Explosion hazards mostly arise from flammable gases and vapors. Instead of avoiding their ignition by explosion protection measurements, it may be preferable to detect them before they become ignitable. Depending on the application, different measuring principles for the detection of gases and vapors can be used, e.g., electrochemical sensors , semiconductor sensors , and point or open-path infrared gas detectors . Optical gas detection by use of infrared radiation (IR) is the leading technique in terms of accuracy, reliability, and economic efficiency. Since most gas measurements are made in the two wavelength ranges of (3… 5) and (8… 14) µm, a broadband IR source with a near-blackbody emission characteristic is necessary. For reliable measurements, the generation of a stable spectrum of infrared radiation is crucial. Very low detection limits can be realized by a long optical path, where more gas molecules are involved in the IR absorption, but this requires a very powerful IR source. According to Planck's law of thermal radiation, the spectrum of a thermal emitter with a blackbody characteristic depends only on its temperature T . The totally emitted radiant power PS is given by the Stefan–Boltzmann law PS=εσAemitT4 with emissivity ε, Stefan–Boltzmann constant σ, and the radiating area Aemit. Consequently, a high radiation power is mainly realized by a high operating temperature of the thermal emitter, but also by a high and wavelength-independent emissivity of infrared radiation. Especially portable gas sensing devices require a low energy consumption of the IR source. For this reason, the radiating element has to be optimized regarding an excellent thermal isolation.

In and , we reported on a novel IR absorber based on a nanostructured NiCr alloy that features a very high and spectrally homogeneous absorptivity. From Kirchhoff's law , αλ,T=ελ,T, it is known that for the maintenance of thermal equilibrium, the absorptivity α at wavelength λ and temperature T must be equal to the emissivity ε. Therefore, good absorbers are also good emitters. Since NiCr is one of the most used materials in heating elements, for instance in toasters and hairdryers, we studied the application of the nanostructured NiCr absorber (Fig. ) in thermal infrared emitters to increase their emittance. By depositing the nanostructured NiCr layer on a thin NiCr foil, the high-temperature stability of the nanostructures could be verified with a first prototype of a thermal IR emitter in a TO-39 package (Fig. ). Thereby, operation in air causes an oxidation of the surface. The speed of oxidation and the thickness of the oxide are increased with increasing temperature. As a consequence, the spectral emissivity of the nanostructured surface is affected. In contrast, an operation in an inert gas atmosphere does not change its optical properties even at very high temperatures. The layout of the first prototype had some disadvantages. First, it had a very low electrical resistance of about 1 Ω, which is not practicable to use. In addition, the temperature distribution was not homogeneous, as can be seen from Fig. . Finally, it had a relatively high power consumption of about 5 W due to the poor thermal isolation of the radiating element.

As already mentioned, an ideal thermal emitter should feature a high electrical resistance and a low heat capacity (also referred to as thermal mass) in order to be modulated electrically. Furthermore, most of the electrical input power should be transformed into radiant power. This can be achieved by a good thermal isolation, a high emissivity near 100 % and a high average temperature of the emitting area. The emissivity and the average temperature, however, do not have any effect on the shape of the heating conductor. So the focus is on improving thermal isolation and electrical resistance. A simplified current-carrying heating conductor is shown in Fig. . In order to improve its geometric shape, the theoretical dependencies need to be considered.

The electrical resistance R of a rectangular conductor is R=ϱellAQ=ϱellbd, where ϱel is the specific electrical resistance of the conductor material and l is the conductor length. The cross section AQ is the product of conductor thickness d and its width b. As given by Eq. (), the radiant power is proportional to the emitting area Aemit; thus, Aemit=bl should be maximized. The conductor material is NiCr. To achieve a high resistance and maximize the emitting area corresponding to Eqs. () and (), the conductor layout can be optimized as follows.

Choose conductor length l as large as possible.

To evaluate the quality of a thermal emitter, further criteria must be analyzed. Thermal emitters are optical devices, so the heating conductor needs to stay in focus while under thermal load, meaning a low deflection is required. Furthermore, the mean element temperature directly affects the emitted radiant power. So temperature should be evenly distributed, leading to a high mean temperature. To assess these characteristics, finite element analyses (FEA) have been carried out. The goal is to compare meander-shaped (Fig. a) and spiral-shaped conductors (Fig. b) with respect to electrical resistance, temperature distribution and out-of-plane deflection (called displacement in FEA).

