proton-proton chain reaction

(redirected from Pp chain)
Proton-proton chain reactionclick for a larger image
Proton-proton chain reaction

proton-proton chain reaction

(p-p chain) A series of nuclear fusion reactions by which energy can be generated in the dense cores of stars. The overall effect of the chain reaction is the conversion of hydrogen nuclei to helium nuclei with the release of an immense amount of energy. The energy maintains the core temperature, some of it flowing to the stellar surface. The p-p chain is an efficient energy source even at core temperatures of only 15 million K; it is therefore believed to be the major source of energy in the Sun and in all main-sequence stars that are cooler (less massive) than the Sun. The carbon cycle predominates in the hotter, somewhat more massive stars.

The sequence of steps of the p-p chain is shown in the illustration, with the principal sequence outlined heavily in black. The possible chains occur simultaneously but in what is thought to be 99.75% of the reactions, the sequence begins with two hydrogen nuclei (i.e. protons) combining to emit a positron and a neutrino and forming a nucleus of deuterium. The deuterium then combines with a proton to yield a nucleus of helium–3 and a photon. About 95% of the helium–3 nuclei will combine together to give helium–4, liberating two protons. Overall, each cycle thus converts four protons into one helium–4 nucleus with the release of energy. High collision speeds (plus a quantum-mechanical process known as tunneling) are required for the protons to fuse, since they have to overcome the mutual repulsion arising from their positive charge.

References in periodicals archive ?
Instead of PP chain degradation upon thermal/oxidation reaction happened under high-temperature conditions, the PP-HP polymer chains form a PP network with increasing mechanical strength and stability.
beta]-Scission reactions are encouraged by PP chain motions, while these motions are prevented when chains are stretched under extensional forces [12].
are introduced to reduce the PP chain scission and prolong the life time of macroradicals in the free-radical grafting reactions, wherein St is the most commonly used.
c] decreased with the increase of content of HBP because HBP destroyed the integrity of PP chain and decreased the activity.
The former brings about the degradation of PP chain, whereas the latter leads to the formation of short-chain structures and even large gels.
Those concerning the silane-grafting and cross-linking of PP are relatively scarce (16-22), most likely because of the nature of PP chain structure.
That is to say, BA and ACO were grafted onto PP chains successfully and formed crosslinked copolymer.
The higher specific surface area corresponding to N-MWNTs (as compared to D-MWNTs) aids in interacting PP chains with N-MWNTs with a higher extent.
This may be due to slightly more freedom of movement of lower-molecular-weight PP chains at 23 [degrees] C, which is above the glass-transition temperature of PP (around 0 [degrees] C).
The addition of ABS-g-MAH seemed to facilitate the intercalation of PP chains between the clay platelets in PP/clay hybrids, due to the increased polarity of the system because of the presence of compatibilizer, leading to a mixed intercalated/exfoliated nanocomposite structure.
It has been suggested that the introduction of the clay particles into the matrix decreased the mobility of the polymer molecules, as a result of a confined PP chains in narrow spacing between clay particles.