Current World Environment

Introduction:

The refrigeration sectorshave a significant and growing influence in the global economy, making substantial contributions to areas such as food preservation, healthcare, energy efficiency, and environmental protection, which policymakers need to thoroughly understand and consider.Didier Coulomb et. al reported that therewere approximately 3 billion refrigeration, air-conditioning, and heat pump systems in operation worldwide, including 1.5 billion domestic refrigerators and almost 17% of the world's total electricity usage has been consumed in this sector in the informatory note on refrigeration technology in November 2015.¹One of the pressing environmental concerns related to refrigeration systems is their contribution to global warming. Approximately 20% of the environmental impact of these systems is attributed to direct emissions, such as release of fluorocarbons (CFCs, HCFCs, and HFCs). The remaining 80% of the impact stems from indirect emissions resulting from the electricity generation needed to power these systems, which is largely derived from fossil fuel power plants. Despite economic growth often taking precedence over environmental considerations in many developing countries, it is imperative to adopt eco-innovative approaches to achieve long-term environmental sustainability. This requires finding innovative solutions that balance economic growth with environmental protection and prioritizing sustainable practices in the refrigeration industry and beyond.^2,3

The conventional method of refrigeration, the vapour compression method, uses refrigerants that contain chlorofluorocarbons (CFC) and hydrochlorofluorocarbons (HCFC) which produces ozone-depleting gases and also greenhouse gases. However, the use of CFCs, HCFCs, and other harmful refrigerants has been banned under the Montreal Protocol, which was implemented on September 16, 1987, to achieve zero ozone depletion effect. Additionally, the Kyoto Protocol, an international treaty set on December 11, 1997, in Kyoto, Japan, aims to reduce greenhouse gas emissions. This has led to the development of alternative sustainable methods of refrigeration that are environmentally friendly and have low global warming potential (GWP) and zero ozone depletion potential (ODP). One such method that has made significant progress in recent years is magnetic refrigeration, which offers several advantages:⁴

(i) use of solid refrigerant material with zero ODP

(ii) less energy consumption with less GWP.

(iii) higher efficiency (30-60%) compared to the vapour compression method (5-10%).

Magnetic refrigeration technology has been developed on the basis of magnetic solid materials which can be acted as a refrigerant by magneto-caloric effect (MCE). Advantages of this technique over the conventional one are :(i) no release of any kind ofODP/GWP gases and(ii) high energy efficiencies. The magneto caloric effect is an inherent characteristic of certain of specific magnetic materials like rare earth alloys and compounds ^[5-10]. These materials exhibit a temperature change when exposed to an external magnetic field, both during its application and subsequent removal.So, the essential requirements for a magnetic cooling machine are (i) a powerful magnetic source and (ii) a material with a significantly high refrigerant capacity. As the entropy as well as adiabatic temperature change increases with application of external magnetic field, a source to generate high value of magnetic field becomes the key parameter of a magnetic refrigerator.  For a large value of magnetic field generation, we have to implement the superconducting magnet which can be utilized for industrial application but not for domestic purpose. So, fabrication of different functional materials with low-field (within 1T) MCE is mandatory for all purpose use of magnetic refrigeration. In addition to find materials with higher MCE, current research trends also prioritize other crucial properties, including thermal hysteresis, cost-effectiveness, aging resistance, low toxicity, and recyclability.¹¹

From the beginning, gadolinium (Gd), has been widely considered as a promising magnetic refrigerant for magnetic refrigerators operating at room temperature.¹² However, its commercial application is somewhat constrained due to its high cost. As a result, research in the field of magnetic cooling has been concentrated on discovering alternative materials that are more affordable yet exhibit greater magnetic cooling effects (MCEs).^13-15 The unique magnetocaloric properties of rare-earth manganites were not reported until 1996, despite being known for over 50 years.¹⁶ However, these materials offer advantages over previously studied materials due to their lack of thermal and field hysteresis and lower cost. Such a proposition was made over a decade ago by Zhang et al., wherein magnetocaloric measurements were conducted onLa_0.67Ca_0.33MnO₃ and La_0.60Ca_0.33Y_0.07MnO_3.¹⁷ For better adjustment of temperature region and value of MCE, three types of doping such as A-site, B-site and vacancy doping has been investigated.^18-20Anotherinteresting phenomenon that has been observed in some doped manganites is inverse magnetocaloric effect which also has drawn considerable attention as it can serve as a heat sink for dissipating heat generated when a conventional magnetocaloric material is magnetized prior to cooling through adiabatic demagnetization.^21-26 Therefore,the combination of conventional magnetocaloric materials with inverse magnetocaloric effect materials presents an exciting opportunity to enhance the efficiency of room-temperature refrigeration.

In this paper we will discuss about the temperature dependence of magnatocaloric effect for a self-doped manganite La_0.9MnO₃which shows a considerable conventional MCE with application of 1T field. In addition, with we will discuss the IMCE effect observed in 2% Cr doped Pr_0.5Sr_0.5MnO₃.

