CN108503352B

CN108503352B - Garnet-based red fluorescent ceramic material and preparation method thereof

Info

Publication number: CN108503352B
Application number: CN201810259190.7A
Authority: CN
Inventors: 张芸莉; 胡松; 王正娟; 周国红; 王士维
Original assignee: Shanghai Institute of Ceramics of CAS
Current assignee: Shanghai Institute of Ceramics of CAS
Priority date: 2018-03-27
Filing date: 2018-03-27
Publication date: 2021-03-16
Anticipated expiration: 2038-03-27
Also published as: CN108503352A

Abstract

The present invention provides a garnetThe garnet-based red fluorescent ceramic material has a chemical formula as follows: RE₃Al_5‑x‑yMn_xR_yO₁₂Wherein RE is at least one of Y, Lu, La and Ga, R is one of Mg, Ca, K and Li, x is more than or equal to 0.001 and less than or equal to 0.05, Y is more than or equal to 0 and less than or equal to 0.1, and x is luminous ion Mn⁴⁺Mole fraction of doping.

Description

Garnet-based red fluorescent ceramic material and preparation method thereof

Technical Field

The invention relates to a garnet-based red fluorescent ceramic material and a preparation method thereof, belonging to the technical field of luminescent materials.

Background

The high-power WLEDs and the laser illumination technology have the advantages of high efficiency, high brightness, long service life, low energy consumption, environmental protection and the like, and are the research hotspots in the field of solid-state illumination display at present. Commercial WLEDs are obtained by combining blue LED chips and yellow fluorescent powder of YAG: Ce, the preparation technology is mature, the cost is low, but the fluorescent powder needs to be packaged by resin, the resin is easy to age, the thermal conductivity is low, the long-term service performance is seriously influenced, therefore, the ceramic fluorescent body with higher thermal conductivity and without packaging is produced, and the current mainstream Y/LuAG: several efforts have been made with Ce yellow/green fluorescent ceramics. However, white light obtained by mixing blue light and yellow light still has the problems of higher coloring temperature and lower color rendering index due to the lack of red light component in the spectrum. One of the effective methods for solving this problem is to add red phosphor, so we need to prepare efficient and reliable red fluorescent ceramic. At present, very few reports about red fluorescent ceramics are reported, and mainly non-oxide systems such as nitride are published, but the preparation conditions of the fluorescent ceramics of the systems are harsh and the cost is high.

Disclosure of Invention

Aiming at the problems, the invention aims to provide the garnet-based red fluorescent ceramic material with simple process, low cost and high efficiency and the preparation method thereof.

In one aspect, the present invention provides a garnet-based red fluorescent ceramic having a chemical formula: RE₃Al_5-x-yMn_xR_yO₁₂Wherein RE is at least one of Y, Lu, La and Ga, R is one of Mg, Ca, K and Li, x is more than or equal to 0.001 and less than or equal to 0.05, Y is more than or equal to 0 and less than or equal to 0.1, and x is luminous ion Mn⁴⁺Mole fraction of doping.

In the present disclosure, garnet-based red fluorescent ceramics (RE)₃Al_5-x-yMn_xR_yO₁₂) The medium Mn element contains higher oxidation state Mn⁴⁺Therefore, the garnet-based red fluorescent ceramic can emit red light with a wave band of 600-750 nm under the excitation of ultraviolet or blue light with a wavelength of 230-500 nm. In addition, due to the perfect crystallization performance and the pore scattering effect, the absorption of the exciting light is strong, the luminous intensity is high, and the quantum efficiency is high.

Preferably, x is more than or equal to 0.001 and less than or equal to 0.01 or/and y is more than or equal to 0.04 and less than or equal to 0.08.

Also, the density of the garnet-based red fluorescent ceramic is preferably > 75%, and more preferably > 97%. In the present disclosure, the appropriate amount of pores in the garnet-based red fluorescent ceramic is required, and if there are too many pores, the ceramic has poor light transmittance and weak luminescence. If the number of pores is too small, the incident light is directly transmitted, the light conversion efficiency is low, and the compactness is preferably > 97%.

Preferably, the garnet-based red fluorescent ceramic can effectively excite red light with a wave band of 600-750 nm under ultraviolet light or blue light within a range of 230-500 nm.

In another aspect, the present invention further provides a method for preparing the garnet-based red fluorescent ceramic, including:

according to the stoichiometric ratio RE₃Al_5-x-yMn_xR_yO₁₂Weighing an RE source, an Al source, an Mn source and an R source, and mixing to obtain mixed powder;

and pressing and molding the obtained mixed powder, placing the mixed powder in an oxidizing atmosphere, and sintering the mixed powder for 2-15 hours at 1500-1700 ℃ to obtain the garnet-based red fluorescent ceramic.