The boundary conditions for the FEA are the following.

Thermal: heat sink with TU=20 ∘C at the pins of the TO39 header;

Table  shows the results of the analyses. The temperature distribution of the meander-shaped emitter (Fig. a) reproduces the visual appearance shown in Fig. , where hotter areas are brighter. Compared to the meander shape, the spiral-shaped element has far more electrical resistance as well as a uniform temperature distribution (Fig. b). This leads to a higher mean temperature of the spiral, although the maximum element temperatures of both layouts are equal. Due to the higher mean temperature, the spiral-shaped conductor is twice as efficient as the meander-shaped conductor regarding emitted radiant power. The fixed support of the conductor ends forces the elements to buckle under thermal load. Fig.  shows the meander displacement as a result of buckling. After a longer period in operation, we discovered malfunction of the emitter caused by buckling (Fig. ), which is not tolerable. The magnitude of the spiral displacement is much lower (u<0.3 mm), and tolerable.

New prototypes were fabricated that have the spiral shape. By reducing the thickness of the radiating element to about 10 µm, a direct electrical modulation of the emitted radiation becomes possible. This is an important feature for its application in gas sensing devices because the most commonly used pyroelectric IR detectors are only sensitive to AC signals. Accordingly, it eliminates the use of a mechanical chopper for radiation modulation and, therefore, enables miniaturization and allows for more compact gas sensing devices. The spiral radiating elements were fabricated by ion beam etching of a thin NiCr foil and have a radiating area of about 2.8 mm2. Both sides are coated with a 2 µm thick nanostructured layer of high emissivity (Fig. ). The radiating elements are hermetically sealed into common TO-39 housings with a krypton atmosphere (Fig. ). In order to use the backside-emitted radiation, a reflector is integrated into the TO-39 housing. In this way, a 70 % higher signal was achieved.

The electrical and optical properties of the new prototypes were studied. We found an adequate electrical cold resistance of about 31Ω±3Ω and a slightly higher hot resistance of about 33Ω±3Ω (Fig. ). Accordingly, the emitters can be operated at 3.3 V, a common voltage used in portable devices. In this case, the mean electrical power consumption is 330mW±20mW, with an operating temperature of 970 K (Fig. ). For temperature measurement the spectral radiance has been measured with a monochromator as can be seen in Fig. . Then the maximum of the spectral radiance has been determined and converted into a temperature with Wien's displacement law. The temperature error is estimated at ±30 K. Comparable thin-film emitters reach only a maximum temperature of about 800 K at the same electrical input power p. 5.

The thin and low-mass radiating element allows the IR source to be modulated electrically. By applying a square-wave voltage, the emitter is switched on and off. Because a non-contact and fast measurement of the temperature of the electrically modulated source is not possible at the moment, the emitted radiant flux Φ is measured with a broadband and fast IR detector. Another advantage of this approach is the possibility to compare the emitter with commercial IR sources, e.g., from and . The frequency response measured with the radiant flux is called normalized optical emission (NOE): NOE=ΔΦ(fm)ΔΦ(fm→0). With increasing frequency, NOE decreases due to the thermal inertia of the emitter. Consequently, the frequency response has a low-pass characteristic (Fig. ). The measurement results show a maximum modulation frequency of about 3 Hz for 50 % NOE. This is sufficient for most applications. However, the NOE and the modulation depth can be simply increased by a further reduction of the thermal mass.

Finally, the measurement results of the spectral radiance validate the near-blackbody emission characteristic of this thermal infrared source (Fig. ).

In this paper, we presented a novel thermal IR emitter for application in optical gas detection. It features a very high radiant power, near-blackbody emission characteristic and can be modulated electrically. Due the excellent thermal isolation from the heat sink, the electrical power consumption is very low. This enables an application in portable gas sensing devices. Since the used NiCr alloy allows a permanent operation at temperatures of up to 1400 K, the fabricated IR source should feature a long-term stable performance and a long lifetime. Finally, its application in gas analysis will improve the performance and efficiency of gas measurement systems. Further improvements will target the reduction of the thermal mass of the radiating element in order to increase its modulation depth.