Quantum dot glass ceramics and preparation method thereof and method for preparing quantum dot glass ceramics product by applying 3D printing technology
阅读说明：本技术 量子点微晶玻璃及其制备和应用3d打印技术制备量子点微晶玻璃制品的方法 (Quantum dot glass ceramics and preparation method thereof and method for preparing quantum dot glass ceramics product by applying 3D printing technology ) 是由 向卫东 何紫瑶 梁晓娟 于 2020-12-25 设计创作，主要内容包括：本发明公开了量子点微晶玻璃及其制备和应用3D打印技术制备量子点微晶玻璃制品的方法。所述量子点微晶玻璃的微观形貌是Cs-4PbBr-6量子点分散于玻璃基质中,所述Cs-4PbBr-6量子点微晶玻璃由以下原料组分制备得到,各原料组分的含量以摩尔百分比表示如下：SiO-230-40％,B-2O-320-30％,ZnO5-10％,CaO10-15％,Ba-2CO-32-5％,Cs-2CO-3+PbBr-2+NaBr：15-25％,其中,Cs-2CO-3、PbBr-2、NaBr三者的摩尔比满足下列条件：Cs-2CO-3/PbBr-2＝3:1,NaBr/PbBr-2＝4.9。本发明提供的Cs-4PbBr-6量子点微晶玻璃具有高量子效率,本发明提供的应用3D打印技术制备Cs-4PbBr-6量子点微晶玻璃制品的方法,能使制品外观完整并保留量子点微晶玻璃的发光特性。(The invention discloses quantum dot glass ceramics, and preparation and application thereof in preparing quantum by using 3D printing technologyA method for preparing a point microcrystalline glass product. The micro-morphology of the quantum dot glass ceramics is Cs 4 PbBr 6 The quantum dots are dispersed in the glass matrix, and the Cs 4 PbBr 6 The quantum dot glass ceramics are prepared from the following raw material components in percentage by mole: SiO 2 2 30‑40％，B 2 O 3 20‑30％，ZnO5‑10％，CaO10‑15％，Ba 2 CO 3 2‑5％，Cs 2 CO 3 +PbBr 2 + NaBr: 15-25%, wherein Cs 2 CO 3 、PbBr 2 And the molar ratio of NaBr meets the following conditions: cs 2 CO 3 /PbBr 2 ＝3:1，NaBr/PbBr 2 4.9. Cs provided by the invention 4 PbBr 6 The quantum dot glass ceramics have high quantum efficiency, and the invention provides the application of 3D printing technology to prepare Cs 4 PbBr 6 The method of the quantum dot glass-ceramic product can ensure that the product has complete appearance and retains the luminescence characteristic of the quantum dot glass-ceramic.)
1. Cs (volatile organic Compounds)4PbBr6The micro-morphology of the quantum dot microcrystalline glass is Cs4PbBr6The quantum dots are dispersed in the glass matrix, and the Cs4PbBr6The quantum dot glass ceramics are prepared from the following raw material components in percentage by mole:
SiO2：30-40％
B2O3：20-30％
ZnO：5-10％
CaO：10-15％
Ba2CO3：2-5％
Cs2CO3+PbBr2+NaBr：15-25％
wherein, Cs2CO3、PbBr2And the molar ratio of NaBr meets the following conditions:
Cs2CO3：PbBr2＝3:1，NaBr：PbBr2＝4.9。
2. the Cs of claim 14PbBr6The quantum dot glass ceramics are characterized in that: the Cs4PbBr6The quantum dot glass ceramics are prepared from the following raw material components in percentage by mole:
SiO2：35％
B2O3：20％
ZnO：5％
CaO：10％
Ba2CO3：5％
Cs2CO3+PbBr2+NaBr：25％。
3. the Cs of claim 1 or 24PbBr6The preparation method of the quantum dot glass ceramics comprises the following steps:
according to Cs4PbBr6The pure raw material components are accurately weighed and analyzed according to the formula of the quantum dot glass ceramics, then the raw material components are put into an agate mortar, and the mixture is uniformly ground and then placed into a crucible; putting the crucible into a high-temperature resistance furnace, heating to the melting temperature of 1200-1300 ℃, preserving the heat for 10-20 minutes, pouring the glass melt onto a cast iron mold, then putting the cast iron mold into a muffle furnace for annealing at the temperature of 400-500 ℃, preserving the heat for 2-4 hours, then carrying out heat treatment in the muffle furnace at the temperature of 500-600 ℃ for 7-12 hours, and then cooling to the temperature of 1300 ℃ along with the furnaceTaking out the glass at room temperature to obtain transparent Cs4PbBr6Quantum dot glass ceramics.