In the present disclosure, RE is in stoichiometric ratio₃Al_5-x-yMn_xR_yO₁₂And weighing the RE source, the Al source, the Mn source and the R source, and mixing to obtain mixed powder. And then directly sintering the mixture in an oxidizing atmosphere (1500-1700 ℃) for a certain time, wherein the R source (oxide or/and carbonate containing R) exists as valence compensation in the process, and the R source is also a sintering aid, so that the sintering process is promoted, and the fluorescent ceramic obtains higher density.

Preferably, the RE source is an oxide of RE, preferably Y₂O₃、Lu₂O₃、La₂O₃、Ga₂O₃At least one of; the Al source is Al₂O₃(ii) a The Mn source is MnO and MnCO₃、Mn₂O₃At least one of; the R source is oxide or/and carbonate containing R, preferably MgO and MgCO₃、CaO、CaCO₃、K₂CO₃、Li₂CO₃At least one of (1).

Preferably, the compression molding mode comprises dry compression molding or/and cold isostatic pressing, preferably dry compression molding and then cold isostatic pressing.

Preferably, the pressure of the dry pressing is 10 to 15MPa, the pressure maintaining time is 0.5 to 5min, the pressure of the cold isostatic pressing is 180 to 200MPa, and the pressure maintaining time is 1 to 10 min.

Preferably, the oxidizing atmosphere is an air atmosphere or an oxygen atmosphere.

Has the advantages that:

in the disclosure, the garnet-based red fluorescent ceramic has higher density, and has strong absorption of exciting light, high luminous intensity and high quantum efficiency due to good crystallization performance and pore scattering effect;

in the disclosure, the garnet-based red fluorescent ceramic takes residual pores in the body as scattering centers, and enhances the absorption of exciting light, improves the luminous intensity and obtains higher quantum efficiency through the scattering effect of the garnet-based red fluorescent ceramic on light, and emits bright red light under the excitation of ultraviolet light or blue light within the range of 250-500 nm;

in the present disclosure, the garnet-based red fluorescent ceramic is easily compatible with the mainstream y (lu) AG: the yellow-green fluorescent ceramic of Ce is packaged, so that the difference is small, and the long-term stability is good;

in the disclosure, the combination of the garnet-based red fluorescent ceramic and the garnet-based yellow/green fluorescent ceramic can generate white light, thereby effectively reducing the color temperature and improving the color rendering index;

in the disclosure, the garnet-based red fluorescent ceramic has high luminous efficiency, and can be applied to other luminous fields such as WLEDs, laser illumination display and the like;

in the method, the dense garnet-based red fluorescent ceramic is prepared by adopting a high-temperature solid-phase reaction one-step method, so that the method is simple in process, low in cost and suitable for batch production;

in the disclosure, cold isostatic pressing is used in the forming method, appropriate sintering aid content (y is preferably 0.04 or more and less than or equal to 0.08), and higher sintering temperature (more than or equal to 1600 ℃) can obtain the garnet-based red fluorescent ceramic with higher density.

Drawings

FIG. 1 is an XRD pattern of garnet-based red fluorescent ceramics prepared in examples 6 to 9 of the present invention;

fig. 2 is an SEM image of garnet-based red fluorescent ceramic (x ═ 0.003) prepared in example 8 of the present invention;

FIG. 3 is an excitation spectrum of a garnet-based red fluorescent ceramic prepared in examples 6 to 9 of the present invention;

fig. 4 is an emission spectrum of a garnet-based red fluorescent ceramic (x ═ 0.003) prepared in example 8 of the present invention under excitation at 460 nm;

fig. 5 shows the quantum efficiency of the garnet-based red fluorescent ceramic (x ═ 0.003) prepared in example 8 of the present invention;

FIG. 6 is a schematic diagram of garnet-based red fluorescent ceramics prepared in examples 6 to 9 of the present invention.

Detailed Description

The present invention is further illustrated by the following examples, which are to be understood as merely illustrative and not restrictive.

In the present disclosure, the chemical formula of the garnet-based red fluorescent ceramic may be: RE₃Al_5-x-yMn_xR_yO12, wherein RE is at least one of Y, Lu, La and Ga, R is one of Mg, Ca, K and Li, 0.001-0.05 (preferably 0.001-0.01), 0-0.1 (preferably 0.04-0.08), x is luminous ion Mn⁴⁺Mole fraction of doping, y being R ions (e.g. Mg)²⁺) Mole fraction of doping. The garnet-based red fluorescent ceramic can be effectively excited by ultraviolet light and blue light within the range of 230-500 nm and emits bright red light with the wave band of 600-750 nm. In addition, red fluorescent ceramics with different excitation wave bands can be prepared by adjusting the value of x.

