Far-infrared transmitting Te-based chalcogenide glasses

Satish Chandel

Govt. College Ghumarwin, Distt.Bilaspur (H.P.)

*Corresponding Author E-mail:

ABSTRACT:

To discover the signs of life on terrestrial planets, we must accurately detect the existence of CO2, which has two vibrational absorption bands at 4µm and 16 µm. Therefore optical materials be able to transmit light far beyond 20 µm are essential for manufacturing these detectors. This study reports on the synthesis of tellurium-based glasses that have a wide transmission far beyond the second atmospheric window. Several far-infrared(IR) transmitting glass systems including Ge-In-Te, Ge-Ga-Te are reported. Their glass-forming ability, thermal stability, and IR transmitting property are investigated. The results show that the broad absorption peak in the 15–20 µm disappear in the Fourier-transform infrared (spectrometer) spectra when gallium is replaced by indium.Te-based chalcogenide glasses containing metal-halides show superior glass-forming ability and better thermal stability than those containing alkali halides. Among these glasses, the ∆T of glass composition 65GeTe4-17In2Te6-18AgI can be as great as 115 degcel. In ternary system, the glass composition Ge16Te69(AgI)15 (∆T=120 degcel) is stable enough toward crystallization in combination with broad transmission region and good chemical durability to be drawn into optical fibers.

KEYWORDS: Tellurium based chalcogenide glass; Far-infrared; Thermal stability

I. INTRODUCTION:

The European Space Agency Darwin mission aims atdirectly analyzing extra solar earth like planets in order todetect signs life in these exoplanets. The main biological marketsof life are the molecules of water, ozone and carbon dioxide.Carbon dioxide is considered to be the most importantgreenhouse gas responsible for global warming.Consequently,accurate detection of CO2 in the earth’s atmosphereis critical for the study of environmental processes. Thedecrease of its production and its underground storage are importantresearch subjects nowadays. Remote optical detectionof CO2 involves monitoring of its two vibrational absorptionbands at 4.3 and 15 µm and therefore requires advanced infraredtechnologies. Chalcogenide glasses are well known fortheir transparency in the infrared optical range and their abilityto be drawn into fiber. Such optical fibers can transmitlight from 2 to 20µm, in function of the composition of theglass constituting the fiber.Sulphur-based glasses have theirIR cut-off edge in the 12 µm while for Se-based glasses thecut-off edge is shifted toward the 16 µm region. To extend thetransparency toward longer wavelength, it is necessary to usethe heaviest chalcogenTe. It is well known that Te-basedglass fibers are the only glass fibers having a large enough opticalwindow to cover this large absorption peak (15 µm),making the underground monitoring of CO2 possible forexample.However, there are stillsome problems for Te-basedglasses preparation: first, tellurium has strong metalliccharacter in comparison with S, Se and the glass-forming abilityis relatively week; second, the glass transition temperatureof Te-based chalcogenide glasses is low and thermal stabilityis poor; third, due to structure defects, traditional Te-based glasses are synthesized with Se or As, which will make infraredoptical wavelength (<13µm) far less than thetheoreticallevel (20 µm). So the development ofTe-based glasses is restrictedbythesedefects.Previously,Maurugeonet al.reported that Te76Ge21Se3had good thermal stability (∆T=123 degcel), but the IR cut-offedge was limited by Se atom. Yang et al.revealed thatTe50As40Ge10 had also good thermal stability (∆T=125 degcel),but the glass transition temperature (Tg=175 degcel) was lowand the element As is poisonous, will pose a threat on ourenvironment. Danto et al.revealed that the IR transmittingrange of Te75Ga10Ge15 glass sample was from 2 to 25µm,while the value of ∆Twas only 113 degcel. Wilhelm et al.reported that Te73Ge20I7 has good IR transmitting range (2 to20 µm) but I2 is volatile during the glass synthesis process.In order to solve the problems above, some novel Te-basedchalcogenide glass systems containing halides havebeen reported in this paper. Effect of the high-coordinationnumber of heavy-metal such as Ge, Ga, In and halides on theglass formation is studied and the glass-forming region isexplored. Some physical, thermal and optical properties ofthe amorphous materials and their compositional dependencehave been investigated. The final purpose is to explore morecandidate materials suitable for preparation of optical fibersfor CO2 detection.

