The excitation wavelength of conventional Tb3+-activated phosphor is near 270 nm. This study describes novel green-emitting Tb3+-activated amorphous calcium silicate by ultraviolet excitation at 378 nm. The Tb3+-activated amorphous calcium silicate was prepared by heating a sample of Tb3+-activated calcium silicate hydrate (CSH) at 900°C for 30 minutes. The emission wavelength of the resulting phosphor was 544 nm. The optimum excitation wavelength within the range 300–400 nm was 378 nm. The Tb3+-activated amorphous calcium silicate emitted green by ultraviolet irradiation. The optimum initial Tb/Ca atomic ratio of this phosphor was about 0.5. A mechanism for the action of the phosphor is proposed, in which Tb3+ ions existing in the layer of the CSH lead to loss of water molecules and OH groups.
In recent years, white light emitting diodes (LEDs) have been used as an alternative to fluorescent lamps. The method of manufacture of white LEDs is determined by the mode of operation, which can be divided into three types as follows: (1) blue LED + yellow phosphor, (2) blue LED + green and red phosphors, and (3) near-ultraviolet LED + red, green and blue phosphors. However, the emission intensities of the red- or green-emitting LED is weaker than that of the blue-emitting [
In these applications, the Eu3+ ion is mainly used as the phosphor activator for red emitting, and the Tb3+ ion is often known as activator of the green-emitting phosphor. Eu3+ ion emits in the red for irradiation in the near ultraviolet at 395 nm. However, Tb3+ ion does not emit in the green for irradiation in the near ultraviolet at 395 nm. The ultraviolet of near 270 nm was emitted Tb3+ ion as a luminescence center. In this way, the improvement of the phosphor was necessary, since Tb3+-activated phosphor was not emitted in the ultraviolet irradiation at 378 nm. As example, the wavelength for exciting Ce3+ and Tb3+-codoped phosphor was about 320 nm [
The level of doping at which concentration quenching begins differs depending on the types of host crystal. For example, the doping limit before concentration quenching occurs is about 5 mol% in CaCO3 and 0.01 mol% in CaS [
However, we found the Tb3+-activated phosphor which was green-emitting by the ultraviolet irradiation at 378 nm. This study describes synthetic conditions of green emitting Tb3+-activated amorphous calcium silicate phosphor by ultraviolet irradiation for white LEDs.
The raw materials used for synthesis were 95% CaCl2, Na2SiO3, and 99.0% TbCl3·6H2O. All reagents were obtained from Kanto Kagaku Co., Ltd. (Japan).
The initial Tb3+-activated calcium silicate hydrate (CSH) phosphor was synthesized by adding 0.1 mol/dm3 Na2SiO3 solution to a solution of 0.1 mol/dm3 CaCl2 and a given concentration of TbCl3, with the initial Tb/Ca and initial (Ca + Tb)/Si atomic ratio set from 0.1–0.7 and 1, respectively. The mixture was heated at 50°C for 0.5 hours. The as-prepared Tb3+-activated CSH phosphor was then heated at 600–900°C under atmospheric pressure to increase the emission intensity. This heating resulted in the production of Tb3+-activated amorphous calcium silicate phosphor.
The final sample was characterized by X-ray diffraction and inductively coupled plasma (ICP) spectrometry. The fluorescence properties of the sample were measured using a Hitachi F-4500 spectrophotometer. All measurements were carried out at room temperature. In this paper, 100% emission intensity is relative to the 422 nm emission intensity of calcium tungstate, which was irradiated in the ultraviolet at 254 nm. The calcium tungstate reference was prepared by Nakaraitesuku Co., Ltd. (Japan). The emission intensity of calcium tungstate as a standard phosphor was measured at the same time as measurement of emission intensity of the sample phosphor. The internal quantum efficiency was measured by a multichannel photodetector (MCPD-7000, Otsuka electronics Co., Ltd.) with an integrating sphere. To determine Tb/Ca atomic ratio in the phosphor, the Tb3+-activated calcium silicate hydrate was dissolved in hydrochloric acid, and analyzed using ICP spectrometry.