4. The method of claim 3, wherein: the melting temperature is 1300 ℃, and the holding time is 10 minutes.
5. The method of claim 3, wherein: the heat treatment temperature was 540 ℃ and the heat treatment time was 10 hours.
6. Application of 3D printing technology to prepare Cs4PbBr6A method of making a quantum dot glass-ceramic article, comprising the steps of:
(1) according to Cs4PbBr6The raw material formula of the quantum dot glass ceramics accurately weighs and analyzes pure raw material components, then the raw material components are put into an agate mortar, mixed and ground uniformly, and then the mixture is placed in a crucible; putting the crucible into a high-temperature resistance furnace, heating to the melting temperature of 1200-1300 ℃, preserving the heat for 10-20 minutes, pouring the glass melt onto a cast iron mold, then putting the cast iron mold into a muffle furnace for annealing at the temperature of 400-500 ℃, preserving the heat for 2-4 hours, then cooling to the room temperature along with the furnace, and taking out the glass block; the Cs4PbBr6The raw material formula of the quantum dot microcrystalline glass consists of the following raw material components in percentage by mole:
SiO2：30-40％
B2O3：20-30％
ZnO：5-10％
CaO：10-15％
Ba2CO3：2-5％
Cs2CO3+PbBr2+NaBr：15-25％
wherein, Cs2CO3、PbBr2And the molar ratio of NaBr meets the following conditions:
Cs2CO3：PbBr2＝3:1，NaBr：PbBr2＝4.9；
(2) prepared in the step (1)Grinding the glass block into powder by using an agate mortar, sieving the powder by using a 270-plus 325-mesh sieve, uniformly mixing the glass powder and the photosensitive resin material according to a certain mass ratio to obtain 3D printing slurry, and introducing a printed graphic file into 3D printing software for printing by using a 3D printer to manufacture a model with a specific shape; heat-treating the model at the temperature of 540-580 ℃ for 5-10 hours to obtain Cs4PbBr6A quantum dot glass-ceramic product.
7. The method of claim 6, wherein: the Cs4PbBr6In the raw material formula of the quantum dot glass ceramics, the content of each raw material component is expressed by mol percent as follows:
SiO2：35％
B2O3：20％
ZnO：5％
CaO：10％
Ba2CO3：5％
Cs2CO3+PbBr2+NaBr：25％。
8. the method of claim 6 or 7, wherein: in the step (1), the melting temperature is 1300 ℃, and the heat preservation time is 10 minutes.
9. The method of claim 6 or 7, wherein: the mass ratio of the glass powder to the photosensitive resin material in the step (2) is 7:3-6: 4.
10. The method of claim 6 or 7, wherein: the heat treatment temperature of the mold in the step (2) is 540 ℃, and the heat treatment time is 10 hours.
(I) technical field
The invention relates to a Cs4PbBr6Quantum dot microcrystalline glass material, preparation method thereof and application of 3D printing technology to preparation of Cs4PbBr6A method for preparing quantum dot glass-ceramic product.
(II) background of the invention
3D printing is one of the rapid prototyping technologies, which is a technology for constructing an object by using a bondable material such as powdered metal or plastic based on a digital model file and by printing layer by layer. There are many kinds of materials currently used for 3D printing, but luminescent materials are rarely used for 3D printing. Recently, all-inorganic halide perovskite quantum dot materials have become hot research in the field of optical materials due to the advantages of excellent luminescence characteristics, such as narrow-band emission, high color purity, adjustable emission spectrum, and the like.
In 2014, Kovalenko et al synthesized CsPbX for the first time by a hot injection method3And (4) nanocrystals. They found CsPbX3The luminescence range of the quantum well cover the whole visible light range, the half-peak width of the emission peak is narrow, the cubic nanocrystal has the quantum yield of more than 90 percent, and CsPbX3Perovskites have the advantage of being exceptionally thick in the field of luminescence.
