be a vacuum, whereby e.g. an adsorbed gas is removed and a protection layer having a clean surface is obtained, which improves electron emission property. The embodiment of the protective layer of the present invention will be described below.

A first embodiment of the present invention is as shown in Fig. 1. In Fig. 1, Mayenite compound particles 14 wherein a part of oxygen constituting the Mayenite compound is substituted by electrons and anions, are disposed on at least a part of a thin layer 12 of e.g. MgO. The Mayenite compound particles 14 comprise a Mayenite compound wherein a part or all of free oxygen in the cages is substituted by anions of atoms having lower electron affinity than oxygen, such as H^", H₂ ^", H^2", 0^", O₂ ^", F^', Cl^", Br^", I^" or S^2", and wherein the electron density in the cages is at least IxIO¹⁵ cm^"3. In Fig. 1, the thin layer 12 is not particularly limited so long as it is electrically conductive, but in view of a high secondary electron emission efficiency, preferred is a thin film containing at least one compound selected from the group consisting of MgO, SrO, CaO, SrCaO and a Mayenite compound. The thin layer 12 may comprise two or more layers .

The thickness of such a protective layer (the total thickness of the thin layer and the Mayenite compound particles) is not particularly limited. For example, it may be equal to the thickness of a protective layer comprising MgO in a conventional PDP. It may, for example, be from 0.01 to 50 μm, and it is preferably from 0.02 to 20 μm, more preferably from 0.05 to 10 μm. As described above, in a case where the obtained

Mayenite compound is applied to the thin layer 12 by e.g. spin coating, it is required to form the Mayenite compound into a powder. On that occasion, compressive force, shear force and frictional force are mechanically applied to the material to crush it by using a hammer, a roller, a ball or the like of e.g. a metal or a ceramic. On that occasion, by use of a planetary mill using tungsten carbide balls, it is possible to obtain coarse particles having a particle size of at most 50 μm without inclusion of foreign substances in the coarse particles of the Mayenite compound.

The Mayenite compound thus obtained may be further crushed into fine particle having an average particle size of at most 20 μm by using a ball mill or a jet mill. It is possible to mix such particles of at most 20 μm with an organic solvent or a vehicle to prepare a slurry or a paste, but by mixing a Mayenite compound coarsely crushed to at most 50 μm with an organic solvent, followed by crushing with beads, a dispersion solution having a finer Mayenite compound powder having a size as calculated as circles of at most 5 μm dispersed can be prepared. For crushing with beads, for example, zirconium oxide beads may be used.

In a case where an alcohol or an ether, each having one or two carbon atoms and having a hydroxyl group, is used as a solvent at the time of crushing, the electrically conductive Mayenite compound may be reacted therewith and decomposed. Accordingly, when such a solvent is used, preferred is one having at least 3 carbon atoms . A compound having at least 3 carbon atoms and a hydroxyl group, an amide compound or an organic solvent having a sulfur compound dissolved may, for example, be 1-propanol or 2-propanol, 1-butanol or 2- butanol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol isopropyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol isopropyl ether, pentyl alcohol, 1- hexanol, 1-octanol, 1-pentanol, tert-pentyl alcohol, N- methylformamide, N-methylpyrrolidone or dimethyl sulfoxide. Such solvents are used alone or as mixed, whereby crushing will easily be carried out.

To form a Mayenite compound on a protective layer to form the PDP of the present invention, a powder of a Mayenite compound is mixed with a solvent to prepare a slurry or a paste, which is applied to the protective layer and fired. The coating method may, for example, be spray coating, die coating, roll coating, dip coating, curtain coating, spin coating or gravure coating, and spin coating and spray coating are particularly preferred with a view to operating the powder density more easily and accurately. As preferred firing conditions for the coating film, the temperature is preferably from 200 to 800⁰C at which organic substances in the components of the slurry will be decomposed, and the Mayenite compound will be sufficiently fixed on the thin layer. In a case where an electrically conductive Mayenite compound is used as the Mayenite compound, the temperature is preferably such a temperature that the oxidative effect of the electrically conductive Mayenite compound will not be accelerated. In such a case, it is preferably from