II. EXPERIMENTAL:

These bulk glass samples were synthesized by conventionalmelt-quenching method. Elemental raw materials ofhigh purity Ge, Ga, In, Te (all of 5 Nwere carefully weighed and transferredinto quartz ampoules that were then sealed under vacuum at a pressure of 2×1000 Pa. The quartz ampoules containing the raw materials were heated at850 degcel for 17 h in rocking furnaces to ensure the homogenization of mixtures.Then, the ampoules were quenched in water, swiftlymoved to a preheated furnace to anneal at 10 degcel below Tgfor 4 h to minimize inner tension induced by a quenching step.After that, glass rods were obtained by taking them out from the ampoules and finally cut into disks which were then polishedfor testing.The amorphous nature of the as-quenched glasses wasestablished by X-ray power diffraction (XRD).All optical tests were carried outat room temperature.

III. RESULTS AND DISCUSSION:

Stable tellurium-based glass compositions have previouslybeen obtained by combining this element with halogens,giving glasses in the Te-Cl and Te-Br systems.These glasses are stable against crystallization, but sufferfrom low glass transition temperature and low thermal stability.In order to increase the Tg, the high-coordination-numberheavy-metal such as Ge, Ga and Indium elements were introduced in this composition. In respect to the volatility of halogens,Ge-Te and Ge-Ga(In)-Te glass were discussed first.The thermal characteristics of Ge-Ga(In)-Te glass sampleswere investigated by TGA analyses. The obtained data waslisted in the Table I.It is clear from this table that with the Ga contentincreasing, the glass transition temperature (Tg) and the valueof∆T increases. It is the element gallium that has threebonding electrons which can capture lone pair electrons fromTe and reduce the metallic character.The glass composition92.5(GeTe4)-7.5Ga has high ∆T value (110 degcel), whichindicates similar thermal stability as Ge15Ga10Te75 glass sample reported by S. Danto.When Ga is substituted byIn, a heavier element, these Te-based chalcogenide glassesexhibit a relatively poor thermal stability. Only<10 mol%ofIn can be introduced into the system. It reveals that theglass transition temperature and thermal stability will beimproved by the addition of Ga. The addition of In cannot probably improve the glass formabilitybecause of its relatively strong metallic property. However,Ge-In-Te glasses present an excellent transmitting propertyin the Fourier-transform infrared (spectrometer) (FTIR) spectra (Fig. 1) compared with Ge-Ga-Te glasses. It is clearthat there is no absorption in the 15–20 µm, which is beneficial for far-IR transmitting applications.

TABLE I. Characteristic temperatures of Ge-Ga(In)-Te glasses:

Glass composition (mol%) Tg(degcel) Tx (degcel) ∆T (degcel)

GeTe4 132 220 88

95(GeTe4)-5Ga 154 260 106

92.5(GeTe4)-7.5Ga 171 281 110

Ge15Ga10Te75 172 285 113

95(GeTe4)-5In 141 228 87

92.5(GeTe4)-7.5In 140 224 84

90(GeTe4)-10In Partially crystallized

Figure 1

IV. CONCLUSIONS:

A series of novel far-infrared Te-based chalcogenideglasses were prepared and investigated. The glass-formingability and thermal stability of Ge-(In)-Te can be improvedby the addition of heavy-metal halides. With the addition of halides, volatility of halogen during synthesis could beavoided. In quaternary glass system, the glass composition65GeTe4-17In2Te6-18AgI has good thermal stability(∆T=115degcel) and wide transmission range (1.8–25 µm).The 15–20 µm absorptions due to the vibrations of Ga-containingstructural units disappears when Ga is substituted byIn. In ternary glass system, the glass composition Ge16Te69(AgI)15 has relatively good thermal stability (∆T=120degcel),broad transmission region and good chemical durability,which is suitable for fiber drawing.

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Received on 20.01.2014 Accepted on 02.02.2014

Int. J. Tech. 4(1): Jan.-June. 2014; Page 155-157