2016, Chen Daizhi et al reported that a heterogeneous interfacial reaction was used to achieve novel Cs4PbBr6A method for synthesizing quantum dots on a large scale at room temperature. Material toolHas good thermal stability and repeatability, and the quantum yield is 65%.
In 2020 Lishashasha et al, by changing Cs2CO3In the raw material, when the content is Cs2CO3When the molar content of the boron silicate glass is 7.5 to 10 percent, Cs is embedded in the borosilicate glass matrix4PbBr6Quantum dots, and discussing Cs4PbBr6As a 0-dimensional perovskite quantum dot and a traditional 3-dimensional CsPbX3The perovskite quantum dots have different structures, appearances and unique light emitting sources.
Disclosure of the invention
The first technical problem to be solved by the invention is to provide a transparent Cs4PbBr6The quantum dot glass ceramics can obviously improve the quantum efficiency.
The second technical problem to be solved by the invention is to provide a method for preparing the Cs4PbBr6A method for preparing quantum dot glass ceramics.
The third technical problem to be solved by the invention is to provide a method for preparing Cs by applying 3D printing technology4PbBr6The method of quantum dot glass-ceramic product makes the appearance complete and retains the light emitting characteristic of quantum dot glass-ceramic.
The invention adopts the technical scheme for solving the problems as follows:
in a first aspect, the present invention provides a Cs4PbBr6The micro-morphology of the quantum dot microcrystalline glass is Cs4PbBr6The quantum dots are dispersed in the glass matrix, and the Cs4PbBr6The quantum dot glass ceramics are prepared from the following raw material components in percentage by mole:
SiO2：30-40％
B2O3：20-30％
ZnO：5-10％
CaO：10-15％
Ba2CO3：2-5％
Cs2CO3+PbBr2+NaBr：15-25％
wherein, Cs2CO3、PbBr2And the molar ratio of NaBr meets the following conditions:
Cs2CO3：PbBr2＝3:1，NaBr：PbBr2＝4.9。
preferably, the Cs4PbBr6The quantum dot glass ceramics are prepared from the following raw material components in percentage by mole:
SiO2：35％
B2O3：20％
ZnO：5％
CaO：10％
Ba2CO3：5％
Cs2CO3+PbBr2+NaBr：25％。
in a second aspect, the present invention provides said Cs4PbBr6The preparation method of the quantum dot glass ceramics comprises the following steps:
according to Cs4PbBr6The pure raw material components are accurately weighed and analyzed according to the formula of the quantum dot glass ceramics, then the raw material components are put into an agate mortar, and the mixture is uniformly ground and then placed into a crucible; putting the crucible into a high-temperature resistance furnace, heating to the melting temperature of 1200-1300 ℃, preserving the heat for 10-20 minutes, pouring the glass melt onto a cast iron mold, then putting the cast iron mold into a muffle furnace for annealing at the temperature of 400-500 ℃, preserving the heat for 2-4 hours, then carrying out heat treatment in the muffle furnace at the temperature of 500-600 ℃ for 7-12 hours, cooling to the room temperature along with the furnace, taking out the glass to obtain transparent Cs4PbBr6Quantum dot glass ceramics.
In the present invention, the crucible used is preferably a corundum crucible or a platinum crucible.
In the present invention, the melting temperature is preferably 1300 ℃.
In the present invention, the incubation time is preferably 10 minutes.
In the present invention, the heat treatment temperature is preferably 540 ℃.
In the present invention, the glass heat treatment time is preferably 10 hours.
Cs prepared by the invention4PbBr6The shape of the microcrystalline glass of the quantum dot microcrystalline glass material can be various, and the microcrystalline glass can be cut, ground and polished.
