Phân tích nhiệt UserCom 39

Measurement of the boiling point of a liquid at different pressures allows you to determine its vapor pressure curve. Besides this, the enthalpy of vaporization of the liquid can be determined from such measurements using the Clausius–Clapeyron equation. In this article, we show how such measurements are performed in a high-pressure DSC using water as an example.

Introduction

The boiling point of a liquid depends on the pressure of the surroundings: the lower the pressure the lower the temperature at which the liquid boils.

For example, on Mount Everest, the air pressure is only about 325 mbar and water boils at about 70 °C and not at 100 °C like at sea level. The pressure dependence of the boiling point can be easily measured by high-pressure DSC (HPDSC).

The enthalpy of vaporization can also be determined from such measurements using the Clausius–Clapeyron equation [1].

The Clausius–Clapeyron equation describes the relationship between the vapor pressure of a substance, p, and the temperature at which it boils, T:

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References

[1] ASTM E2071, Standard Practice for Calculating Heat of Vaporization or Sublimation from Vapor Pressure Data ( 2010 )

The determination of the composition of ammonium nitrate emulsions is essential for monitoring the quality criteria of the emulsions as well as their stability during storage in order to predict their shelf life. The investigation of these types of emulsions can be time-consuming because a combination of various methods is usually necessary. With the aid of complementary thermal analysis techniques (TGA and DSC), nearly all the constituents of the emulsion can be quantitatively determined with just a minimum of analytical work. The different methods are presented in this article.

Introduction

“Water-in-oil” ammonium nitrate emulsions are the main components of emulsion explosives. In comparison with traditional nitroglycerine-based dynamites, these explosives have several advantages. Following their development in 1962 [1, 2], they have become widely used due to their good safety properties (handling, insensitivity to temperature change), their high detonation rate, and their low basic cost.

The emulsion itself is not an explosive. An effective commercial explosive is obtained when glass or plastic microbubbles, which lower the density of the system, and metal particles (optionally) like aluminum powder as high-energy fuel are mixed into the emulsion. The explosive can be prepared in a mobile unit on site before the final explosive is pumped into the borehole [1].

In the emulsion, the aqueous phase consists of a supersaturated solution of ammonium nitrate (AN), which is metastable due to supersaturation. Sometimes, other salts such as sodium nitrate (SN) or calcium nitrate are added.

The aqueous phase with a droplet size of around 1 μm is emulsified in a small volume of hydrocarbon oil, which forms a thin film around the droplets. The dispersed aqueous phase contains approximately 90 mass percent of the liquid fraction and the remaining 10 mass percent corresponds to the oil phase.

Ammonium nitrate makes up about 60 to 80 percent of the overall composition and is the major ingredient of the emulsion. An emulsifier enables the formation and stabilization of the emulsion, which consists of two phases with very different polarity [1].

During storage, evaporation of water can cause the ammonium nitrate to crystallize. Crystallization leads to breakdown of the emulsion and loss of the explosive properties. On the other hand, a water content that is too high inhibits detonation.

For this reason, stability tests require accurate and precise results for the water content and the state of the ammonium nitrate with respect to its amorphous or crystalline phases [1]. The contents of ammonium nitrate and hydrocarbon oil are important for monitoring the quality control of the emulsion.

Quantitative analysis of the emulsion can be time-consuming because a combination of different methods is usually necessary to detect and determine a range of chemical compounds [3]. With the aid of complementary thermal analysis techniques (TGA and DSC), only a minimum of analytical work is needed to quantitatively determine nearly all the components in ammonium nitrate emulsions. Besides this, the time-consuming separation of the aqueous and oil phases is not necessary so that no errors can arise from this.

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References

[1] H. A. Bampfield and J. Cooper, In: Encyclopedia of Emulsion Technology; P. Becher ( Ed. ); Applications; Marcel Dekker Inc., New York, Vol. 3 ( 1988 ), 282– 306.
[2] C. Oommen et al, J. Hazard. Mater., Vol. A67 ( 1999 ), 253 – 281.
[3] D. T. Burns et al, J. Anal. Chim. Acta, Vol. 375 ( 1998 ), 255 – 260.

Nowadays, natural protein or keratin treatment is used to straighten hair, to stop it from frizzing, and at the same time to make it look shinier. There are, however, risks associated with many types of treatments and it is important to be fully aware of possible adverse effects in order to choose a safe treatment. Thermomechanical analysis (TMA) is a good technique to characterize the mechanical and physical properties of hair.