200 to 600⁰C. Further, the firing time is preferably about 10 minutes. One example of a method for preparing a slurry to be used for formation of the Mayenite compound on the protective layer for formation of the PDP of the present invention, is a method of dehydrating the above solvent having a low moisture content, mixing from 0.01 to 50 mass% of Mayenite compound coarse particles of at most 50 μm with from 50 to 99.99 mass% of the solvent, and mixing zirconium oxide beads in a weight from 2 to 5 times the solvent as crushing mills with the above mixture to carry out crushing with beads, thereby to disperse the electrically conductive Mayenite compound in the solvent. On that occasion, it is preferred to use zirconia oxide beads having a size of from 0.01 to 0.5 mm in diameter, whereby a slurry containing an electrically conductive Mayenite compound powder having an average particle size of at most 5 μm can be obtained.

In the slurry of the present invention, the average particle size of the particles of the Mayenite compound to be used for the PDP is preferably as small as possible, but it is difficult to obtain a powder having an average particle size less than 0.002 μm. Further, such a size is about the same as the size of the unit cell of the Mayenite compound, and accordingly, when an electrically conductive Mayenite compound is used as the Mayenite compound, the compound may not keep electrical conductivity. Therefore, the average particle size is preferably at least 0.002 μm. Further, if the average particle size of the powder exceeds 5 μm, no sufficient effect as an electron emitter will be obtained. In a case where the powder is used for a PDP, the average particle size of the Mayenite compound powder is preferably at most 5 μm, considering downsizing of the device and electric power saving. The average particle size of the electrically conductive Mayenite compound can be determined by a particle size distribution measuring apparatus by means of laser diffraction scattering method (light scattering method) .

The electron emission efficiency depends on the particle size of the Mayenite compound particles on the protective layer and their density per unit area. In order to obtain a high secondary electron emission efficiency, the density of the Mayenite compound particles on the protective layer per unit area of the protective layer is preferably at least 0.001/R² (particle/μm²) and at most 0.5/R² (particle/μm²) to the size R (μm) of the cross section of the particles as calculated as circles. The size as calculated as circles is defined as a value double the square root of a value obtained by dividing the cross sectional area (area of the cross section when a powder is cut at a plane in parallel with a substrate) measured by a known method utilizing image analysis by the number π. However, the average particle size may be determined by a particle size distribution measuring apparatus by means of light scattering method, which is regarded as the size R as calculated as circles.

The standard deviation σ of the particle size distribution of particles which contribute to electron emission is preferably as small as possible. This is because even when a powder is disposed at an optimum distribution concentration relative to the average of the particle sizes, particles having particle sizes larger than the average have short distances with adjacent particles, and accordingly the electric field concentration effects are offset by each other and decrease, whereby no electron emission may occur . Further, particles having different particle sizes strictly have different electric field concentration effects, and accordingly, the electron emission may occur only from particles having high electric field concentration effects, whereby the total emission current of the entire PDP may decrease. Accordingly, σ of the particle size distribution is preferably at most 3 R, more preferably at most 2 R, furthermore preferably at most 1.5 R to the size R as calculated as circles.

When the unit of the size R as calculated as circles is represented by μm, the density of the particles which contribute to electron emission in the PDP of the present invention is preferably at least 0.001/R² particle and at most 0.5/R² particle per 1 μm² of the substrate surface. If it is less than O.OOl/R² particle, the density of the particles which contribute to electron emission is too low, and the electron emission amount obtained as a device tends to be small. On the other hand, if it exceeds 0.5/R² particle, the electric field concentration effects may be offset since the distance between particles is small, whereby the number of electrons emitted from particles will decrease. It is more preferably at least 0.005/R² particle and at most 0.1/R² particle, more preferably at least 0.01/R² particle and at most 0.05/R² particle.