Experimental details and methods of calculations:

Both the samples La_0.9MnO₃ and Pr_0.5Sr_0.5Mn_0.98Cr_0.02O₃ was prepared by the chemicalroute as described our earlier report ^20,21. In order to calculate MCE, isothermal magnetization (M-H) curve has been measured at different temperatures from 0 to 10 kOe (1T) field using commercial superconducting quantum interference devicemagnetometer (Quantum Design).

The MCE can be measured through direct or indirect methods.²⁷ Direct measurement involves observing the change in temperature (?T) during the application or removal of a magnetic field, while indirect methods involve measuring either the heat capacity (C) or magnetization (M). In this study, we employed the conventional indirect method of measuring the MCE through magnetization measurement. Firstly, we generated the temperature-dependent magnetization curve by simulating the experimental data obtained from the isothermal M-H curve. Subsequently, we calculated the magnetic entropy change (?S_M) due to variations in the magnetic field at constant temperature using the Maxwell equation of classical thermodynamics: …. (1)

Results and Discussions

Large MCE in self-doped La_0.9MnO₃ with low field change

From the isothermal M-H curve temperature dependent magnetization curve (M-T) has been drawn under different magnetic field. As shown in dashed line of figure 1 shows M-T curve for self-doped La_0.9MnO₃ for 10 kOe field. A paramagnetic to ferromagnetic transition occurs at 252 K, which is similar to our earlier reported data. Figure 1 shows the temperature dependent MCE for the change in magnetic field from0 to 10 kOe. Maximum MCE has been observed around T_C with value 2.5 J/Kg-K which is reasonably high value for such a low field.

Figure 1: Temperature dependence magnetization (dashed line) and magnetocaloric effect (MCE) for La0.9MnO3.   Click here to view Figure

Large IMCE in half-doped Pr_0.5Sr_0.5Mn_0.98Cr_0.02O₃ with low field change

The addition of a small amount of Chromium (Cr) to the Manganese (Mn) site has been observed to have a significant impact on the Magnetocaloric Effect (MCE) and Inverse Magnetocaloric Effect (IMCE) in Pr0.5Sr0.5MnO3, as discussed in our earlier findings.²¹ Out of various doping concentration we have found that maximum change in temperature happens when 2% Cr is doped. So here we have calculated the temperature dependence MCE and IMCE for

Figure 2: Temperature dependence magnetization (dashed line) and magnetocaloric effect (MCE) for Pr0.5Sr0.5Mn0.98Cr0.02O3.   Click here to view Figure

Pr_0.5Sr_0.5Mn_0.98Cr_0.02O₃in 10 kOe field. Figure 2 shows the M-T curve and ?S_M-T curve for this sample. From the M-T curve two distinct magnetic transition exists for the hole doped sample. A paramagnetic to ferromagnetic transition occurs around 220 K where we get a broad peak in conventional MCE with a comparatively peak value (0.55 J/Kg-K). And in the lower temperature region a sharp dip occurs for inverse magnetocaloric effect around 180 K i.e, the ferromagnetic to antiferromagnetic transition temperature with a value of 1.26 J/Kg-K.

In order to assess the magnetocaloric effect and its potential viability in magnetic refrigeration applications, we have calculated the relative cooling power (RCP). The RCP is determined by multiplying the maximum value of entropy change (?S_Max) by the temperature change at half maximum (?T_FWHM)i.e, . Table 1 shows the maximum magnetic entropy change and RCP of these two materials around transition temperatures.

Table 1: Calculated values of  and RCP for these two materials with change of 10kOe field

 So as shown in table 1, both samples exhibit considerable amount of RCP values to be used as a magnetocaloric materials. ²⁸

Conclusion

In conclusion, to develop environment-friendly cooling technology, it is necessary to seek magnetocaloric materials that exhibit large MCE, no thermal hysteresis, nontoxicity, and cost-effectiveness. In this study, we calculated the temperature-dependent - and RCP around transition temperature for La_0.9MnO₃ and Pr_0.5Sr_0.5Mn_0.98Cr_0.02O₃. A comparatively large MCE in self-doped La_0.9MnO₃has been observed with application or withdrawal of 10kOe magnetic field which can be generated by a reasonably lightweight and compact permanent magnet.We also observeda reasonable amount of IMCE (1.26J/Kg-K) in Pr_0.5Sr_0.5Mn_0.98Cr_0.02O₃ for the application or withdrawal of 10 kOe magnetic field. Both samples displayed significant MCE and RCP values without exhibiting any thermal hysteresis or aging effects. Furthermore, these samples are non-toxic, making them ideal for use in magnetic refrigeration applications. Among these samples, Pr_0.5Sr_0.5Mn_0.98Cr_0.02O₃, with its large MCE and IMCE, is suitable for cooling during both magnetization and demagnetization processes.Thus, this study is a crucial step to find a suitable refrigeration material which can contribute to sustainable practices and help reduce the negative impact of refrigeration on the environment.

Acknowledgement 

Author would like to acknowledge Dr. Asim Ghosh, Department of Physics, Raghunathpur College for continuous support and motivation for the research work. 

Conflict of interest

The author(s) don’t have any conflict of interest in article of research “Magnetocaloric effect in half-doped and self-doped manganites: a study to green refrigeration”. 

Funding Sources 

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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