*2.2.1 Bi2Fe4O9*

The Spin frustrated Mullite type Bi2Fe4O9 (BFO), yielded as an undesired secondary phase from the production of BiFeO3 [61], is being extensively evaluated due to its range of industrial applications in the form of catalyst for oxidation of ammonia, sensors for magnetic field detection, and even as memory storage devices [62–66]. However, the most common issue experienced in this single-phase multiferroic system is the occurrence of weak magnetic/electric fields at room temperature. The Fe6 atoms that are synchronized in an octahedral fashion, interact ferromagnetically with one another while the Fe4 atoms synchronized tetrahedrally towards the inner axis of the unit cell interact in antiferromagnetic order thus ensuring a spin frustrated configuration with weak magnetic order beyond 264 K and weak ferroelectric ordering below 250 K [67]. As documented in reports, partially doping any transition metal ion or rare-earth (RE) ion into either of the sites of BFO can enhance the gas sensing parameters such as its conductivity,

*Fiber Optic Sensors for Gas Detection: An Overview on Spin Frustrated Multiferroics DOI: http://dx.doi.org/10.5772/intechopen.106863*

**Figure 9.** *Crystal structure of Bi2Fe4O9 and the bond angles and bond lengths of the material [71].*

catalytic activity required for oxygen adsorption, electron mobility, chemical stability, and enhanced sensitivity [68–71]. Given the usage of a magnetic dopant, it is important to understand the nature of magnetism in this sample Bi2Fe4O9 samples exhibited significant changes in magnetic and electrical characteristics, which are necessary for improved photocatalytic and gas sensing applications [68, 70]. Bi2Fe4O9 crystal structure consists of two Fe sites, one consisting of the Fe in the tetrahedral coordination (Fe1) and other Fe in octahedral coordination (Fe2) as shown in **Figure 9**. Bi2Fe4O9 is widely used for the detection of VOC test gases at laboratory scale owing to their good selectivity, good long-term stability, low synthesis cost and most importantly its ability to sense at room temperature. Recently Subha et al. [55] reported rare earth Nd doped Bi2Fe4O9 (BNFO3) as a gas sensing material using Fiber optic evanescent wave absorption set up to detect VOC's such as ammonia, ethanol, methanol and acetone at room temperature as shown in **Figure 7**. The improved sensing ability in the Nd-doped Bi2Fe4O9 sample was seen towards ammonia vapours as shown in **Figure 10** with an extremely short response and recovery time of 40 sec and 48 sec, respectively, thereby making them an efficient fiber optic ammonia gas sensor. The same group also investigated room temperature gas sensing ability in Gd doped Bi2Fe4O9 sample, and observed enhanced sensing capability towards ethanol gas at 500 ppm. The clad modified fiber optic gas sensing studies suggested that Gd doping improved ethanol gas sensing ability from 0 to 500 ppm at room temperature compared to host Bi2Fe4O9 with high sensitivity, quick reaction and recovery period of around 38 s and 67 s, respectively. All of these findings highlight the Bi2Fe4O9 with enhanced gas sensing capability suitable for industrial applications [26].

#### **Figure 10.**

*(a) Sensitivity of BNFO3 towards various VOC gas vapors; (b) response analysis of BNNFO3 sample; and (c) gas sensing mechanism in BNFO3 sample [55].*

## *2.2.2 YMnO3*

The usage of YMnO3 perovskite materials in gas sensing properties has recently garnered attention. Such system shows high reactive and stable behaviour over multiple oxidation/reduction states in the experimental cycles. Yttrium and rare-earth manganites in RMnO3 oxide stabilises in two structural phases. The hexagonal phase (space group P63cm, Z = 6) forms with R = Ho, Er, Tm, Yb, Lu, or Y, which have a small ionic radius whereas the orthorhombic phase (space group Pnma, Z = 4) forms with R = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, or Dy, which possess a larger ionic radius. The orthorhombic phase exhibits ferromagnetic ordering, whereas the hexagonal phase shows both ferromagnetic and ferroelectric ordering [72–74]. Hexagonal YMnO3 oxide, with an orthorhombic crystal structure (**Figure 11**) is an excellent material for non-volatile memory and metal-ferroelectric-semiconductor (MFS) devices, because of their coupled magnetic and ferroelectric behaviour [76–79]. Such electromagnetic multiferroics, which exhibit simultaneous ferroelectricity and magnetism, can be exploited in both electrical and magnetic applications. To date, many reports have focused on YMnO3 based materials because of their unique dielectric, ferroelectric, and magnetic properties [75, 80–83].