This means, for example, when a PDP is prepared using particles having a size R as calculated as circles of 0.5 μm, the particle density is preferably at least 0.004 particle/μm² and at most 2.0 particles/μm², more preferably at least 0.02 particle/μm² and at most 0.4 particle/μm², most preferably at least 0.04 particle/μm² and at most 0.2 particle/μm². A second embodiment of the present invention resides in a protective layer 22 as shown in Fig. 2, which comprises e.g. MgO as a base material, and contains Mayenite compound particles 24 wherein a part of oxygen constituting the Mayenite compound is substituted by electrons and anions. The Mayenite compound has high sputtering resistance to Ne ions as compared with MgO and has secondary electron emission function equal to MgO, and accordingly it is possible to form a protective layer made of only a Mayenite compound. Further, the protective layer may be formed by a mixture of a Mayenite compound, MgO, SrO, CaO and SrCaO. The Mayenite compound particles 24 comprises an electrically conductive Mayenite compound wherein the density of electrons and anions of atoms having lower electron affinity than oxygen is at least lxlθ^is cm^"3.

The content of the Mayenite compound in the total volume of materials forming the protective layer is preferably at least 5 vol%, more preferably at least 10 vol% . Such a protective layer, which has high plasma resistance and is hardly plasma-etched, has high performance to protect the discharge electrodes and the dielectric layer in a PDP. The content of the electrically conductive Mayenite compound is preferably less than 25% to the total volume of materials forming the protective layer, from the viewpoint of electrification properties. The Mayenite compound has high sputtering resistance to Ne ions as compared with MgO and has secondary electron emission function equal to MgO, and accordingly it is possible to form a protective layer made of only a Mayenite compound . As a material other than the Mayenite compound constituting the protective layer, a metal oxide may be used. It is preferred to use an alkaline earth metal oxide, which has favorable electrification properties, whereby a low discharge voltage is obtained. More preferably, MgO can be used. Further, the protective layer may comprise two or more layers. Since the secondary electron emission coefficient γ when Xe is used as excitation ions is high, the surface layer of the protective layer preferably contains a Mayenite compound.

The thickness of the protective layer (the total thickness of all the layers in the case of two or more layers) containing the Mayenite compound is not particularly limited. For example, the thickness of the protective layer may be about the same as the protective layer comprising MgO of a conventional PDP. It may, for example, be from 0.01 to 50 μm, and it is preferably from 0.02 to 20 μm, more preferably from 0.05 to 5 μm. In the PDP of the present invention, the thickness of the protective layer is the average thickness measured by a feeler type surface roughness meter.

For formation of the protective layer containing a Mayenite compound, various methods such as a deposition method and a screen printing method comprising coating a dielectric layer with an ink containing a powder of a Mayenite compound prepared by a method similar to formation of an ink containing an electrically conductive Mayenite compound as described above, may be used. As the vapor deposition method, a physical vapor deposition method (PVD) may, for example, be a vacuum deposition method, an electron beam deposition method, an ion plating method, an ion beam deposition method or a sputtering method. The sputtering method may, for example, be a DC sputtering method, an RF sputtering method, a magnetron sputtering method, an ECR sputtering method or an ion beam sputtering method (laser ablation method) . Further, a chemical vapor deposition method (CVD) may, for example, be thermal CVD, plasma CVD or photo CVD. It is possible to form two layers by binary deposition or by depositing MgO or the like first and then depositing a Mayenite compound. Among them, the sputtering method and the ion plating method are preferred since the film thickness can be precisely controlled, and a transparent film can be formed. Further, an electron beam deposition method and CVD are preferred with a view to obtaining transparent and high quality crystals.

Further, for the protective layer of the present invention, it is possible to use an amorphous material containing Ca or Sr and Al in the same compositional ratio as the Mayenite compound. A part of Al contained in the amorphous material may be substituted by the same number of atoms of Si, Ge or Ga. EXAMPLES Now, the present invention will be described in further detail with reference to Examples and Comparative Examples. However, the following Examples are only to more definitely describe the present invention, and the present invention is by no means restricted to the following Examples. Examples 1 to 4 are Examples of the present invention, and Example 5 is a Comparative Example . EXAMPLE 1

Calcium carbonate and aluminum oxide were mixed in a molar ratio of 12:7 and held in the air at 1,300⁰C for 6 hours to prepare a 12CaO- 7Al₂O₃ compound. The powder was formed into a molded product by a uniaxial pressing machine, and the molded product was held in the air at