In a third aspect, the invention provides a method for preparing Cs by applying a 3D printing technology4PbBr6A method of making a quantum dot glass-ceramic article, comprising the steps of:
(1) according to Cs4PbBr6The raw material formula of the quantum dot glass ceramics accurately weighs and analyzes pure raw material components, then the raw material components are put into an agate mortar, mixed and ground uniformly, and then the mixture is placed in a crucible; putting the crucible into a high-temperature resistance furnace, heating to the melting temperature of 1200-1300 ℃, preserving the heat for 10-20 minutes, pouring the glass melt onto a cast iron mold, then putting the cast iron mold into a muffle furnace for annealing at the temperature of 400-500 ℃, preserving the heat for 2-4 hours, then cooling to the room temperature along with the furnace, and taking out the glass block; the Cs4PbBr6The raw material formula of the quantum dot microcrystalline glass consists of the following raw material components in percentage by mole:
SiO2：30-40％
B2O3：20-30％
ZnO：5-10％
CaO：10-15％
Ba2CO3：2-5％
Cs2CO3+PbBr2+NaBr：15-25％
wherein, Cs2CO3、PbBr2And the molar ratio of NaBr meets the following conditions:
Cs2CO3：PbBr2＝3:1，NaBr：PbBr2＝4.9；
(2) grinding the glass blocks prepared in the step (1) into powder by using an agate mortar, sieving the powder by using a 270-plus 325-mesh sieve, uniformly mixing the glass powder and the photosensitive resin material according to a certain mass ratio to obtain 3D printing slurry, and introducing the printed graphic file into 3D printing software for printing by using a 3D printer to manufacture a model with a specific shape; model is set at 5Heat treating at 40-580 deg.C for 5-10 hr to obtain Cs4PbBr6A quantum dot glass-ceramic product.
The photosensitive resin material is DLP desktop-level 3D printing photosensitive resin.
Preferably, the Cs4PbBr6In the raw material formula of the quantum dot glass ceramics, the content of each raw material component is expressed by mol percent as follows:
SiO2：35％
B2O3：20％
ZnO：5％
CaO：10％
Ba2CO3：5％
Cs2CO3+PbBr2+NaBr：25％。
preferably, the crucible used in step (1) is a corundum crucible or a platinum crucible.
Preferably, the melting temperature in step (1) is 1300 ℃.
Preferably, the heat-retention time in step (1) is 10 minutes.
Preferably, the size of the sieve sieved in step (2) is 300 meshes.
Preferably, the mass ratio of the glass frit to the photosensitive resin material in step (2) is 7:3 to 6:4, more preferably 6: 4.
Preferably, the heat treatment temperature of the mold in the step (2) is 540 ℃.
Preferably, the heat treatment time of the mold in the step (2) is 10 hours.
Step (2) of the invention, Cs is subjected to 3D printing and appropriate subsequent heat treatment conditions4PbBr6The quantum dot glass ceramic product has complete appearance and retains the luminescence property of the quantum dot glass ceramic.
Compared with the prior art, the invention has the beneficial effects that:
(1) the invention changes PbBr2To adjust Cs2CO3And PbBr2When the molar ratio of Cs to the total amount of2CO3：PbBr2Successfully prepared when the ratio is 3:1Cs4PbBr6Quantum dot glass ceramics. The method of the invention changes the raw material proportion, the melting condition and the heat treatment condition to convert Cs into Cs4PbBr6The quantum efficiency of the quantum dot glass ceramics is improved to 58.9 percent, and Cs is greatly improved4PbBr6The stability of the quantum dots enables the quantum dots to still maintain the original luminous efficiency of more than 85% after being soaked in water for more than two months.
(2) The invention is realized by adding Cs4PbBr6The proper temperature and time are searched in the optimal heat treatment range of the quantum dot glass ceramics, the model after 3D printing and forming is subjected to heat treatment, a glass product with complete appearance and retained luminescent property of the glass ceramics is obtained, and Cs is greatly simplified4PbBr6The application of the quantum dot glass ceramics shows that the quantum dot glass ceramics has wide prospect.
(IV) description of the drawings
FIG. 1 shows Cs produced at different heat treatment temperatures4PbBr6X-ray diffraction (XRD) patterns of quantum dot glass-ceramic samples.
FIG. 2 shows Cs obtained in example 8 and heat-treated at 540 ℃ for 10 hours4PbBr6TEM image of quantum dot glass-ceramic sample.
FIG. 3 shows Cs produced at different heat treatment temperatures4PbBr6Fluorescence plot of quantum dot glass-ceramic sample.
FIG. 4 shows Cs produced at different heat treatment temperatures4PbBr6And (3) photographs of the quantum dot glass ceramic sample under normal light and ultraviolet light.
FIG. 5 shows Cs obtained in example 84PbBr6Water stability plot of quantum dot glass-ceramic sample.
FIG. 6 shows Cs prepared by 3D printing4PbBr6Sample diagram of quantum dot glass-ceramic product.