Garnet-based red fluorescent ceramic (Mn) of the present invention⁴⁺Doped garnet-based red fluorescent ceramic) has simple preparation process and low cost, can solve the problem of fluorescent powder packaging, has higher quantum efficiency due to perfect crystallization performance and air hole scattering effect, and can be better applied to the field of illumination display. In one embodiment of the present disclosure, the garnet-based red fluorescent ceramic can be directly prepared by a high-temperature solid-phase reaction one-step method. The following exemplarily illustrates a method for preparing the garnet-based red fluorescent ceramic.

According to the stoichiometric ratio RE₃Al_5-x-yMn_xR_yO₁₂And weighing the RE source, the Al source, the Mn source and the R source, and mixing to obtain mixed powder. In alternative embodiments, the RE source may be an oxide of RE (RE)₂O₃) E.g. Y₂O₃、Lu₂O₃、La₂O₃、Ga₂O₃And the like. In an alternative embodiment, the source of Al may be Al₂O₃And the like. In alternative embodiments, the source of Mn may be MnO, MnCO₃、Mn₂O₃And the like. In alternative embodiments, the R source may be an oxide or/and carbonate containing R, such as MgO, MgCO₃、CaO、CaCO₃、K₂CO₃、Li₂CO₃Etc. which can provide valence compensating cations while being present as sintering aids. As an example, in accordance with RE₃Al_5-x-yMn_xR_yO₁₂(x is more than or equal to 0.001 and less than or equal to 0.05 and y is more than 0 and less than or equal to 0.1) according to the stoichiometric ratio₂O₃Powder of Al₂O₃Mixing the powder, MnO powder and oxide powder containing R, grinding and sieving to obtain mixed powder. The purities of all the raw materials (RE source, Al source, Mn source and R source) are not less than 99.5%.

And pressing and molding the mixed powder to obtain a biscuit. The compression molding mode comprises dry compression molding or/and cold isostatic pressing, and preferably comprises dry compression molding and then cold isostatic pressing. In an optional embodiment, the pressure of the dry pressing can be 10 to 15Mpa, and the pressure maintaining time is 0.5 to 5 min. In an alternative embodiment, the pressure of the cold isostatic pressing may be 180 to 200MPa, and the dwell time is 1 to 10 min.

And (3) placing the biscuit in an oxidizing atmosphere, and sintering at 1500-1700 ℃ for 2-15 hours (preferably 3-15 hours) to obtain the garnet-based red fluorescent ceramic. The oxidizing atmosphere may be an oxygen atmosphere or an air atmosphere. And then carrying out plane grinding and polishing treatment on the garnet-based red fluorescent ceramic to a proper thickness.

As an example of one of the methods for preparing the garnet-based red fluorescent ceramic, the following are included:

(1) according to RE₃Al_5-x-yMn_xMg_yO₁₂(x is more than or equal to 0.001 and less than or equal to 0.01, and y is more than or equal to 0 and less than or equal to 0.1) according to the stoichiometric ratio₂O₃Powder of Al₂O₃Mixing, grinding and sieving powder, MgO powder and Mn source powder, wherein the MgO powder is used as a sintering aid and also provides Mg for valence state compensation²⁺(ii) a (2) Dry pressing the mixture for molding and carrying out cold isostatic pressing treatment; (3) and sintering the biscuit in a muffle furnace at high temperature to obtain the red fluorescent ceramic. RE in step (1)₂O₃The powder is Y₂O₃、Lu₂O₃、La₂O₃、Ga₂O₃At least one of Mn source powder is MnO and MnCO₃、Mn₂O₃One kind of (1). The purity of all raw material powder is not less than 99.5%. And (3) carrying out high-temperature solid-phase reaction in the step (3) in an oxidizing atmosphere, preferably directly sintering in an air atmosphere, wherein the sintering temperature is 1550-1700 ℃, and the heat preservation time is 3-15 hours.

In the present disclosure, the garnet-based red fluorescent ceramic has more complete ceramic grain development, good crystallization performance, fewer defects, and higher luminescence performance. Moreover, some air holes still exist in the garnet-based red fluorescent ceramic, and due to the scattering effect of the air holes, the absorption of the ceramic body on incident light and the extraction of emitted light are increased, so that the quantum efficiency is favorably improved.