1,350⁰C for 3 hours to prepare a sintered product. This sintered product was white, and when the conductivity was measured by a voltage-current meter, it was confirmed to be an insulant showing no electrical conductivity. The 12CaO'7Al₂O₃ compound obtained was left in a mixed gas (hydrogen gas: 20 vol%, nitrogen gas: 80 vol%) of hydrogen gas and nitrogen gas at 1,300⁰C for 2 hours, and was quickly cooled to a room temperature in the same atmosphere at a cooling speed higher than 50°C/min, to prepare a Mayenite compound in which a part of free oxygen ions was substituted by H^" (hereinafter referred to as sample A) . The sample A was confirmed to be a Mayenite compound in which a part of free oxygen ions was substituted by H^" from the fact that the total amount of H^" density quantified by SiMS subtracted by the density of OH^" quantified by IR showed the presence of H^" .

A surface of the sample A was ground by a diamond grinder, and irradiated with ultraviolet rays (having Hg spectrum of from 254 nm to 436 nm centering at 365 nm) by a high-pressure mercury lamp (model HLRlOOOF) manufactured by Sen Lights Corp., to obtain an electrically conductive Mayenite compound containing cages including electrons and cages including H^" (hereinafter referred to as sample B) . From its optical- absorption spectrum, the electron density of the electrically conductive Mayenite compound was about

10¹⁹/cm³. Meanwhile, the density of H^" was confirmed to be about 10¹⁹/cm³ from the value obtained by subtracting the density of OH^" quantified by IR from the total value of H^" concentration quantified by SIMS in the same manner as above.

Fig. 3 schematically shows the measurement apparatus of secondary electron emission coefficient in Example 1. A target (sample to be measured) placed in a vacuum chamber was irradiated with Ne⁺ by using an ion gun, and a capturing collector voltage of from -20 to 100 V was applied to a collector placed in the vicinity of the target to capture secondary electrons .

The sample B was placed in the secondary electron emission property measurement apparatus as a target. The vacuum in the apparatus was set to about 10-5 Pa, and the sample was irradiated with Ne⁺ at an acceleration voltage of 600 V, and as a result, the secondary electron emission property (hereinafter referred to as γ_Ne) against Ne⁺ as shown in Fig. 4 was obtained. Since the Y value saturates at a collector voltage of about 70 V, it is understandable that all of secondary electrons emitted were captured. As shown in Fig. 4, the secondary electron emission coefficient γ_Ne at this time was 0.23 at a collector voltage of 70 V. When the sample B was subjected to a heat treatment in the air at 460⁰C for 10 minutes, and thereafter, the value of γ_Ne was measured by using the above method, and as a result, the value γ_Ne was 0.21 at a collector voltage of 70 V. From this result, it was understood that an electrically conductive Mayenite compound containing cages including H^" and cages including electrons was excellent in oxidation resistance and thermal stability. EXAMPLE 2

The sample A of Example 1 was left in the air at 460°C for 10 minutes to carry out a heat treatment, and thereafter, UV light irradiation was carried out in the same manner as Example 1 (sample C) . Namely, the sample C was prepared in the same manner as the sample B except that a heat treatment was carried out in the air.

The H^" concentrations of the sample A and the sample C were measured in the same manner as Example 1, but the concentrations did not change between before and after the heat treatment in which the samples were left in the air at 460⁰C for 10 minutes. This result shows that H^" in the Mayenite compound in which a part of free oxygen ions is substituted by H^" is stable against a heat treatment of high temperature in the air, and is excellent in oxidation resistance and thermal stability. The electron density of the sample C was confirmed to be about 10¹⁹/cm³ from its optical-absorption spectrum. Meanwhile, the concentration of H^" of the sample C was confirmed to be about 10¹⁹/cm³ from the value obtained by subtracting the concentration of OH^" quantified by IR from the total amount of H^" concentration quantified by SIMS in the same manner as Example 1. From this result, it was confirmed that the sample C after the irradiation of UV light was an electrically conductive Mayenite compound containing cages including electrons and H^~ . With respect to the sample C after UV irradiation, the secondary electron emission property was measured in the same manner as Example 1, and as a result, the value of secondary electron emission coefficient γ_Ne was 0.16 as shown in Fig. 5. Further, the sample C was subjected to a heat treatment in the same manner as Example 1, and the value γ_Ne was measured, and as a result, it was 0.16 at a collector voltage of 100 V. EXAMPLE 3