Introduction

Thermomechanical Analysis (TMA) measures the expansion and shrinkage behavior of materials as a function of time or temperature while they are subjected to a defined force [1, 2].

Different measurement modes with suitable clamping accessories are available. These allow bars, films or thin fibers like hair to be analyzed.

Even very fine hairs can be mounted in the tension accessory with the aid of copper clips.

In this application example, two different hairs, an untreated natural hair and a natural hair that had been subjected to a shine and bond hair treatment, were measured in the tension mode up to a temperature of 230 °C. The hair samples were compared with regard to their shrinkage and expansion behavior and thermal stability.

Hair from the head of a human being is a natural fiber produced by keratin, a protein containing a high concentration of sulfur originating from the amino acid cysteine. The cortex occupies most of the hair area. The keratin fibers of the cortex make the hair strong; its long chains are compressed and form a regular structure. The most important physical property of hair is resistance to tear, but elasticity and hydrophilic properties are also important.

In general, hair treatments are commonly used to obtain a desired appearance, shape or texture. However, treatments can cause damage to the hair on a molecular level resulting in hair that looks dry, thin and weak. The desire for products that improve the appearance and feel of hair has created a huge industry for hair care.

Characterization of the structure and physical and mechanical properties of hair are essential for the development of innovative cosmetic products. Techniques like TMA can be very useful for this type of work.

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References

[1] PET, Physical curing by DLTMA, UserCom 5, 15
[2] Interpreting TMA curves, UserCom 14, 1

The heat capacity of a material can be determined with high accuracy using temperature-modulated DSC. Measurements up to 700 °C can be performed by conventional DSC. In this article, we show how good results can also be obtained above 1000 °C using the TGA/DSC 1. To do this, we present ADSC measurements of nickel, sapphire and molybdenum in the temperature range 900 to 1400 °C. The work originated as part of an interlaboratory test organized in 2012/2013 by the Thermophysics Group of the German Society for Thermal Analysis (Gefta).

Introduction

Several different procedures for determining the specific heat capacity by DSC are available, such as for example the sapphire method [1, 2]. The maximum temperature of the DSC is however 700 °C. The article in reference [3] describes how specific heat capacities can be determined using the sapphire method up to a temperature of 1400 °C with an accuracy of about 10%. Application examples are presented in reference [4].

When the TGA/DSC is used to determine specific heat capacity, the heat flow signal measured simultaneously with the TGA signal is evaluated just as in DSC measurements. Other DSC methods that are available include the temperature-modulated techniques IsoStep^® [5], Steady-State [3], ADSC [6] and TOPEM^® [7, 8]. TOPEM^® is currently not implemented for TGA/DSC.

Compared with the sapphire method, temperature-modulated methods have the advantage that they are less affected by drift and can therefore achieve accuracies of up to 2%. Heat capacities can also be determined for isothermal conditions using Steady-State, ADSC and TOPEM^®.

In addition, information is obtained about the contributions of sensible and latent heat capacity. However, these methods require considerably longer measurement times compared with the sapphire method. In this article, we present specific heat capacity measurements determined by ADSC in the temperature range 900 to 1400 °C. The measurements were performed using the TGA/DSC 1.

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References

[1] Measuring specific heat capacity, UserCom 7, 1 – 5.
[2] G. W. H. Höhne, W. Hemminger and H.-J. Flammersheim: Differential Scanning Calorimetry, Springer Verlag, 1996, 118.
[3] Heat capacity determination at high temperatures by TGA / DSC, Part 1: DSC standard procedures, UserCom 27, 1 – 4.
[4] Heat capacity determination at high temperatures by TGA / DSC, Part 2: Applications, UserCom 28, 1 – 4.
[5] Iso-Step™: UserCom 15, 8, The investigation of curing reactions with IsoStep™ : UserCom 15, 18 – 19.
[6] Alternating differential scanning calorimetry, ADSC, opens new possibilities, UserCom 2, 5 – 6.
[7] J. Schawe, The separation of sensible and latent heat flow using TOPEM^®, UserCom 22, 16 – 19.
[8] J. Schawe et. al., Stochastic temperature modulation: A new technique in temperature modulated DSC, Thermochimica Acta 446 (2006) 147.