The present invention will be described in detail by way of examples. It is also to be understood that the following examples are illustrative of the present invention and are not to be construed as limiting the scope of the invention, and that certain insubstantial modifications and adaptations of the invention by those skilled in the art may be made in light of the above teachings. The specific process parameters and the like of the following examples are also only one example of suitable ranges, i.e., those skilled in the art can select the appropriate ranges through the description herein, and are not limited to the specific values exemplified below.

The chemical formula of the garnet-based red fluorescent ceramic prepared in the following example is: lu (Lu)₃Al_5-x- _yMn_xMg_yO₁₂X is more than or equal to 0.001 and less than or equal to 0.01, and y is more than or equal to 0 and less than or equal to 0.1. The following steps were followed for any of the examples described below: accurately weighing Lu by a four-digit balance according to the stoichiometric ratio₂O₃、Al₂O₃、MgO、MnCO₃Adding raw material powder into a ball milling tank, ball milling the mixed material for 12h, drying the mixed material in an oven for 24h, sieving the mixed material by using a 200-mesh sieve, pressing the obtained powder into a wafer with the diameter of 20mm by using a steel die under the pressure of 12MPa, and carrying out cold isostatic pressing treatment under the pressure of 200MPa to obtain a biscuit; placing the biscuit in an alumina crucible, and sintering in a high-temperature muffle furnace; the obtained ceramic is double-sidedGround to a thickness of 0.2 mm.

Example 1

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.001. The sintering temperature is 1600 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The resulting ceramic body had a relative density of 97.85% and emitted red light under ultraviolet or blue excitation.

Example 2

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.002. The sintering temperature is 1600 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The resulting ceramic body had a relative density of 97.88% and emitted red light upon excitation with ultraviolet or blue light.

Example 3

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.003. The sintering temperature is 1600 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The resulting ceramic body had a relative density of 97.86% and emitted red light under excitation by ultraviolet or blue light and red light under excitation by ultraviolet or blue light.

Example 4

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.005. The sintering temperature is 1600 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The resulting ceramic body had a relative density of 97.74% and emitted red light upon excitation with ultraviolet or blue light.

Example 5

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.008. The sintering temperature is 1600 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The resulting ceramic body had a relative density of 97.87% and emitted red light under ultraviolet or blue excitation.

Example 6

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.001. The sintering temperature was 1650 ℃, the time was 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 98.95% and emitted red light under ultraviolet or blue light excitation.

Example 7

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.002. The sintering temperature was 1650 ℃, the time was 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 99.29% and emitted red light upon excitation with ultraviolet or blue light.

Example 8

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.003. The sintering temperature was 1650 ℃, the time was 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 99.19% and emitted red light under uv or blue excitation.

Example 9

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.005. The sintering temperature was 1650 ℃, the time was 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 98.65% and emitted red light under uv or blue excitation.

Referring to FIG. 1, XRD patterns of garnet-based red fluorescent ceramics prepared in examples 6 to 9, in comparison with a standard card, were all Lu₃Al₅O₁₂Phase, showing that Mn ion doping has no influence on the matrix structure;

the SEM image in FIG. 2 shows that the garnet-based red fluorescent ceramic prepared in example 8 has good crystallinity, a small amount of pores and second phases exist, and the density is up to 99.19% by the Archimedes method;

as shown in fig. 3, the garnet-based red fluorescent ceramic prepared in examples 6 to 9 has two strong excitation peaks in the range of 230 to 500nm, and the excitation peak in the ultraviolet region is significantly broadened with the increase of Mn content;

FIG. 4 shows that the garnet-based red fluorescent ceramic prepared in example 8 emits red light in the wavelength range of 600-750 nm under the excitation of 460 nm;

fig. 5 is a spectrum of the garnet-based red fluorescent ceramic prepared in example 8 (x ═ 0.003) measured by an integrating sphere, and calculated to have a Quantum Efficiency (QE) as high as 47.8%;

FIG. 6 is a diagram of a real object of the garnet-based red fluorescent ceramic prepared in examples 6 to 9, in which the writing under the sample can be seen implicitly, which shows that the sample has a certain light transmittance. The sample emitted bright red light under 365nm ultraviolet lamp illumination.

Example 10

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.001. The sintering temperature is 1700 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The main structural morphology, excitation spectrum and emission spectrum of the sample prepared in the embodiment are similar to those of the samples prepared in the embodiments 6-9.

Example 11

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.002. The sintering temperature is 1700 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The main structural morphology, excitation spectrum and emission spectrum of the sample prepared in the embodiment are similar to those of the samples prepared in the embodiments 6-9.