Figure
Structure of CSH.
X-ray diffraction patterns of products prepared by heating the Tb3+-activated CSH phosphor are shown in Figure
X-ray diffraction patterns of products prepared by heating Tb3+-activated calcium silicate hydrate (CSH) phosphor. Tb/Ca atomic ratio, (a) 0, (b)–(e) 0.5. Heating temperature (°C), (a), (b) room temperature, (c) 800, (d) 900, and (e) 1000.
As expected, the initial Tb3+-activated CSH phosphor barely emitted in the green under near-ultraviolet irradiation. However, it was shown that the emission intensity of the CSH phosphors increased with increasing temperature.
We next examined the effect of the initial Tb/Ca atomic ratio on the emission intensity of the phosphor heated at 700°C, as shown in Figure
Effect of initial Tb/Ca atomic ratio on the emission intensity of the phosphor heated at 700°C.
Actually, the chemical composition of Tb3+-activated CSH phosphor was measured using ICP. In the case of initial Tb/Ca atomic ratio of 0.3, the chemical composition of the CSH phosphor was CaO/Tb2O3/SiO2/H2O = 21.1/29.3/38.5/17.3 mass%. The chemical composition of the phosphor obtained for an initial Tb/Ca atomic ratio of 0.5 was Na2O/CaO/Tb2O3/SiO2/H2O = 0.4/12.5/31.8/28.0/17.3 mass%. The content of Na+ increased with the increase in the Tb/Ca atomic ratio. The above results indicated the compositions for the phosphor of Ca0.59Tb0.25SiO2.97 and Na0.07Ca0.37Tb0.27SiO2.76, respectively.
The spectra of excitation and emission of heated Tb3+-activated calcium silicate phosphor at room temperature are shown in Figure
Spectra of excitation and emission of heated Tb3+-activated calcium silicate phosphor at room temperature.
The effect of heating temperature, on emission intensity of Tb3+-activated amorphous calcium silicate phosphor is shown in Figure
Effect of heating temperature on emission intensity of Tb3+-activated amorphous calcium silicate phosphor.
Figure
Appearance of green-emitting Tb3+-activated amorphous calcium silicate phosphor by black-light irradiation. (a) before black-light irradiation, (b) during black-light irradiation.
The internal quantum efficiency commercial phosphor such as red-emitting CASN (CaAlSiN3:Eu2+), blue-emitting BAM (BaMgAl10O17:Eu2+), and green-emitting BSON (Ba3Si6O12N2:Eu2+) was 80%, 83%, and 84%, respectively. The internal quantum efficiency of Tb3+-activated amorphous calcium silicate phosphor in this paper was 37% of the half of commercial phosphors. The half width of CASN, BAM, BSON, and phosphor in the present paper was 89, 63, 56, and 12 nm. That of amorphous calcium silicate phosphor was very narrow. The internal quantum efficiency depended on peak area of emission spectrum and emission intensity. When emission peak was wide, the emission color was not bright. When emission peak was narrow, the emission color was bright.
We reported on synthesis and fluorescence properties of Eu3+-activated amorphous calcium silicate phosphor. In this paper, it used Tb3+ ion instead of Eu3+ ion. Tb3+-activated calcium silicate hydrate (CSH) phosphor was synthesized by liquid-liquid reaction. The amorphous state phosphor was obtained by heating Tb3+-activated CSH at 900°C. Then, the emission intensity of Tb3+-activated amorphous calcium silicate phosphor was a maximum at initial Tb/Ca atomic ratio of 0.50. The excitation wavelength of Tb3+-activated amorphous calcium silicate phosphor was 378 nm. The study on the phosphor with excitation wavelength of 378 nm was very few. Since there were Tb3+ ions in the layer of CSH structure, the excitation wavelength of Tb3+-activated phosphor was changed to ultraviolet at 378 nm. The emission wavelength was 544 nm, and the emission color was green.