With respect to the sample B, the secondary electron emission property (hereinafter referred to as γ_xe) was measured in the same manner as Example 1 except that Xe was used as excitation ions, and as a result, γ_Xe was 0.13 as shown in Fig. 6. As shown in Fig. 6, an electrically conductive Mayenite compound containing cages including H^" and cages including electrons, has high secondary electron emission coefficient γ_Xe against Xe ions as well as the coefficient against Ne ions. The sample B was subjected to a heating treatment in the same manner as Example 1 and the value of γ_Xe was measured, and as a result, it was 0.09 at a collector voltage of 70 V. EXAMPLE 4

With respect to the sample C, the secondary electron emission property was measured in the same manner as Example 1 except that Xe was used as excitation ions, and as a result, the secondary electron emission coefficient γ_Xe was 0.08 as shown in Fig. 7. As shown in Fig. 7, the Mayenite compound containing hydrogen anions has high secondary electron emission coefficient γ_Xe against Xe ions as well as the coefficient against Ne ions. When the sample C was subjected to a heat treatment in the same manner as Example 1 and the value of γ_Xe was measured, it was 0.08 at a collector voltage of 100 V. EXAMPLE 5

Calcium carbonate and aluminum oxide were blended at a molar ratio of 12:7, and the blended product was left in the air at 1,300⁰C for 6 hours to prepare 12CaO" 7Al₂O₃ compound. The powder was formed into a formed product by using an uniaxial pressing machine, and the formed product was left in the air at 1,350⁰C for 3 hours to prepare a sintered product. The sintered product was white and when its conductivity was measured by using a current voltage meter, it was an insulator showing no conductivity. The sintered product was placed in an alumina container with a cap together with metal aluminum, and they were heated in a vacuum furnace at 300⁰C and held for 10 hours, and gradually cooled to a room temperature. A heat treatment product obtained showed a black brown color and it was confirmed by an X- ray diffraction measurement to be a Mayenite compound. As shown in Fig. 8, the optical-absorption spectrum measured by using U3500 manufactured by Hitachi showed that the heat treatment product had an electron density of 1.4xlO²¹/cm³, and the result of van der Pauw method showed that it had a conductivity of 120 S/cm. Further, as shown in Fig. 9, electron spin resonance (hereinafter referred to as ESR) signal of the heat treatment product obtained was measured by JES-TE300 manufactured by JEOL, and as a result, it was confirmed to be an asymmetric form having a g value of 1.994 that is typically observed in a conductive Mayenite compound having high electron density of more than 10²¹/cm³ (hereinafter the sample is referred to as sample D) . The sample D was placed in a secondary electron emission property measurement apparatus as a target. When the sample D was irradiated with Ne⁺ or Xe⁺ at an acceleration voltage of 600 V under the vacuum degree of about 10^"5 Pa, and as a result, a secondary electron emission property shown in Fig. 10 was obtained. The Y value saturates at a collector voltage of about 70 V, which indicates that all of secondary electrons emitted were captured. As shown in Fig. 10, the value of secondary electron emission coefficient Y at this time was, at a collector voltage of 70 V, 0.31 by Ne⁺ excitation and 0.22 by Xe⁺ excitation. The sample D was subjected to a heat treatment in the air at 460°C for 10 minutes, and its secondary electron emission property was measured in the same manner as Example 1, but no significant value was obtained.

From Examples 1 to 4, a bulk of an electrically conductive Mayenite compound in which a part of oxygen constituting the Mayenite compound is substituted by electrons and H^" shows good secondary electron emission coefficient even after it was subjected to a heat treatment in the air at a high temperature of 460°C. This result indicates that the electrically conductive Mayenite compound is excellent in oxidation resistance . The γ values shown in Table 1 are values of secondary electron emission property at a collector voltage of 70 V. Table 1 shows γ_Ne and γ_Xe values before and after the heat treatment in the air at 460°C for 10 minutes of each of Examples 1 to 5, and their results.