Example 12

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.003. The sintering temperature is 1700 ℃, the time is 5 hours, and the atmosphere is air atmosphere. The main structural morphology, excitation spectrum and emission spectrum of the sample prepared in the embodiment are similar to those of the samples prepared in the embodiments 6-9.

Example 13

Fixing the content of Mg, and the chemical formula is Lu₃Al_5-x-0.04Mn_xMg_0.04O₁₂Wherein x is 0.005. The sintering temperature is 1700 ℃ for a period of timeFor 5 hours, the atmosphere was air. The main structural morphology, excitation spectrum and emission spectrum of the sample prepared in the embodiment are similar to those of the samples prepared in the embodiments 6-9.

Example 14

The content of the luminescent ions Mn is fixed, and the chemical formula is Lu₃Al_5-0.003-yMn_0.003Mg_yO₁₂Wherein y is 0. The sintering temperature was fixed at 1650 ℃ for 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 78.47%.

Example 15

The content of the luminescent ions Mn is fixed, and the chemical formula is Lu₃Al_5-0.003-yMn_0.003Mg_yO₁₂Wherein y is 0.02. The sintering temperature was fixed at 1650 ℃ for 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 88.36%.

Example 16

The content of the luminescent ions Mn is fixed, and the chemical formula is Lu₃Al_5-0.003-yMn_0.003Mg_yO₁₂Wherein y is 0.06. The sintering temperature was fixed at 1650 ℃ for 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 98.58%.

Example 17

The content of the luminescent ions Mn is fixed, and the chemical formula is Lu₃Al_5-0.003-yMn_0.003Mg_yO₁₂Wherein y is 0.08. The sintering temperature was fixed at 1650 ℃ for 5 hours, and the atmosphere was air. The resulting ceramic body had a relative density of 98.87%.

The samples prepared in examples 14-17 have similar main structure appearance, excitation spectrum and emission spectrum as those of examples 6-9. The samples obtained in examples 15 to 17 were not doped with Mg in example 14²⁺Compared with the sample doped with Mg²⁺The density and the luminous efficiency of the sample are obviously improved.

Table 1 shows the composition and performance parameters of the garnet-based red fluorescent ceramics prepared in examples 1 to 17 of the present invention:

Claims

1. the garnet-based red fluorescent ceramic is characterized by having the chemical formula: RE₃Al_5-x-yMn_xR_yO₁₂Wherein RE is at least one of Y, Lu, La and Ga, R is one of Mg, Ca, K and Li, x is more than or equal to 0.001 and less than or equal to 0.01, Y is more than or equal to 0.04 and less than or equal to 0.08, and x is luminous ion Mn⁴⁺The mole fraction of doping;

the garnet-based red fluorescent ceramic is prepared by the following steps: according to the stoichiometric ratio RE₃Al_5-x-yMn_xR_yO₁₂Weighing an RE source, an Al source, an Mn source and an R source, and mixing to obtain mixed powder; pressing and molding the obtained mixed powder, placing the mixed powder in an oxidizing atmosphere, and sintering the mixed powder for 2-15 hours at 1500-1700 ℃ to obtain the garnet-based red fluorescent ceramic;

the RE source is an oxide of RE; the Al source is Al₂O₃(ii) a The Mn source is MnO and MnCO₃、Mn₂O₃At least one of; the R source is oxide or/and carbonate containing R.

2. The garnet-based red fluorescent ceramic according to claim 1, wherein the density of the garnet-based red fluorescent ceramic is more than 75%.

3. The garnet-based red fluorescent ceramic according to claim 1, wherein the garnet-based red fluorescent ceramic effectively excites red light of a wavelength band of 600 to 750nm under ultraviolet light or blue light in a range of 230 to 500 nm.

4. According to the rightThe garnet-based red fluorescent ceramic according to any one of claims 1 to 3, wherein the RE source Y is₂O₃、Lu₂O₃、La₂O₃、Ga₂O₃At least one of; the R source is MgO and MgCO₃、CaO、CaCO₃、K₂CO₃、Li₂CO₃At least one of (1).

5. The method for preparing the garnet-based red fluorescent ceramic according to any one of claims 1 to 4, wherein the compression molding is dry compression molding or/and cold isostatic pressing.

6. The method according to claim 5, wherein the press molding is performed by dry press molding followed by cold isostatic press molding.

7. The production method according to claim 5 or 6, wherein the dry-pressing molding pressure is 10 to 15MPa, and the dwell time is 0.5 to 5 minutes; the pressure of the cold isostatic pressing is 180-200 MPa, and the pressure maintaining time is 1-10 minutes.

8. The production method according to claim 5 or 6, wherein the oxidizing atmosphere is an air atmosphere or an oxygen atmosphere.