Recently, gas sensing properties of YMnO3 was tested on LPG, H2, CO and H2S gases and among the gases it showed maximum sensitivity to H2S gas when compared to other gases. The gas sensor fabrication is shown in **Figure 12a** wherein spin frustrated

*Fiber Optic Sensors for Gas Detection: An Overview on Spin Frustrated Multiferroics DOI: http://dx.doi.org/10.5772/intechopen.106863*

**Figure 11.** *The orthorhombic crystal structure of YMnO3, showing the magnetic element Mn [75].*

#### **Figure 12.**

*(a) Schematic diagram of YMnO3 sensor element; and (b) sensor response towards 500 ppm of reducing gases as a function of operating temperature [78].*

YMnO3 of 50 μm thickness is coated inside a cylindrical tube of 8 mm length and 2 mm diameter. Nichrome wire was fixed inside the tube to heat the chamber and to carry out the temperature dependent sensitivity measurement. Chromel alumel thermocouple was used for monitoring the temperature of the tube [78]. Voltage drop in with the application of 10 V was used for sensing the gas. The sensing property of YMnO3 was measured in the presence gases such as CO, H2, H2S and LPG at a concentration of 500 ppm. **Figure 12b** shows the dynamic sensing properties as a function of operating temperature for the YMnO3 different gases. The response of the sensor increases with temperature up to 100°C, and then it decreases with temperature. The YMnO3 sensor element shows a 96% response for an operating temperature of 100°C for H2S gas. Other reducing gases such as CO, H2, and LPG show comparatively low responses [78].

### *2.2.3 Ni3V2O8*

Ni3V2O8 is an important system of spin frustrated magnet at low temperatures it undergoes a series of competing magnetic ordering. Ni3V2O8 has orthorhombic crystal structure with space group Cmca, which is characterized by triangular lattices and short range antiferromagnetic interactions [84]. In Ni3V2O8, the magnetic

#### **Figure 14.**

*(a) Schematic view of exhaust gases after treatment system; and (b) response transients curve for NH3 in the range of 50–500 ppm at 650°C [87].*

*Fiber Optic Sensors for Gas Detection: An Overview on Spin Frustrated Multiferroics DOI: http://dx.doi.org/10.5772/intechopen.106863*

lattice is made up of magnetic ion Ni2+ (S = 1, d8), based on an anisotropic Kagomé lattice. Non-magnetic VO4 tetrahedra layer separates the magnetic layers. Due to this separation the geometric frustration is reduced and this allows long-range magnetic ordering in the material. Ni2+ has two kind of positions that is "spine" and "cross tie" correspondingly. The deviation from the ideal Kagomé geometry introduces many new interactions that relieve the frustration of underlying Kagomé antiferromagnet interacting. The structure of Ni3V2O8 is shown in **Figure 13**.

Recently, Ni3V2O8 is used as important materials for gas sensing application [85–87]. Ni3V2O8 showed the high sensitivity to Ammonia gas. To control the amount NH3 that is sent to the air, a powerful closed loop control system based on the NH3 is required and the schematic of the automobile based sensors are given in **Figure 14a**. The sensor showed the optimal behaviour at 650°C beyond this temperature the behaviour decreases (**Figure 14b**). The sensor 90% response and recovery times of the sensor to 500 ppm NH3 were approximately 2 and 10 s, respectively [87].

### **3. Applications and challenging aspects of fiber optic sensors**

Fiber optic sensors (FOS) have been employed in a variety of harsh environmental applications where other sensor types typically fail, due to its unique characteristics such as electromagnetic immunity, intrinsic safety, chemical and heat resistance, and exceptionally small size. Some innovative FOS solutions have been used in high-temperature, high-pressure, and possibly explosive environments, such as oil and gas wells [88], pipelines [89], and in turbines testing and engine testing [90–92]. It is well recognized that fiber optic sensors play a vital role in many applications. Fiber optic sensors are expected to be utilised to improve the efficacy and cost-effectiveness of many electronic goods, given the vast variety of benefits that fiber optic sensing offers in multiple industries. The usage of fiber optic sensors in environmental monitoring is quadrupling, which is critical for guaranteeing adequate food and water supplies, identifying potential airborne contaminants, and safeguarding structures from corrosion. Water safety, agriculture, transportation, smart structure protection, and biomedical monitoring are all projected to see an increase in environmental monitoring. Further, the future prospects of fiber optic sensors towards various technological aspects are represented in **Figure 15**,

#### **Figure 15.** *Future prospects of fiber optic sensors towards various technological aspects.*

wherein this technology would be employed in smart city initiatives, created specifically for tough and challenging atmosphere. Despite its interesting solutions in environmental monitoring, Clinical environment is still facing challenges in some ways, particularly if FOS is poorly constructed. In a surgical room with a critically ill patient, for example, the instrumentation must be as simple as possible so that the medical personnel may focus on the emergency rather than how the sensor should be attached or the system configured. With a "plug and play" philosophy, it should be as simple as it is with other existing electrical devices. Most practitioners do not yet have the background associated with FOS technologies, and they should not only gain complete confidence in this new technology that is slowly gaining traction in their environment, but also be completely at ease with the potential addition of new steps to their daily medical procedures. In practise, it signifies that an efficacious incorporation of FOS technology is highly reliant on a thorough understanding of medical applications and related clinical measures so as to benefit more from the optical sensing technology.

Despite the multiple challenges and impediments to sensor deployment in smart technology, R&D breakthroughs would result in the widespread availability of low-cost and precise sensors for monitoring water, soil nutrition, temperature, and humidity. Fiber optic sensor technology will continue to grow slowly and steadily over the next few years. Researchers in the field of photonics will continue to be fascinated by fiber optic sensors. They're looking forward to developing new technologies and seeing what these sensors can do for the sensing and instrumentation industries. We recognise that this study may not be exhaustive in all categories, but it is an attempt to provide readers with an overview and the most straightforward way when developing and researching Fiber optic gas sensors.