FIG. 7 shows different Cs2CO3/PbBr2Cs prepared in proportion4PbBr6XRD patterns of quantum dot glass ceramics samples.
FIG. 8 is a fluoroscopic image of a 3D printed model.
(V) detailed description of the preferred embodiments
The invention is described in detail below with reference to examples, which are intended to illustrate the invention further and are not to be construed as limiting the invention. Insubstantial modifications and adaptations of the foregoing disclosure may be made.
Examples 1 to 5
Accurately weighing SiO with the mol percentage of 35 percent2、20％B2O3、5％ZnO、10％CaO、5％Ba2CO3After the base glass component is prepared, Cs is accurately weighed according to the perovskite quantum dot formula in the table 12CO3、NaBr、PbBr2Putting into a corundum crucible, and mixing and grinding the corundum crucible and the base glass component uniformly. Placing in a high temperature furnace, heating to 1200 deg.C, maintaining for 20min, pouring into a mold with preset temperature, annealing in a muffle furnace at 410 deg.C for 3 hr, heat treating in a muffle furnace at 520 deg.C for 10 hr to obtain Cs4PbBr6Quantum dot glass ceramic samples.
Table 1: perovskite quantum dot formula
Cs2CO3
NaBr
PbBr2
Example 1
9.2％
13.5％
2.3％
Example 2
8.4％
13.8％
2.8％
Example 3
8.2％
13.6％
3.2％
Example 4
8％
13％
4％
Example 5
7.8％
12.6％
4.6％
The percentages in table 1 are in mole percent.
The XRD patterns of examples 1, 2 and 4 are shown in FIG. 7, when Cs is present2CO3And PbBr2The ratio of (A) to (B) is 2: 1 (example 4), the main feature of the diffraction peak is CsPbBr, cubic3The phase (JCPDSNo.54-0752) has better plane fit. With Cs2CO3And PbBr2The ratio of (d) was changed to 3:1 (example 2), Cs4PbBr6Diffraction peaks appear in the phase, and the main diffraction peak is attributed to rhombus Cs4PbBr6The (110), (300), (131), (214), (223) and (330) planes (JCPDSNo.73-2478) of (A) show that pure Cs with high crystallinity is obtained4PbBr6NCS glass. When Cs is2CO3With PbBr2The ratio of (A) to (B) is 4: 1 (example 1), the predominant crystalline phase is CsBr (JCPDSNo. 78-0615). The quantum efficiencies of examples 1-5 were 26.2%, 58.9%, 32, respectively.5％、22.1％、18.9％。
Example 6
With reference to example 2, according to 35% SiO2、20％B2O3、5％ZnO、10％CaO、5％Ba2CO38.4% Cs2CO3, 13.8% NaBr, 2.8% PbBr2The quantum dot component is prepared by accurately weighing raw materials, then placing the raw materials into a corundum crucible, and uniformly mixing and grinding the mixture with the base glass component. Placing in a high temperature furnace, heating to 1300 deg.C, maintaining for 20min, pouring into a mold with preset temperature, annealing in a muffle furnace at 410 deg.C for 3 hr, heat treating in a muffle furnace at 520 deg.C for 10 hr to obtain Cs4PbBr6Quantum dot glass ceramic samples.
The glass liquid can be melted more thoroughly by increasing the melting temperature, cracks in the glass body are reduced, and the mechanical property of the glass is improved. Meanwhile, the glass liquid has stronger flowing property, and the quantum dots are more uniformly distributed in the glass.
Example 7
With reference to example 2, according to 35% SiO2、20％B2O3、5％ZnO、10％CaO、5％Ba2CO3Base glass component of (1), 8.4% Cs2CO3、13.8％NaBr、2.8％PbBr2The quantum dot component is prepared by accurately weighing raw materials, then placing the raw materials into a corundum crucible, and uniformly mixing and grinding the mixture with the base glass component. Placing in a high temperature furnace, heating to 1300 deg.C, maintaining for 10min, pouring into a mold with preset temperature, annealing in a muffle furnace at 410 deg.C for 3 hr, heat treating in a muffle furnace at 520 deg.C for 10 hr to obtain Cs4PbBr6Quantum dot glass ceramic samples.
The volatilization of the quantum dots in a high-temperature environment can be reduced by reducing the heat preservation time of the glass liquid, so that the content of the quantum dots in the glass is higher, and the uniformity and the luminous intensity of the glass are effectively improved.