TABLE 1

INDUSTRIAL APPLICABILITY

According to the present invention, on a protective layer, there are disposed particles of Mayenite compound in which a part of oxygen constituting the conductive Mayenite compound is substituted by electrons and anions of atoms having smaller electron affinity than oxygen, and having an electron density of at least IxIO¹⁵ cm³, or such a Mayenite compound is contained in the protective layer, whereby production of PDP is simplified and a PDP having high secondary electron emission coefficient not only against Ne ions but also against Xe ions, and thus having good discharge property, can be obtained and power saving of PDP is realized.

The entire disclosure of Japanese Patent Application No. 2007-218603 filed on August 24, 2007 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.

Claims

1. A plasma display panel comprising a front substrate and a rear substrate facing each other via a discharge space, discharge electrodes formed on at least one of the front substrate and the rear substrate, a dielectric layer covering the discharge electrodes, and a protective layer covering the dielectric layer, wherein the protective layer contains a Mayenite compound, at least a part of oxygen constituting the Mayenite compound is substituted by electrons, a part of the electrons is further substituted by anions of atoms having lower electron affinity than that of oxygen, the Mayenite compound has an electron density of at least IxIO¹⁵ cm^"3, and the secondary electron emission coefficients excited by Ne or Xe ions at an accelerating voltage of 600 V, are respectively at least 0.05 at a secondary electron collector voltage at which secondary electrons can be sufficiently captured.

2. The plasma display panel according to Claim 1, wherein the secondary electron emission coefficient excited by Ne ions is at least 0.05 at a secondary electron collector voltage at which secondary electrons can be sufficiently captured.

3. The plasma display panel according to Claim 1, wherein the secondary electron emission coefficient excited by Xe ions is at least 0.05 at a secondary electron collector voltage at which secondary electrons can be sufficiently captured.

4. The plasma display panel according to any one of Claims 1 to 3 , wherein the Mayenite compound is 12CaO- 7Al₂O₃ compound, 12SrO- 7Al₂O₃ compound, a mixed crystal compound of these, or an isomorphous compound of any one of these .

5. The plasma display panel according to any one of Claims 1 to 4, wherein the Mayenite compound has a part of Al substituted by Si, Ge, B or Ga.

6. The plasma display panel according to any one of

Claims 1 to 5 , wherein the protective layer has a thin layer having a conductivity of at most 1. OxICT⁵ S/cm on the dielectric layer, and on a part of the thin layer, there is disposed the Mayenite compound wherein at least a part of oxygen constituting the Mayenite compound is substituted by electrons, a part of the electrons is further substituted by anions of atoms having lower electron affinity than that of oxygen, and the Mayenite compound has an electron density of at least IxIO¹⁵ cm^"3.

7. The plasma display panel according to any one of Claims 1 to 6 , wherein the thin layer is a layer containing at least one compound selected from the group consisting of MgO, SrO, CaO, SrCaO and a Mayenite compound.

8. The plasma display panel according to any one of Claims 1 to 7, wherein the content of the Mayenite compound is at least 5 vol% to the total volume of the materials forming the protective layer.

9. A process for producing a plasma display panel comprising a front substrate and a rear substrate facing each other via a discharge space, discharge electrodes formed on at least one of the front substrate and the rear substrate, a dielectric layer covering the discharge electrodes, and a protective layer covering the dielectric layer; which comprises a step of disposing a Mayenite compound on the dielectric layer, wherein at least a part of oxygen constituting the Mayenite compound is substituted by electrons, a part of the electrons is further substituted by anions of atoms having lower electron affinity than that of oxygen, and the Mayenite compound has an electron density of at least IxIO¹⁵ cm^"3.

10. The process for producing a plasma display panel according to Claim 9, wherein the thin layer is a layer containing at least one compound selected from the group consisting of MgO, SrO, CaO, SrCaO and a Mayenite compound .