Thermogravimetry (TG) is a technique for measuring mass change of a sample with temperature. A sample to be measured is placed in a furnace and its mass change is monitored by thermobalance Analyze the material decomposition and thermal stability through mass change as a function of temperature in scanning mode or as a function of time in the isothermal mode. TG curves are plotted as mass change expressed in percent versus temperature or time. Instrument Set up The Cahn microbalance senses the vertical displacement of a sample due to mass change using an optical system. The optical system includes a light source, a ﬂag, a light tube and a photodiode. The ﬂag beneath the balance arm interferes with light propagating from the source to the light detector (photodiode) when the mass change is sensed by the balance beam. A feedback control system adjusts the current in a magneticcoil system and maintains the balance beam in its original horizontal position even if the sample mass keeps changing. sample is connected to the microbalance Furnace designed as cylindrical tube with heating elements on its wall Diameter of the tube accommodates the sample with relatively little open space. Thermocouple location eliminates temperature gradient among the sample, thermocouple and heating elements Protective gas will ﬂow into and out of the furnace tube to maintain an inert atmosphere during heating Sample Dimension • Endothermic decomposition: high heating rate will increase the starting and ﬁnishing temperatures of decomposition. • Temperature range from start to ﬁnish will be wider at a higher heating rate than a lower heating rate • High heating rate :temperature difference between the sample and thermocouple junction. • Real sample temperature may lag behind that of the thermocouple • Decomposition with volatile products: products take time to diffuse out of the sample and be carried away by ﬂowing gas. A low heating rate is more likely to generate thermal equilibrium and give a reproducible result for the analysis horizontal line :no decomposition with mass loss of volatile products indicates a rapid mass loss at the initial stage of a TA curve (Drying or desorption) one-stage decomposition curve: Stability Limit of Sample multistage decomposition with stable intermediates multistage decomposition but with no stable intermediates: High heating rate version of (iv) indicates that a chemical reaction with mass gain: say oxidation of metal mass-gain reaction occurs and then a mass-loss reaction occurs Derivative Thermogravimetric Curve(DTG) A peak in a DTG curve represents a maximum of mass change rate. DTG does not contain any new information other than the original TG curve however, it clearly identiﬁes the temperature at which mass loss is at a maximum The temperature at which decomposition starts is deﬁned as the intersection of initial line tangent and tangent of line portion when the slope changed. The ﬁnishing temperature of decomposition is deﬁned in a similar manner, here midpoint temperature between the starting and ﬁnishing temperature can be deﬁned as TB • Crystal water loss occurs at separated temperatures. • Correspondingly, the structure of a sample goes through several stages of change during crystal water loss. • TG curves do not always decomposition temperatures show obvious • desirable to plot DTG curves with TG curves to reveal the decomposition temperatures of polymers. • Only DTG curves could clearly indicate the decomposition temperature of natural rubber, as well as the higher decomposition temperature of butadiene rubber. Composition Analysis Demonstrates that the amount of ceramics (hydroxyapatite) in composites can be accurately determined Measure the weight differences in composite samples before and after the polymer (ultrahighmolecular weight polyethylene) Kinetic Analysis : How to obtain a DTG equation Here α For a linear heating rate Eq. 1 Taking derivative of Eq 1, we get Integrate both sides to get the TG curve (theoretical) Kinetics of most reactions under isothermal conditions can be summarized by the general equation: The various mathematical equations that relate with the rate of some solid-state reaction as given in the previous slides. g(α) and f(α) uses TG data in their integral or differentiated forms, respectively. The former uses data directly from the TG curve, and the differentiated form uses data from the DTG curve. Each equation is associated with a particular model for the progress of the reaction, such as gas diffusion into a solid particle or the appearance of product nuclei with subsequent growth of those nuclei. The shape of the TG curve will vary with the particular mechanism involved, as will the temperature at which dα/dt is a maximum The traditional method of conducting kinetic studies is the isothermal method: loading the sample into the TG apparatus, rapidly heating it to some speciﬁc temperature, and monitoring the mass change. The experiment is repeated with a new sample at another temperature, and a family of curves is generated The total mass loss for the reaction is found by heating the sample in a rising-temperature experiment until the mass loss is complete. A series of α and dα/dt values can then be calculated for a given temperature data are ﬁtted to the expressions as given earlier, and the best ﬁt suggests the model that is likely to apply to the reaction under test. From this expression, the average rate constant k can be calculated at each temperature. A plot of log k vs. 1=T should provide a straight-line graph of slope E/R from which E can be calculated, and the intercept gives the preexponential factor A 1. It is difﬁcult to determine exactly the time and temperature of the beginning of the reaction. 2. It requires several experiments and so is time consuming. 3. Each experiment requires a new sample, which must react in exactly the same way as all the other samples. This requires careful attention to standardization of the experimental procedures, which includes, e.g., the use of the same sample pan with the same sample packing and gas exchange. Decomposition of a solid material :Affected by the partial pressure of product gas Using values of the Gibbs free-energy change (ΔG°), the enthalpy change (ΔH°), and the entropy change (ΔS°), are the decomposition temperature T can be calculated for any partial pressure P of CO2: Hence, under equilibrium conditions, the decomposition temperature is lowered by 138 C. The temperatures found by thermal methods are always higher because these are dynamic techniques, although for small sample sizes and slow heating rates the correspondence is close.