Examples 8 to 11
With reference to example 2, according to 35% SiO2、20％B2O3、5％ZnO、10％CaO、5％Ba2CO3Base glass component of (1), 8.4% Cs2CO3、13.8％NaBr、2.8％PbBr2The quantum dot component is prepared by accurately weighing raw materials, then placing the raw materials into a corundum crucible, and uniformly mixing and grinding the mixture with the base glass component. Placing in a high temperature furnace, heating to 1300 deg.C, maintaining for 10min, pouring into a mold with preset temperature, annealing in a muffle furnace at 410 deg.C for 3h, heat treating in the muffle furnace at 540 deg.C (example 8), 560 deg.C (example 9), 580 deg.C (example 10), and 600 deg.C (example 11), and maintaining for 10h to obtain Cs4PbBr6Quantum dot glass ceramic samples.
Obtained Cs4PbBr6The photo of the quantum dot glass-ceramic sample under normal light and ultraviolet light is shown in fig. 4, and the sample is transparent glass under normal light. Obtained Cs4PbBr6The XRD pattern of the quantum dot microcrystalline glass sample is shown in figure 1, and Cs is seen from figure 1 when the heat treatment temperature is 560 DEG C4PbBr6The diffraction peak of (a) appears clearly, and as the temperature increases, the intensity of the diffraction peak increases. Obtained Cs4PbBr6The fluorescence emission pattern of the quantum dot glass-ceramic sample is shown in fig. 3, and it can be seen from fig. 3 that the emission wavelength shifts to a long wavelength as the heat treatment temperature increases, and the emission intensity of the glass samples obtained at 540 c and 560 c is the best. Wherein the heat treatment temperature is 540 deg.C4PbBr6An electron microscope image of the quantum dot microcrystalline glass sample is shown in fig. 2, and as can be seen from fig. 2, quantum dots are uniformly distributed in the base glass.
Example 12
According to 35% SiO2、20％B2O3、5％ZnO、10％CaO、5％Ba2CO3Base glass component of (1), 8.4% Cs2CO3、13.8％NaBr、2.8％PbBr2The quantum dot component is prepared by accurately weighing raw materials, then placing the raw materials into a corundum crucible, and uniformly mixing and grinding the mixture with the base glass component. Placing in a high temperature furnace, heating to 1300 deg.C, maintaining for 10min, pouring into a mold with preset temperature, annealing in a muffle furnace at 410 deg.C for 3 hr to obtain Cs4PbBr6And (4) precursor glass.
Mixing Cs4PbBr6The precursor glass is directly ground into powder in an agate mortar, sieved by a 280-mesh sieve, and then mixed with 40% (mass ratio) DLP (digital light processing) desktop photosensitive resin and 60% Cs (Cs)4PbBr6And mixing and ball-milling the precursor glass powder to obtain precursor slurry.
The 3D structure of the university of wenzhou school badge was designed from 3ds MAX (Autodesk) to obtain a Stereolithography (STL) file. The precursor slurry was polymerized by DLP-based 3D printer as STL file. After printing, the sample was separated from the prototype platform, ultrasonically cleaned in alcohol for 30 seconds to remove uncured resin and obtain a model.
And (3) carrying out heat treatment on the model at 560 ℃ for 10h to obtain the 3D printing model with the light-emitting property.
Examples 13 to 14
Referring to example 12, a 3D printed model having a light emitting property was obtained by changing the heat treatment temperature of the model to 540 ℃ (example 13) and 580 ℃ (example 14). The softening and cracking conditions of the resin can be controlled by changing the heat treatment temperature, and Cs in the model can be controlled4PbBr6And the quantum dots are separated out, so that the luminous intensity and the integrity of the model are improved. As shown in fig. 8, 540 ℃ is the optimum heat treatment temperature.
Example 15
The 540 ℃ heat-treated glass ceramic sample obtained in example 8 was ground into powder and stored in water to test Cs4PbBr6Stability of the glass ceramics in water. The storage time of the glass frit in water was changed, and as a result, as shown in fig. 5, the fluorescence intensity of the glass frit was slightly decreased as the storage time was gradually increased from 10 days to 60 days, but finally remained around 85% of the original luminous efficiency.
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